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HTTP Message Signatures
draft-ietf-httpbis-message-signatures-10

Document Type Active Internet-Draft (httpbis WG)
Authors Annabelle Backman , Justin Richer , Manu Sporny
Last updated 2022-05-26
Replaces draft-richanna-http-message-signatures, draft-cavage-http-signatures
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draft-ietf-httpbis-message-signatures-10
HTTP                                                     A. Backman, Ed.
Internet-Draft                                                    Amazon
Intended status: Standards Track                               J. Richer
Expires: 27 November 2022                            Bespoke Engineering
                                                               M. Sporny
                                                          Digital Bazaar
                                                             26 May 2022

                        HTTP Message Signatures
                draft-ietf-httpbis-message-signatures-10

Abstract

   This document describes a mechanism for creating, encoding, and
   verifying digital signatures or message authentication codes over
   components of an HTTP message.  This mechanism supports use cases
   where the full HTTP message may not be known to the signer, and where
   the message may be transformed (e.g., by intermediaries) before
   reaching the verifier.  This document also describes a means for
   requesting that a signature be applied to a subsequent HTTP message
   in an ongoing HTTP exchange.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpbis-message-
   signatures/.

   Discussion of this document takes place on the HTTP Working Group
   mailing list (mailto:ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/.  Working Group
   information can be found at https://httpwg.org/.

   Source for this draft and an issue tracker can be found at
   https://github.com/httpwg/http-extensions/labels/signatures.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Discussion . . . . . . . . . . . . . . . . .   5
     1.2.  HTTP Message Transformations  . . . . . . . . . . . . . .   6
     1.3.  Conventions and Terminology . . . . . . . . . . . . . . .   7
     1.4.  Application of HTTP Message Signatures  . . . . . . . . .  10
   2.  HTTP Message Components . . . . . . . . . . . . . . . . . . .  11
     2.1.  HTTP Fields . . . . . . . . . . . . . . . . . . . . . . .  12
       2.1.1.  Canonicalized Structured HTTP Fields  . . . . . . . .  14
       2.1.2.  Dictionary Structured Field Members . . . . . . . . .  14
     2.2.  Derived Components  . . . . . . . . . . . . . . . . . . .  15
       2.2.1.  Signature Parameters  . . . . . . . . . . . . . . . .  17
       2.2.2.  Method  . . . . . . . . . . . . . . . . . . . . . . .  18
       2.2.3.  Target URI  . . . . . . . . . . . . . . . . . . . . .  19
       2.2.4.  Authority . . . . . . . . . . . . . . . . . . . . . .  19
       2.2.5.  Scheme  . . . . . . . . . . . . . . . . . . . . . . .  20
       2.2.6.  Request Target  . . . . . . . . . . . . . . . . . . .  20
       2.2.7.  Path  . . . . . . . . . . . . . . . . . . . . . . . .  22
       2.2.8.  Query . . . . . . . . . . . . . . . . . . . . . . . .  22
       2.2.9.  Query Parameters  . . . . . . . . . . . . . . . . . .  23
       2.2.10. Status Code . . . . . . . . . . . . . . . . . . . . .  25
     2.3.  Request-Response Signature Binding  . . . . . . . . . . .  25
     2.4.  Creating the Signature Base . . . . . . . . . . . . . . .  28
   3.  HTTP Message Signatures . . . . . . . . . . . . . . . . . . .  30
     3.1.  Creating a Signature  . . . . . . . . . . . . . . . . . .  31
     3.2.  Verifying a Signature . . . . . . . . . . . . . . . . . .  33

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       3.2.1.  Enforcing Application Requirements  . . . . . . . . .  35
     3.3.  Signature Algorithm Methods . . . . . . . . . . . . . . .  36
       3.3.1.  RSASSA-PSS using SHA-512  . . . . . . . . . . . . . .  37
       3.3.2.  RSASSA-PKCS1-v1_5 using SHA-256 . . . . . . . . . . .  37
       3.3.3.  HMAC using SHA-256  . . . . . . . . . . . . . . . . .  38
       3.3.4.  ECDSA using curve P-256 DSS and SHA-256 . . . . . . .  38
       3.3.5.  EdDSA using curve edwards25519  . . . . . . . . . . .  39
       3.3.6.  JSON Web Signature (JWS) algorithms . . . . . . . . .  40
   4.  Including a Message Signature in a Message  . . . . . . . . .  40
     4.1.  The Signature-Input HTTP Field  . . . . . . . . . . . . .  41
     4.2.  The Signature HTTP Field  . . . . . . . . . . . . . . . .  41
     4.3.  Multiple Signatures . . . . . . . . . . . . . . . . . . .  42
   5.  Requesting Signatures . . . . . . . . . . . . . . . . . . . .  45
     5.1.  The Accept-Signature Field  . . . . . . . . . . . . . . .  46
     5.2.  Processing an Accept-Signature  . . . . . . . . . . . . .  47
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
     6.1.  HTTP Signature Algorithms Registry  . . . . . . . . . . .  48
       6.1.1.  Registration Template . . . . . . . . . . . . . . . .  48
       6.1.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  49
     6.2.  HTTP Signature Metadata Parameters Registry . . . . . . .  49
       6.2.1.  Registration Template . . . . . . . . . . . . . . . .  49
       6.2.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  50
     6.3.  HTTP Signature Derived Component Names Registry . . . . .  50
       6.3.1.  Registration Template . . . . . . . . . . . . . . . .  51
       6.3.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  51
     6.4.  HTTP Signature Component Parameters Registry  . . . . . .  53
       6.4.1.  Registration Template . . . . . . . . . . . . . . . .  53
       6.4.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  53
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  54
     7.1.  Signature Verification Skipping . . . . . . . . . . . . .  55
     7.2.  Use of TLS  . . . . . . . . . . . . . . . . . . . . . . .  55
     7.3.  Signature Replay  . . . . . . . . . . . . . . . . . . . .  55
     7.4.  Insufficient Coverage . . . . . . . . . . . . . . . . . .  56
     7.5.  Cryptography and Signature Collision  . . . . . . . . . .  56
     7.6.  Key Theft . . . . . . . . . . . . . . . . . . . . . . . .  57
     7.7.  Modification of Required Message Parameters . . . . . . .  57
     7.8.  Mismatch of Signature Parameters from Message . . . . . .  58
     7.9.  Multiple Signature Confusion  . . . . . . . . . . . . . .  58
     7.10. Signature Labels  . . . . . . . . . . . . . . . . . . . .  58
     7.11. Symmetric Cryptography  . . . . . . . . . . . . . . . . .  59
     7.12. Canonicalization Attacks  . . . . . . . . . . . . . . . .  59
     7.13. Key Specification Mix-Up  . . . . . . . . . . . . . . . .  60
     7.14. HTTP Versions and Component Ambiguity . . . . . . . . . .  60
     7.15. Key and Algorithm Specification Downgrades  . . . . . . .  60
     7.16. Parsing Structured Field Values . . . . . . . . . . . . .  61
     7.17. Choosing Message Components . . . . . . . . . . . . . . .  61
     7.18. Confusing HTTP Field Names for Derived Component Names  .  62
     7.19. Non-deterministic Signature Primitives  . . . . . . . . .  62

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     7.20. Choosing Signature Parameters and Derived Components over
            HTTP Fields  . . . . . . . . . . . . . . . . . . . . . .  63
     7.21. Semantically Equivalent Field Values  . . . . . . . . . .  63
     7.22. Message Content . . . . . . . . . . . . . . . . . . . . .  64
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  65
     8.1.  Identification through Keys . . . . . . . . . . . . . . .  65
     8.2.  Signatures do not provide confidentiality . . . . . . . .  65
     8.3.  Oracles . . . . . . . . . . . . . . . . . . . . . . . . .  66
     8.4.  Required Content  . . . . . . . . . . . . . . . . . . . .  66
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  66
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  66
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  68
   Appendix A.  Detecting HTTP Message Signatures  . . . . . . . . .  69
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  69
     B.1.  Example Keys  . . . . . . . . . . . . . . . . . . . . . .  69
       B.1.1.  Example Key RSA test  . . . . . . . . . . . . . . . .  70
       B.1.2.  Example RSA PSS Key . . . . . . . . . . . . . . . . .  71
       B.1.3.  Example ECC P-256 Test Key  . . . . . . . . . . . . .  72
       B.1.4.  Example Ed25519 Test Key  . . . . . . . . . . . . . .  73
       B.1.5.  Example Shared Secret . . . . . . . . . . . . . . . .  73
     B.2.  Test Cases  . . . . . . . . . . . . . . . . . . . . . . .  73
       B.2.1.  Minimal Signature Using rsa-pss-sha512  . . . . . . .  74
       B.2.2.  Selective Covered Components using rsa-pss-sha512 . .  75
       B.2.3.  Full Coverage using rsa-pss-sha512  . . . . . . . . .  76
       B.2.4.  Signing a Response using ecdsa-p256-sha256  . . . . .  77
       B.2.5.  Signing a Request using hmac-sha256 . . . . . . . . .  78
       B.2.6.  Signing a Request using ed25519 . . . . . . . . . . .  78
     B.3.  TLS-Terminating Proxies . . . . . . . . . . . . . . . . .  79
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  81
   Document History  . . . . . . . . . . . . . . . . . . . . . . . .  82
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  86

1.  Introduction

   Message integrity and authenticity are important security properties
   that are critical to the secure operation of many HTTP applications.
   Application developers typically rely on the transport layer to
   provide these properties, by operating their application over [TLS].
   However, TLS only guarantees these properties over a single TLS
   connection, and the path between client and application may be
   composed of multiple independent TLS connections (for example, if the
   application is hosted behind a TLS-terminating gateway or if the
   client is behind a TLS Inspection appliance).  In such cases, TLS
   cannot guarantee end-to-end message integrity or authenticity between
   the client and application.  Additionally, some operating
   environments present obstacles that make it impractical to use TLS,
   or to use features necessary to provide message authenticity.
   Furthermore, some applications require the binding of an application-

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   level key to the HTTP message, separate from any TLS certificates in
   use.  Consequently, while TLS can meet message integrity and
   authenticity needs for many HTTP-based applications, it is not a
   universal solution.

   This document defines a mechanism for providing end-to-end integrity
   and authenticity for components of an HTTP message.  The mechanism
   allows applications to create digital signatures or message
   authentication codes (MACs) over only the components of the message
   that are meaningful and appropriate for the application.  Strict
   canonicalization rules ensure that the verifier can verify the
   signature even if the message has been transformed in any of the many
   ways permitted by HTTP.

   The signing mechanism described in this document consists of three
   parts:

   *  A common nomenclature and canonicalization rule set for the
      different protocol elements and other components of HTTP messages,
      used to create the signature base.

   *  Algorithms for generating and verifying signatures over HTTP
      message components using this signature base through application
      of cryptographic primitives.

   *  A mechanism for attaching a signature and related metadata to an
      HTTP message, and for parsing attached signatures and metadata
      from HTTP messages.  To facilitate this, this document defines the
      "Signature-Input" and "Signature" fields.

   This document also provides a mechanism for a potential verifier to
   signal to a potential signer that a signature is desired in one or
   more subsequent messages.  This optional negotiation mechanism can be
   used along with opportunistic or application-driven message
   signatures by either party.  To facilitate this, this document
   defines the "Accept-Signature" field.

1.1.  Requirements Discussion

   HTTP permits and sometimes requires intermediaries to transform
   messages in a variety of ways.  This may result in a recipient
   receiving a message that is not bitwise equivalent to the message
   that was originally sent.  In such a case, the recipient will be
   unable to verify a signature over the raw bytes of the sender's HTTP
   message, as verifying digital signatures or MACs requires both signer
   and verifier to have the exact same signature base.  Since the exact
   raw bytes of the message cannot be relied upon as a reliable source
   for a signature base, the signer and verifier must independently

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   create the signature base from their respective versions of the
   message, via a mechanism that is resilient to safe changes that do
   not alter the meaning of the message.

   For a variety of reasons, it is impractical to strictly define what
   constitutes a safe change versus an unsafe one.  Applications use
   HTTP in a wide variety of ways, and may disagree on whether a
   particular piece of information in a message (e.g., the message
   content, the method, or the Date header field) is relevant.  Thus a
   general purpose solution must provide signers with some degree of
   control over which message components are signed.

   HTTP applications may be running in environments that do not provide
   complete access to or control over HTTP messages (such as a web
   browser's JavaScript environment), or may be using libraries that
   abstract away the details of the protocol (such as the Java
   HTTPClient library (https://openjdk.java.net/groups/net/httpclient/
   intro.html)).  These applications need to be able to generate and
   verify signatures despite incomplete knowledge of the HTTP message.

1.2.  HTTP Message Transformations

   As mentioned earlier, HTTP explicitly permits and in some cases
   requires implementations to transform messages in a variety of ways.
   Implementations are required to tolerate many of these
   transformations.  What follows is a non-normative and non-exhaustive
   list of transformations that may occur under HTTP, provided as
   context:

   *  Re-ordering of header fields with different header field names
      (Section 3.2.2 of [HTTP1]).

   *  Combination of header fields with the same field name
      (Section 3.2.2 of [HTTP1]).

   *  Removal of header fields listed in the Connection header field
      (Section 6.1 of [HTTP1]).

   *  Addition of header fields that indicate control options
      (Section 6.1 of [HTTP1]).

   *  Addition or removal of a transfer coding (Section 5.7.2 of
      [HTTP1]).

   *  Addition of header fields such as Via (Section 5.7.1 of [HTTP1])
      and Forwarded (Section 4 of [RFC7239]).

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   Based on the definition of HTTP and the requirements described above,
   we can identify certain types of transformations that should not
   prevent signature verification, even when performed on message
   components covered by the signature.  The following list describes
   those transformations:

   *  Combination of header fields with the same field name.

   *  Reordering of header fields with different names.

   *  Conversion between different versions of the HTTP protocol (e.g.,
      HTTP/1.x to HTTP/2, or vice-versa).

   *  Changes in casing (e.g., "Origin" to "origin") of any case-
      insensitive components such as header field names, request URI
      scheme, or host.

   *  Addition or removal of leading or trailing whitespace to a header
      field value.

   *  Addition or removal of obs-folds.

   *  Changes to the request-target and Host header field that when
      applied together do not result in a change to the message's
      effective request URI, as defined in Section 5.5 of [HTTP1].

   Additionally, all changes to components not covered by the signature
   are considered safe.

1.3.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The terms "HTTP message", "HTTP request", "HTTP response", absolute-
   form, absolute-path, "effective request URI", "gateway", "header
   field", "intermediary", request-target, "sender", and "recipient" are
   used as defined in [HTTP1].

   The term "method" is to be interpreted as defined in Section 4 of
   [HTTP].

   For brevity, the term "signature" on its own is used in this document
   to refer to both digital signatures (which use asymmetric
   cryptography) and keyed MACs (which use symmetric cryptography).

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   Similarly, the verb "sign" refers to the generation of either a
   digital signature or keyed MAC over a given input string.  The
   qualified term "digital signature" refers specifically to the output
   of an asymmetric cryptographic signing operation.

   In addition to those listed above, this document uses the following
   terms:

   HTTP Message Signature:
      A digital signature or keyed MAC that covers one or more portions
      of an HTTP message.  Note that a given HTTP Message can contain
      multiple HTTP Message Signatures.

   Signer:
      The entity that is generating or has generated an HTTP Message
      Signature.  Note that multiple entities can act as signers and
      apply separate HTTP Message Signatures to a given HTTP Message.

   Verifier:
      An entity that is verifying or has verified an HTTP Message
      Signature against an HTTP Message.  Note that an HTTP Message
      Signature may be verified multiple times, potentially by different
      entities.

   HTTP Message Component:
      A portion of an HTTP message that is capable of being covered by
      an HTTP Message Signature.

   HTTP Message Component Name:
      A name that identifies an HTTP Message Component.

   HTTP Message Component Identifier:
      The combination of an HTTP Message Component Name and any
      parameters that uniquely identifies a specific HTTP Message
      Component in respect to a particular HTTP Message Signature and
      the HTTP Message it applies to.

   HTTP Message Component Value:
      The value associated with a given component identifier within the
      context of a particular HTTP Message.  Component values are
      derived from the HTTP Message and are usually subject to a
      canonicalization process.

   Covered Components:

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      An ordered set of HTTP message component identifiers for fields
      (Section 2.1) and derived components (Section 2.2) that indicates
      the set of message components covered by the signature, never
      including the @signature-params identifier itself.  The order of
      this set is preserved and communicated between the signer and
      verifier to facilitate reconstruction of the signature base.

   Signature Base:
      The sequence of bytes processed by the cryptographic algorithm to
      produce or verify the HTTP Message Signature.  The signature base
      is generated by the signer and verifier using the covered
      components set and the HTTP Message.

   HTTP Message Signature Algorithm:
      A cryptographic algorithm that describes the signing and
      verification process for the signature, defined in terms of the
      HTTP_SIGN and HTTP_VERIFY primitives described in Section 3.3.

   Key Material:
      The key material required to create or verify the signature.  The
      key material is often identified with an explicit key identifier,
      allowing the signer to indicate to the verifier which key was
      used.

   Creation Time:
      A timestamp representing the point in time that the signature was
      generated, as asserted by the signer.

   Expiration Time:
      A timestamp representing the point in time after which the
      signature should no longer be accepted by the verifier, as
      asserted by the signer.

   The term "Unix time" is defined by [POSIX.1], Section 4.16
   (http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
   V1_chap04.html#tag_04_16).

   This document contains non-normative examples of partial and complete
   HTTP messages.  Some examples use a single trailing backslash '' to
   indicate line wrapping for long values, as per [RFC8792].  The \
   character and leading spaces on wrapped lines are not part of the
   value.

   This document uses the following terminology from Section 3 of
   [STRUCTURED-FIELDS] to specify syntax and parsing: List, Inner List,
   Dictionary, String, Integer, Byte Sequence, and Boolean.  This
   document uses the following ABNF rules from [STRUCTURED-FIELDS] where
   applicable: sf-string, key, parameters.

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1.4.  Application of HTTP Message Signatures

   HTTP Message Signatures are designed to be a general-purpose security
   mechanism applicable in a wide variety of circumstances and
   applications.  In order to properly and safely apply HTTP Message
   Signatures, an application or profile of this specification MUST
   specify all of the following items:

   *  The set of component identifiers (Section 2) that are expected and
      required.  For example, an authorization protocol could mandate
      that the Authorization field be covered to protect the
      authorization credentials and mandate the signature parameters
      contain a created parameter, while an API expecting semantically
      relevant HTTP message content could require the Content-Digest
      header to be present and covered.

   *  A means of retrieving the key material used to verify the
      signature.  An application will usually use the keyid parameter of
      the signature parameters (Section 2.2.1) and define rules for
      resolving a key from there, though the appropriate key could be
      known from other means.

   *  A means of determining the signature algorithm used to verify the
      signature is appropriate for the key material.  For example, the
      process could use the alg parameter of the signature parameters
      (Section 2.2.1) to state the algorithm explicitly, derive the
      algorithm from the key material, or use some pre-configured
      algorithm agreed upon by the signer and verifier.

   *  A means of determining that a given key and algorithm presented in
      the request are appropriate for the request being made.  For
      example, a server expecting only ECDSA signatures should know to
      reject any RSA signatures, or a server expecting asymmetric
      cryptography should know to reject any symmetric cryptography.

   An application using signatures also has to ensure that the verifier
   will have access to all required information to re-create the
   signature base.  For example, a server behind a reverse proxy would
   need to know the original request URI to make use of identifiers like
   @target-uri.  Additionally, an application using signatures in
   responses would need to ensure that clients receiving signed
   responses have access to all the signed portions, including any
   portions of the request that were signed by the server.

   The details of this kind of profiling are the purview of the
   application and outside the scope of this specification, however some
   additional considerations are discussed in Section 7.

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2.  HTTP Message Components

   In order to allow signers and verifiers to establish which components
   are covered by a signature, this document defines component
   identifiers for components covered by an HTTP Message Signature, a
   set of rules for deriving and canonicalizing the values associated
   with these component identifiers from the HTTP Message, and the means
   for combining these canonicalized values into a signature base.  The
   values for these items MUST be accessible to both the signer and the
   verifier of the message, which means these are usually derived from
   aspects of the HTTP message or signature itself.

   Some HTTP message components can undergo transformations that change
   the bitwise value without altering meaning of the component's value
   (for example, the merging together of header fields with the same
   name).  Message component values must therefore be canonicalized
   before it is signed, to ensure that a signature can be verified
   despite such intermediary transformations.  This document defines
   rules for each component identifier that transform the identifier's
   associated component value into such a canonical form.

   Component identifiers are serialized using the production grammar
   defined by [STRUCTURED-FIELDS], Section 4.  The component identifier
   has a component name, which is a String value serialized using the
   sf-string ABNF rule.  The component identifier MAY also include
   defined parameters which are serialized using the parameters rule.

   component-identifier = component-name parameters
   component-name = sf-string

   Note that this means the serialization of the component name itself
   is encased in double quotes, with parameters following as a
   semicolon-separated list, such as "cache-control", "date", or
   "@signature-params", and "example-dictionary";key="foo".

   One component identifier is distinct from another if either the
   component name or its parameters differ.  Within a single list of
   covered components, each component identifier MUST be distinct from
   every other component identifier.  Multiple component identifiers
   having the same component name MAY be included if they have
   parameters that make them distinct.

   The component value associated with a component identifier is defined
   by the identifier itself.  Component values MUST NOT contain newline
   (\n) characters.

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   The following sections define component identifier types, their
   parameters, their associated values, and the canonicalization rules
   for their values.  The method for combining component identifiers
   into the signature base is defined in Section 2.4.

2.1.  HTTP Fields

   The component name for an HTTP field is the lowercased form of its
   field name.  While HTTP field names are case-insensitive,
   implementations MUST use lowercased field names (e.g., content-type,
   date, etag) when using them as component names.

   Unless overridden by additional parameters and rules, the HTTP field
   value MUST be canonicalized as a single combined value as defined in
   Section 5.2 of [HTTP].

   If the combined value is not available for a given header, the
   following algorithm will produce canonicalized results for an
   implementation:

   1.  Create an ordered list of the field values of each instance of
       the field in the message, in the order that they occur (or will
       occur) in the message.

   2.  Strip leading and trailing whitespace from each item in the list.
       Note that since HTTP field values are not allowed to contain
       leading and trailing whitespace, this will be a no-op in a
       compliant implementation.

   3.  Remove any obsolete line-folding within the line and replace it
       with a single space (" "), as discussed in Section 5.2 of
       [HTTP1].  Note that this behavior is specific to [HTTP1] and does
       not apply to other versions of the HTTP specification.

   4.  Concatenate the list of values together with a single comma (",")
       and a single space (" ") between each item.

   The resulting string is the canonicalized component value.

   Note that some HTTP fields have values with multiple valid
   serializations that have equivalent semantics.  Applications signing
   and processing such fields MUST consider how to handle the values of
   such fields to ensure that the signer and verifier can derive the
   same value, as discussed in Section 7.21.

   Following are non-normative examples of canonicalized values for
   header fields, given the following example HTTP message fragment:

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   Host: www.example.com
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   X-OWS-Header:   Leading and trailing whitespace.
   X-Obs-Fold-Header: Obsolete
       line folding.
   Cache-Control: max-age=60
   Cache-Control:    must-revalidate
   Example-Dict:  a=1,    b=2;x=1;y=2,   c=(a   b   c)

   The following example shows canonicalized values for these example
   header fields, presented using the signature base format discussed in
   Section 2.4:

   "host": www.example.com
   "date": Tue, 20 Apr 2021 02:07:56 GMT
   "x-ows-header": Leading and trailing whitespace.
   "x-obs-fold-header": Obsolete line folding.
   "cache-control": max-age=60, must-revalidate
   "example-dict": a=1,    b=2;x=1;y=2,   c=(a   b   c)

   Since empty HTTP header fields are allowed, they are also able to be
   signed when present in a message.  The canonicalized value is the
   empty string.  This means that the following empty header:

   NOTE: '\' line wrapping per RFC 8792

   X-Empty-Header: \

   Is serialized by the signature base generation algorithm
   (Section 2.4) with an empty string value following the colon and
   space added after the content identifier.

   NOTE: '\' line wrapping per RFC 8792

   "x-empty-header": \

   Note: these are shown here using the line wrapping algorithm in
   [RFC8792] due to limitations in the document format that strips
   trailing spaces from diagrams.

   Any HTTP field component identifiers MAY have the following
   parameters in specific circumstances.

   sf  A boolean flag indicating that the field value is to be
      canonicalized using strict encoding of the structured field value.
      Section 2.1.1

   key  A string parameter used to select a single member value from a

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      dictionary structured field.  Section 2.1.2

2.1.1.  Canonicalized Structured HTTP Fields

   If the value of the the HTTP field in question is a structured field
   ([STRUCTURED-FIELDS]), the component identifier MAY include the sf
   parameter to indicate it is a known structured field.  If this
   parameter is included with a component identifier, the HTTP field
   value MUST be serialized using the rules specified in Section 4 of
   [STRUCTURED-FIELDS] applicable to the type of the HTTP field.  Note
   that this process will replace any optional internal whitespace with
   a single space character, among other potential transformations of
   the value.

   For example, the following dictionary field is a valid serialization:

   Example-Dict:  a=1,    b=2;x=1;y=2,   c=(a   b   c)

   If included in the input string as-is, it would be:

   "example-dict": a=1,    b=2;x=1;y=2,   c=(a   b   c)

   However, if the sf parameter is added, the value is re-serialized as
   follows:

   "example-dict";sf: a=1, b=2;x=1;y=2, c=(a b c)

   The resulting string is used as the component value in Section 2.1.

2.1.2.  Dictionary Structured Field Members

   If a given field is known by the application to be a Dictionary
   structured field, an individual member in the value of that
   Dictionary is identified by using the parameter key and the
   Dictionary member key as a String value.

   An individual member value of a Dictionary Structured Field is
   canonicalized by applying the serialization algorithm described in
   Section 4.1.2 of [STRUCTURED-FIELDS] on the member_value and its
   parameters, without the dictionary key.  Specifically, the value is
   serialized as an Item or Inner List (the two possible values of a
   Dictionary member).

   Each parameterized key for a given field MUST NOT appear more than
   once in the signature base.  Parameterized keys MAY appear in any
   order in the signature base, regardless of the order they occur in
   the source Dictionary.

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   If a Dictionary key is named as a covered component but it does not
   occur in the Dictionary, this MUST cause an error in the signature
   base generation.

   Following are non-normative examples of canonicalized values for
   Dictionary structured field members given the following example
   header field, whose value is known by the application to be a
   Dictionary:

   Example-Dict:  a=1, b=2;x=1;y=2, c=(a   b    c), d

   The following example shows canonicalized values for different
   component identifiers of this field, presented using the signature
   base format discussed in Section 2.4:

   "example-dict";key="a": 1
   "example-dict";key="d": ?1
   "example-dict";key="b": 2;x=1;y=2
   "example-dict";key="c": (a b c)

   Note that the value for key="c" has been re-serialized according to
   the strict member_value algorithm.

2.2.  Derived Components

   In addition to HTTP fields, there are a number of different
   components that can be derived from the control data, processing
   context, or other aspects of the HTTP message being signed.  Such
   derived components can be included in the signature base by defining
   a component name, possible parameters, and the derivation method for
   its component value.

   Derived component names MUST start with the "at" @ character.  This
   differentiates derived component names from HTTP field names, which
   cannot contain the @ character as per Section 5.1 of [HTTP].
   Processors of HTTP Message Signatures MUST treat derived component
   names separately from field names, as discussed in Section 7.18.

   This specification defines the following derived components:

   @signature-params  The signature metadata parameters for this
      signature.  (Section 2.2.1)

   @method  The method used for a request.  (Section 2.2.2)

   @target-uri  The full target URI for a request.  (Section 2.2.3)

   @authority  The authority of the target URI for a request.

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      (Section 2.2.4)

   @scheme  The scheme of the target URI for a request.  (Section 2.2.5)

   @request-target  The request target.  (Section 2.2.6)

   @path  The absolute path portion of the target URI for a request.
      (Section 2.2.7)

   @query  The query portion of the target URI for a request.
      (Section 2.2.8)

   @query-param  A parsed query parameter of the target URI for a
      request.  (Section 2.2.9)

   @status  The status code for a response.  (Section 2.2.10).

   Additional derived component names MAY be defined and registered in
   the HTTP Signatures Derived Component Name Registry.  (Section 6.3)

   Derived component values are taken from the context of the target
   message for the signature.  This context includes information about
   the message itself, such as its control data, as well as any
   additional state and context held by the signer.  In particular, when
   signing a response, the signer can include any derived components
   from the originating request by using the request-response signature
   binding parameter (Section 2.3).

   request:  Values derived from and results applied to an HTTP request
      message as described in Section 3.4 of [HTTP].

   response:  Values derived from and results applied to an HTTP
      response message as described in Section 3.4 of [HTTP].

   A derived component definition MUST define all targets to which it
   can be applied.

   The component value MUST be derived from the HTTP message being
   signed or the context in which the derivation occurs.  The derived
   component value MUST be of the following form:

   derived-component-value = *VCHAR

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2.2.1.  Signature Parameters

   HTTP Message Signatures have metadata properties that provide
   information regarding the signature's generation and verification,
   such as the set of covered components, a timestamp, identifiers for
   verification key material, and other utilities.

   The signature parameters component name is @signature-params.  This
   message component's value is REQUIRED as part of the signature base
   (Section 2.4) but the component identifier MUST NOT be enumerated
   within the set of covered components itself.

   The signature parameters component value is the serialization of the
   signature parameters for this signature, including the covered
   components set with all associated parameters.  These parameters
   include any of the following:

   *  created: Creation time as an Integer UNIX timestamp value.  Sub-
      second precision is not supported.  Inclusion of this parameter is
      RECOMMENDED.

   *  expires: Expiration time as an Integer UNIX timestamp value.  Sub-
      second precision is not supported.

   *  nonce: A random unique value generated for this signature as a
      String value.

   *  alg: The HTTP message signature algorithm from the HTTP Message
      Signature Algorithm Registry, as a String value.

   *  keyid: The identifier for the key material as a String value.

   Additional parameters can be defined in the HTTP Signature Parameters
   Registry (Section 6.2.2).

   The signature parameters component value is serialized as a
   parameterized inner list using the rules in Section 4 of
   [STRUCTURED-FIELDS] as follows:

   1.  Let the output be an empty string.

   2.  Determine an order for the component identifiers of the covered
       components, not including the @signature-params component
       identifier itself.  Once this order is chosen, it cannot be
       changed.  This order MUST be the same order as used in creating
       the signature base (Section 2.4).

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   3.  Serialize the component identifiers of the covered components,
       including all parameters, as an ordered inner-list according to
       Section 4.1.1.1 of [STRUCTURED-FIELDS] and append this to the
       output.

   4.  Determine an order for any signature parameters.  Once this order
       is chosen, it cannot be changed.

   5.  Append the parameters to the inner-list in the chosen order
       according to Section 4.1.1.2 of [STRUCTURED-FIELDS], skipping
       parameters that are not available or not used for this message
       signature.

   6.  The output contains the signature parameters component value.

   Note that the Inner List serialization from Section 4.1.1.1 of
   [STRUCTURED-FIELDS] is used for the covered component value instead
   of the List serialization from Section 4.1.1 of [STRUCTURED-FIELDS]
   in order to facilitate parallelism with this value's inclusion in the
   Signature-Input field, as discussed in Section 4.1.

   This example shows a canonicalized value for the parameters of a
   given signature:

   NOTE: '\' line wrapping per RFC 8792

   ("@target-uri" "@authority" "date" "cache-control")\
     ;keyid="test-key-rsa-pss";alg="rsa-pss-sha512";\
     created=1618884475;expires=1618884775

   Note that an HTTP message could contain multiple signatures
   (Section 4.3), but only the signature parameters used for a single
   signature are included in an entry.

2.2.2.  Method

   The @method derived component refers to the HTTP method of a request
   message.  The component value is canonicalized by taking the value of
   the method as a string.  Note that the method name is case-sensitive
   as per [HTTP], Section 9.1, and conventionally standardized method
   names are uppercase US-ASCII.  If used, the @method component
   identifier MUST occur only once in the covered components.

   For example, the following request message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

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   Would result in the following @method component value:

   POST

   And the following signature base line:

   "@method": POST

2.2.3.  Target URI

   The @target-uri derived component refers to the target URI of a
   request message.  The component value is the full absolute target URI
   of the request, potentially assembled from all available parts
   including the authority and request target as described in [HTTP],
   Section 7.1.  If used, the @target-uri component identifier MUST
   occur only once in the covered components.

   For example, the following message sent over HTTPS:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   Would result in the following @target-uri component value:

   https://www.example.com/path?param=value

   And the following signature base line:

   "@target-uri": https://www.example.com/path?param=value

2.2.4.  Authority

   The @authority derived component refers to the authority component of
   the target URI of the HTTP request message, as defined in [HTTP],
   Section 7.2.  In HTTP 1.1, this is usually conveyed using the Host
   header, while in HTTP 2 and HTTP 3 it is conveyed using the
   :authority pseudo-header.  The value is the fully-qualified authority
   component of the request, comprised of the host and, optionally, port
   of the request target, as a string.  The component value MUST be
   normalized according to the rules in [HTTP], Section 4.2.3.  Namely,
   the host name is normalized to lowercase and the default port is
   omitted.  If used, the @authority component identifier MUST occur
   only once in the covered components.

   For example, the following request message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

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   Would result in the following @authority component value:

   www.example.com

   And the following signature base line:

   "@authority": www.example.com

   The @authority derived component SHOULD be used instead of signing
   the Host header directly, see Section 7.20.

2.2.5.  Scheme

   The @scheme derived component refers to the scheme of the target URL
   of the HTTP request message.  The component value is the scheme as a
   string as defined in [HTTP], Section 4.2.  While the scheme itself is
   case-insensitive, it MUST be normalized to lowercase for inclusion in
   the signature base.  If used, the @scheme component identifier MUST
   occur only once in the covered components.

   For example, the following request message requested over plain HTTP:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   Would result in the following @scheme component value:

   http

   And the following signature base line:

   "@scheme": http

2.2.6.  Request Target

   The @request-target derived component refers to the full request
   target of the HTTP request message, as defined in [HTTP],
   Section 7.1.  The component value of the request target can take
   different forms, depending on the type of request, as described
   below.  If used, the @request-target component identifier MUST occur
   only once in the covered components.

   For HTTP 1.1, the component value is equivalent to the request target
   portion of the request line.  However, this value is more difficult
   to reliably construct in other versions of HTTP.  Therefore, it is
   NOT RECOMMENDED that this component be used when versions of HTTP
   other than 1.1 might be in use.

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   The origin form value is combination of the absolute path and query
   components of the request URL.  For example, the following request
   message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   Would result in the following @request-target component value:

   /path?param=value

   And the following signature base line:

   "@request-target": /path?param=value

   The following request to an HTTP proxy with the absolute-form value,
   containing the fully qualified target URI:

   GET https://www.example.com/path?param=value HTTP/1.1

   Would result in the following @request-target component value:

   https://www.example.com/path?param=value

   And the following signature base line:

   "@request-target": https://www.example.com/path?param=value

   The following CONNECT request with an authority-form value,
   containing the host and port of the target:

   CONNECT www.example.com:80 HTTP/1.1
   Host: www.example.com

   Would result in the following @request-target component value:

   www.example.com:80

   And the following signature base line:

   "@request-target": www.example.com:80

   The following OPTIONS request message with the asterisk-form value,
   containing a single asterisk * character:

   OPTIONS * HTTP/1.1
   Host: www.example.com

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   Would result in the following @request-target component value:

   *

   And the following signature base line:

   "@request-target": *

2.2.7.  Path

   The @path derived component refers to the target path of the HTTP
   request message.  The component value is the absolute path of the
   request target defined by [RFC3986], with no query component and no
   trailing ? character.  The value is normalized according to the rules
   in [HTTP], Section 4.2.3.  Namely, an empty path string is normalized
   as a single slash / character, and path components are represented by
   their values after decoding any percent-encoded octets.  If used, the
   @path component identifier MUST occur only once in the covered
   components.

   For example, the following request message:

   POST /path?param=value HTTP/1.1
   Host: www.example.com

   Would result in the following @path component value:

   /path

   And the following signature base line:

   "@path": /path

2.2.8.  Query

   The @query derived component refers to the query component of the
   HTTP request message.  The component value is the entire normalized
   query string defined by [RFC3986], including the leading ? character.
   The value is normalized according to the rules in [HTTP],
   Section 4.2.3.  Namely, percent-encoded octets are decoded.  If used,
   the @query component identifier MUST occur only once in the covered
   components.

   For example, the following request message:

   POST /path?param=value&foo=bar&baz=batman HTTP/1.1
   Host: www.example.com

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   Would result in the following @query component value:

   ?param=value&foo=bar&baz=batman

   And the following signature base line:

   "@query": ?param=value&foo=bar&baz=batman

   The following request message:

   POST /path?queryString HTTP/1.1
   Host: www.example.com

   Would result in the following @query component value:

   ?queryString

   And the following signature base line:

   "@query": ?queryString

   If the query string is absent from the request message, the value is
   the leading ? character alone:

   ?

   Resulting in the following signature base line:

   "@query": ?

2.2.9.  Query Parameters

   If a request target URI uses HTML form parameters in the query string
   as defined in HTMLURL, Section 5 [HTMLURL], the @query-param derived
   component allows addressing of individual query parameters.  The
   query parameters MUST be parsed according to HTMLURL, Section 5.1
   [HTMLURL], resulting in a list of (nameString, valueString) tuples.
   The REQUIRED name parameter of each component identifier contains the
   nameString of a single query parameter as a String value.  Several
   different named query parameters MAY be included in the covered
   components.  Single named parameters MAY occur in any order in the
   covered components.

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   The component value of a single named parameter is the the
   valueString of the named query parameter defined by HTMLURL,
   Section 5.1 [HTMLURL], which is the value after percent-encoded
   octets are decoded.  Note that this value does not include any
   leading ? characters, equals sign =, or separating & characters.
   Named query parameters with an empty valueString are included with an
   empty string as the component value.

   If a query parameter is named as a covered component but it does not
   occur in the query parameters, this MUST cause an error in the
   signature base generation.

   For example for the following request:

   POST /path?param=value&foo=bar&baz=batman&qux= HTTP/1.1
   Host: www.example.com

   Indicating the baz, qux and param named query parameters in would
   result in the following @query-param component values:

   _baz:_ batman

   _qux:_ an empty string

   _param:_ value

   And the following signature base lines:

   NOTE: '\' line wrapping per RFC 8792

   "@query-param";name="baz": batman
   "@query-param";name="qux": \

   "@query-param";name="param": value

   If a parameter name occurs multiple times in a request, all parameter
   values of that name MUST be included in separate signature base lines
   in the order in which the parameters occur in the target URI.  Note
   that in some implementations, the order of parsed query parameters is
   not stable, and this situation could lead to unexpected results.  If
   multiple parameters are common within an application, it is
   RECOMMENDED to sign the entire query string using the @query
   component identifier defined in Section 2.2.8.

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2.2.10.  Status Code

   The @status derived component refers to the three-digit numeric HTTP
   status code of a response message as defined in [HTTP], Section 15.
   The component value is the serialized three-digit integer of the HTTP
   status code, with no descriptive text.  If used, the @status
   component identifier MUST occur only once in the covered components.

   For example, the following response message:

   HTTP/1.1 200 OK
   Date: Fri, 26 Mar 2010 00:05:00 GMT

   Would result in the following @status component value:

   200

   And the following signature base line:

   "@status": 200

   The @status component identifier MUST NOT be used in a request
   message.

2.3.  Request-Response Signature Binding

   When a request message results in a signed response message, the
   signer can include portions of the request message in the signature
   base by adding the req parameter to the component identifier.

   req  Indicates that the component value is derived from the request
      that triggered this response message and not from the response
      message directly.

   This parameter can be applied to both HTTP fields and derived
   components with the same semantics.  The component value for a
   message component using this parameter is calculated in the same
   manner as it is normally, but data is pulled from the request
   message.

   Note that the same component name MAY be included with and without
   the req parameter in a single signature base, indicating the same
   named component from both the request and response message.

   The req parameter MAY be combined with other parameters as
   appropriate for the component identifier, such as the key parameter
   for a dictionary field.

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   For example, when serving a response for this signed request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature:  sig1=:LAH8BjcfcOcLojiuOBFWn0P5keD3xAOuJRGziCLuD8r5MW9S0\
     RoXXLzLSRfGY/3SF8kVIkHjE13SEFdTo4Af/fJ/Pu9wheqoLVdwXyY/UkBIS1M8Br\
     c8IODsn5DFIrG0IrburbLi0uCc+E2ZIIb6HbUJ+o+jP58JelMTe0QE3IpWINTEzpx\
     jqDf5/Df+InHCAkQCTuKsamjWXUpyOT1Wkxi7YPVNOjW4MfNuTZ9HdbD2Tr65+BXe\
     TG9ZS/9SWuXAc+BZ8WyPz0QRz//ec3uWXd7bYYODSjRAxHqX+S1ag3LZElYyUKaAI\
     jZ8MGOt4gXEwCSLDv/zqxZeWLj/PDkn6w==:

   {"hello": "world"}

   This would result in the following unsigned response message:

   HTTP/1.1 503 Service Unavailable
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62

   {"busy": true, "message": "Your call is very important to us"}

   To cryptographically link the response to the request, the server
   signs the response with its own key and includes the signature of
   sig1 from the request in the covered components of the response.  The
   signature base for this example is:

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   NOTE: '\' line wrapping per RFC 8792

   "@status": 503
   "content-length": 62
   "content-type": application/json
   "signature";req;key="sig1": :LAH8BjcfcOcLojiuOBFWn0P5keD3xAOuJRGziC\
     LuD8r5MW9S0RoXXLzLSRfGY/3SF8kVIkHjE13SEFdTo4Af/fJ/Pu9wheqoLVdwXyY\
     /UkBIS1M8Brc8IODsn5DFIrG0IrburbLi0uCc+E2ZIIb6HbUJ+o+jP58JelMTe0QE\
     3IpWINTEzpxjqDf5/Df+InHCAkQCTuKsamjWXUpyOT1Wkxi7YPVNOjW4MfNuTZ9Hd\
     bD2Tr65+BXeTG9ZS/9SWuXAc+BZ8WyPz0QRz//ec3uWXd7bYYODSjRAxHqX+S1ag3\
     LZElYyUKaAIjZ8MGOt4gXEwCSLDv/zqxZeWLj/PDkn6w==:
   "@signature-params": ("@status" "content-length" "content-type" \
     "signature";req;key="sig1");created=1618884479\
     ;keyid="test-key-ecc-p256"

   The signed response message is:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 503 Service Unavailable
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62
   Signature-Input: reqres=("@status" "content-length" "content-type" \
     "signature";req;key="sig1");created=1618884479\
     ;keyid="test-key-ecc-p256"
   Signature: reqres=:vR1E+sDgh0J3dZyVdPc7mK0ZbEMW3N47eDpFjXLE9g95Gx1K\
     QLpdOmDQfedgdLzaFCqfD0WPn9e9/jubyUuZRw==:

   {"busy": true, "message": "Your call is very important to us"}

   Since the request's signature value itself is not repeated in the
   response, the requester MUST keep the original signature value around
   long enough to validate the signature of the response that uses this
   component identifier.

   Note that the ECDSA algorithm in use here is non-deterministic,
   meaning a different signature value will be created every time the
   algorithm is run.  The signature value provided here can be validated
   against the given keys, but newly-generated signature values are not
   expected to match the example.  See Section 7.19.

   The req parameter MUST NOT be used with a request message.

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2.4.  Creating the Signature Base

   The signature base is a US-ASCII string containing the canonicalized
   HTTP message components covered by the signature.  The input to the
   signature base creation algorithm is the list of covered component
   identifiers and their associated values, along with any additional
   signature parameters.  The output is the ordered set of bytes that
   form the signature base, which conforms to the following ABNF:

   signature-base = *( signature-base-line LF ) signature-params-line
   signature-base-line = component-identifier ":" SP
       ( derived-component-value / field-value )
   signature-params-line = DQUOTE "@signature-params" DQUOTE ":" SP inner-list

   To create the signature base, the signer or verifier concatenates
   together entries for each identifier in the signature's covered
   components (including their parameters) using the following
   algorithm:

   1.  Let the output be an empty string.

   2.  For each message component item in the covered components set (in
       order):

       1.  Append the component identifier for the covered component
           serialized according to the component-identifier rule.  Note
           that this serialization places the component name in double
           quotes and appends any parameters outside of the quotes.

       2.  Append a single colon :

       3.  Append a single space " "

       4.  Determine the component value for the component identifier.

           *  If the component name starts with an "at" character (@),
              derive the component's value from the message according to
              the specific rules defined for the derived component, as
              in Section 2.2.  If the derived component name is unknown
              or the value cannot be derived, produce an error.

           *  If the component name does not start with an "at"
              character (@), canonicalize the HTTP field value as
              described in Section 2.1.  If the value cannot be
              calculated, produce an error.

       5.  Append the covered component's canonicalized component value.

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       6.  Append a single newline \n

   3.  Append the signature parameters component (Section 2.2.1) as
       follows:

       1.  Append the component identifier for the signature parameters
           serialized according to the component-identifier rule, i.e.
           "@signature-params"

       2.  Append a single colon :

       3.  Append a single space " "

       4.  Append the signature parameters' canonicalized component
           value as defined in Section 2.2.1

   4.  Return the output string.

   If covered components reference a component identifier that cannot be
   resolved to a component value in the message, the implementation MUST
   produce an error and not create an input string.  Such situations are
   included but not limited to:

   *  The signer or verifier does not understand the derived component
      name.

   *  The component name identifies a field that is not present in the
      message or whose value is malformed.

   *  The component identifier indicates that a structured field
      serialization is used (via the sf parameter), but the field in
      question is known to not be a structured field or the type of
      structured field is not known to the implementation.

   *  The component identifier is a dictionary member identifier that
      references a field that is not present in the message, is not a
      Dictionary Structured Field, or whose value is malformed.

   *  The component identifier is a dictionary member identifier or a
      named query parameter identifier that references a member that is
      not present in the component value, or whose value is malformed.
      E.g., the identifier is "example-dict";key="c" and the value of
      the Example-Dict header field is a=1, b=2, which does not have the
      c value.

   In the following non-normative example, the HTTP message being signed
   is the following request:

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   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18

   {"hello": "world"}

   The covered components consist of the @method, @authority, and @path
   derived components followed by the Content-Digest, Content-Length,
   and Content-Type HTTP header fields, in order.  The signature
   parameters consist of a creation timestamp of 1618884473 and a key
   identifier of test-key-rsa-pss.  Note that no explicit alg parameter
   is given here since the verifier is assumed by the application to
   correctly use the RSA PSS algorithm based on the identified key.  The
   signature base for this message with these parameters is:

   NOTE: '\' line wrapping per RFC 8792

   "@method": POST
   "@authority": example.com
   "@path": /foo
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "content-length": 18
   "content-type": application/json
   "@signature-params": ("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884473;keyid="test-key-rsa-pss"

               Figure 1: Non-normative example Signature Base

   Note that the example signature base here, or anywhere else within
   this specification, does not include the final newline that ends the
   displayed example.

3.  HTTP Message Signatures

   An HTTP Message Signature is a signature over a string generated from
   a subset of the components of an HTTP message in addition to metadata
   about the signature itself.  When successfully verified against an
   HTTP message, an HTTP Message Signature provides cryptographic proof
   that the message is semantically equivalent to the message for which
   the signature was generated, with respect to the subset of message

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   components that was signed.

3.1.  Creating a Signature

   Creation of an HTTP message signature is a process that takes as its
   input the message and the requirements for the application.  The
   output is a signature value and set of signature parameters that can
   be applied to the message.

   In order to create a signature, a signer MUST follow the following
   algorithm:

   1.  The signer chooses an HTTP signature algorithm and key material
       for signing.  The signer MUST choose key material that is
       appropriate for the signature's algorithm, and that conforms to
       any requirements defined by the algorithm, such as key size or
       format.  The mechanism by which the signer chooses the algorithm
       and key material is out of scope for this document.

   2.  The signer sets the signature's creation time to the current
       time.

   3.  If applicable, the signer sets the signature's expiration time
       property to the time at which the signature is to expire.  The
       expiration is a hint to the verifier, expressing the time at
       which the signer is no longer willing to vouch for the safety of
       the signature.

   4.  The signer creates an ordered set of component identifiers
       representing the message components to be covered by the
       signature, and attaches signature metadata parameters to this
       set.  The serialized value of this is later used as the value of
       the Signature-Input field as described in Section 4.1.

       *  Once an order of covered components is chosen, the order MUST
          NOT change for the life of the signature.

       *  Each covered component identifier MUST be either an HTTP field
          in the message Section 2.1 or a derived component listed in
          Section 2.2 or its associated registry.

       *  Signers of a request SHOULD include some or all of the message
          control data in the covered components, such as the @method,
          @authority, @target-uri, or some combination thereof.

       *  Signers SHOULD include the created signature metadata
          parameter to indicate when the signature was created.

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       *  The @signature-params derived component identifier is not
          explicitly listed in the list of covered component
          identifiers, because it is required to always be present as
          the last line in the signature base.  This ensures that a
          signature always covers its own metadata.

       *  Further guidance on what to include in this set and in what
          order is out of scope for this document.

   5.  The signer creates the signature base using these parameters and
       the signature base creation algorithm.  (Section 2.4)

   6.  The signer uses the HTTP_SIGN primitive function to sign the
       signature base with the chosen signing algorithm using the key
       material chosen by the signer.  The HTTP_SIGN primitive and
       several concrete applications of signing algorithms are defined
       in in Section 3.3.

   7.  The byte array output of the signature function is the HTTP
       message signature output value to be included in the Signature
       field as defined in Section 4.2.

   For example, given the HTTP message and signature parameters in the
   example in Section 2.4, the example signature base is signed with the
   test-key-rsa-pss key in Appendix B.1.2 and the RSA PSS algorithm
   described in Section 3.3.1, giving the following message signature
   output value, encoded in Base64:

   NOTE: '\' line wrapping per RFC 8792

   HIbjHC5rS0BYaa9v4QfD4193TORw7u9edguPh0AW3dMq9WImrlFrCGUDih47vAxi4L2\
   YRZ3XMJc1uOKk/J0ZmZ+wcta4nKIgBkKq0rM9hs3CQyxXGxHLMCy8uqK488o+9jrptQ\
   +xFPHK7a9sRL1IXNaagCNN3ZxJsYapFj+JXbmaI5rtAdSfSvzPuBCh+ARHBmWuNo1Uz\
   VVdHXrl8ePL4cccqlazIJdC4QEjrF+Sn4IxBQzTZsL9y9TP5FsZYzHvDqbInkTNigBc\
   E9cKOYNFCn4D/WM7F6TNuZO9EgtzepLWcjTymlHzK7aXq6Am6sfOrpIC49yXjj3ae6H\
   RalVc/g==

              Figure 2: Non-normative example signature value

   Note that the RSA PSS algorithm in use here is non-deterministic,
   meaning a different signature value will be created every time the
   algorithm is run.  The signature value provided here can be validated
   against the given keys, but newly-generated signature values are not
   expected to match the example.  See Section 7.19.

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3.2.  Verifying a Signature

   Verification of an HTTP message signature is a process that takes as
   its input the message (including Signature and Signature-Input
   fields) and the requirements for the application.  The output of the
   verification is either a positive verification or an error.

   In order to verify a signature, a verifier MUST follow the following
   algorithm:

   1.  Parse the Signature and Signature-Input fields as described in
       Section 4.1 and Section 4.2, and extract the signatures to be
       verified.

       1.  If there is more than one signature value present, determine
           which signature should be processed for this message based on
           the policy and configuration of the verifier.  If an
           applicable signature is not found, produce an error.

       2.  If the chosen Signature value does not have a corresponding
           Signature-Input value, produce an error.

   2.  Parse the values of the chosen Signature-Input field as a
       parameterized Inner List to get the ordered list of covered
       components and the signature parameters for the signature to be
       verified.

   3.  Parse the value of the corresponding Signature field to get the
       byte array value of the signature to be verified.

   4.  Examine the signature parameters to confirm that the signature
       meets the requirements described in this document, as well as any
       additional requirements defined by the application such as which
       message components are required to be covered by the signature.
       (Section 3.2.1)

   5.  Determine the verification key material for this signature.  If
       the key material is known through external means such as static
       configuration or external protocol negotiation, the verifier will
       use that.  If the key is identified in the signature parameters,
       the verifier will dereference this to appropriate key material to
       use with the signature.  The verifier has to determine the
       trustworthiness of the key material for the context in which the
       signature is presented.  If a key is identified that the verifier
       does not know, does not trust for this request, or does not match
       something preconfigured, the verification MUST fail.

   6.  Determine the algorithm to apply for verification:

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       1.  If the algorithm is known through external means such as
           static configuration or external protocol negotiation, the
           verifier will use this algorithm.

       2.  If the algorithm is explicitly stated in the signature
           parameters using a value from the HTTP Message Signatures
           registry, the verifier will use the referenced algorithm.

       3.  If the algorithm can be determined from the keying material,
           such as through an algorithm field on the key value itself,
           the verifier will use this algorithm.

       4.  If the algorithm is specified in more than one location, such
           as through static configuration and the algorithm signature
           parameter, or the algorithm signature parameter and from the
           key material itself, the resolved algorithms MUST be the
           same.  If the algorithms are not the same, the verifier MUST
           fail the verification.

   7.  Use the received HTTP message and the signature's metadata to
       recreate the signature base, using the algorithm defined in
       Section 2.4.  The value of the @signature-params input is the
       value of the Signature-Input field for this signature serialized
       according to the rules described in Section 2.2.1, not including
       the signature's label from the Signature-Input field.

   8.  If the key material is appropriate for the algorithm, apply the
       appropriate HTTP_VERIFY cryptographic verification algorithm to
       the signature, recalculated signature base, key material,
       signature value.  The HTTP_VERIFY primitive and several concrete
       algorithms are defined in Section 3.3.

   9.  The results of the verification algorithm function are the final
       results of the cryptographic verification function.

   If any of the above steps fail or produce an error, the signature
   validation fails.

   For example, verifying the signature with the key sig1 of the
   following message with the test-key-rsa-pss key in Appendix B.1.2 and
   the RSA PSS algorithm described in Section 3.3.1:

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   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884473;keyid="test-key-rsa-pss"
   Signature: sig1=:HIbjHC5rS0BYaa9v4QfD4193TORw7u9edguPh0AW3dMq9WImrl\
     FrCGUDih47vAxi4L2YRZ3XMJc1uOKk/J0ZmZ+wcta4nKIgBkKq0rM9hs3CQyxXGxH\
     LMCy8uqK488o+9jrptQ+xFPHK7a9sRL1IXNaagCNN3ZxJsYapFj+JXbmaI5rtAdSf\
     SvzPuBCh+ARHBmWuNo1UzVVdHXrl8ePL4cccqlazIJdC4QEjrF+Sn4IxBQzTZsL9y\
     9TP5FsZYzHvDqbInkTNigBcE9cKOYNFCn4D/WM7F6TNuZO9EgtzepLWcjTymlHzK7\
     aXq6Am6sfOrpIC49yXjj3ae6HRalVc/g==:

   {"hello": "world"}

   With the additional requirements that at least the method, authority,
   path, content-digest, content-length, and content-type be signed, and
   that the signature creation timestamp is recent enough at the time of
   verification, the verification passes.

3.2.1.  Enforcing Application Requirements

   The verification requirements specified in this document are intended
   as a baseline set of restrictions that are generally applicable to
   all use cases.  Applications using HTTP Message Signatures MAY impose
   requirements above and beyond those specified by this document, as
   appropriate for their use case.

   Some non-normative examples of additional requirements an application
   might define are:

   *  Requiring a specific set of header fields to be signed (e.g.,
      Authorization, Digest).

   *  Enforcing a maximum signature age from the time of the created
      time stamp.

   *  Rejection of signatures past the expiration time in the expires
      time stamp.  Note that the expiration time is a hint from the
      signer and that a verifier can always reject a signature ahead of
      its expiration time.

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   *  Prohibition of certain signature metadata parameters, such as
      runtime algorithm signaling with the alg parameter when the
      algorithm is determined from the key information.

   *  Ensuring successful dereferencing of the keyid parameter to valid
      and appropriate key material.

   *  Prohibiting the use of certain algorithms, or mandating the use of
      a specific algorithm.

   *  Requiring keys to be of a certain size (e.g., 2048 bits vs. 1024
      bits).

   *  Enforcing uniqueness of a nonce value.

   Application-specific requirements are expected and encouraged.  When
   an application defines additional requirements, it MUST enforce them
   during the signature verification process, and signature verification
   MUST fail if the signature does not conform to the application's
   requirements.

   Applications MUST enforce the requirements defined in this document.
   Regardless of use case, applications MUST NOT accept signatures that
   do not conform to these requirements.

3.3.  Signature Algorithm Methods

   HTTP Message signatures MAY use any cryptographic digital signature
   or MAC method that is appropriate for the key material, environment,
   and needs of the signer and verifier.

   Each signature algorithm method takes as its input the signature base
   defined in Section 2.4 as a byte array (M), the signing key material
   (Ks), and outputs the signature output as a byte array (S):

   HTTP_SIGN (M, Ks)  ->  S

   Each verification algorithm method takes as its input the
   recalculated signature base defined in Section 2.4 as a byte array
   (M), the verification key material (Kv), and the presented signature
   to be verified as a byte array (S) and outputs the verification
   result (V) as a boolean:

   HTTP_VERIFY (M, Kv, S) -> V

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   This section contains several common algorithm methods.  The method
   to use can be communicated through the explicit algorithm signature
   parameter alg defined in Section 2.2.1, by reference to the key
   material, or through mutual agreement between the signer and
   verifier.

3.3.1.  RSASSA-PSS using SHA-512

   To sign using this algorithm, the signer applies the RSASSA-PSS-SIGN
   (K, M) function [RFC8017] with the signer's private signing key (K)
   and the signature base (M) (Section 2.4).  The mask generation
   function is MGF1 as specified in [RFC8017] with a hash function of
   SHA-512 [RFC6234].  The salt length (sLen) is 64 bytes.  The hash
   function (Hash) SHA-512 [RFC6234] is applied to the signature base to
   create the digest content to which the digital signature is applied.
   The resulting signed content byte array (S) is the HTTP message
   signature output used in Section 3.1.

   To verify using this algorithm, the verifier applies the RSASSA-PSS-
   VERIFY ((n, e), M, S) function [RFC8017] using the public key portion
   of the verification key material ((n, e)) and the signature base (M)
   re-created as described in Section 3.2.  The mask generation function
   is MGF1 as specified in [RFC8017] with a hash function of SHA-512
   [RFC6234].  The salt length (sLen) is 64 bytes.  The hash function
   (Hash) SHA-512 [RFC6234] is applied to the signature base to create
   the digest content to which the verification function is applied.
   The verifier extracts the HTTP message signature to be verified (S)
   as described in Section 3.2.  The results of the verification
   function indicate if the signature presented is valid.

   Note that the output of RSA PSS algorithms are non-deterministic, and
   therefore it is not correct to re-calculate a new signature on the
   signature base and compare the results to an existing signature.
   Instead, the verification algorithm defined here needs to be used.
   See Section 7.19.

   Use of this algorithm can be indicated at runtime using the rsa-pss-
   sha512 value for the alg signature parameter.

3.3.2.  RSASSA-PKCS1-v1_5 using SHA-256

   To sign using this algorithm, the signer applies the RSASSA-
   PKCS1-V1_5-SIGN (K, M) function [RFC8017] with the signer's private
   signing key (K) and the signature base (M) (Section 2.4).  The hash
   SHA-256 [RFC6234] is applied to the signature base to create the
   digest content to which the digital signature is applied.  The
   resulting signed content byte array (S) is the HTTP message signature
   output used in Section 3.1.

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   To verify using this algorithm, the verifier applies the RSASSA-
   PKCS1-V1_5-VERIFY ((n, e), M, S) function [RFC8017] using the public
   key portion of the verification key material ((n, e)) and the
   signature base (M) re-created as described in Section 3.2.  The hash
   function SHA-256 [RFC6234] is applied to the signature base to create
   the digest content to which the verification function is applied.
   The verifier extracts the HTTP message signature to be verified (S)
   as described in Section 3.2.  The results of the verification
   function are compared to the http message signature to determine if
   the signature presented is valid.

   Use of this algorithm can be indicated at runtime using the rsa-
   v1_5-sha256 value for the alg signature parameter.

3.3.3.  HMAC using SHA-256

   To sign and verify using this algorithm, the signer applies the HMAC
   function [RFC2104] with the shared signing key (K) and the signature
   base (text) (Section 2.4).  The hash function SHA-256 [RFC6234] is
   applied to the signature base to create the digest content to which
   the HMAC is applied, giving the signature result.

   For signing, the resulting value is the HTTP message signature output
   used in Section 3.1.

   For verification, the verifier extracts the HTTP message signature to
   be verified (S) as described in Section 3.2.  The output of the HMAC
   function is compared to the value of the HTTP message signature, and
   the results of the comparison determine the validity of the signature
   presented.

   Use of this algorithm can be indicated at runtime using the hmac-
   sha256 value for the alg signature parameter.

3.3.4.  ECDSA using curve P-256 DSS and SHA-256

   To sign using this algorithm, the signer applies the ECDSA algorithm
   [FIPS186-4] using curve P-256 with the signer's private signing key
   and the signature base (Section 2.4).  The hash SHA-256 [RFC6234] is
   applied to the signature base to create the digest content to which
   the digital signature is applied, (M).  The signature algorithm
   returns two integer values, r and s.  These are both encoded in big-
   endian unsigned integers, zero-padded to 32-octets each.  These
   encoded values are concatenated into a single 64-octet array
   consisting of the encoded value of r followed by the encoded value of
   s.  The resulting concatenation of (r, s) is byte array of the HTTP
   message signature output used in Section 3.1.

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   To verify using this algorithm, the verifier applies the ECDSA
   algorithm [FIPS186-4] using the public key portion of the
   verification key material and the signature base re-created as
   described in Section 3.2.  The hash function SHA-256 [RFC6234] is
   applied to the signature base to create the digest content to which
   the signature verification function is applied, (M).  The verifier
   extracts the HTTP message signature to be verified (S) as described
   in Section 3.2.  This value is a 64-octet array consisting of the
   encoded values of r and s concatenated in order.  These are both
   encoded in big-endian unsigned integers, zero-padded to 32-octets
   each.  The resulting signature value (r, s) is used as input to the
   signature verification function.  The results of the verification
   function indicate if the signature presented is valid.

   Note that the output of ECDSA algorithms are non-deterministic, and
   therefore it is not correct to re-calculate a new signature on the
   signature base and compare the results to an existing signature.
   Instead, the verification algorithm defined here needs to be used.
   See Section 7.19.

   Use of this algorithm can be indicated at runtime using the ecdsa-
   p256-sha256 value for the alg signature parameter.

3.3.5.  EdDSA using curve edwards25519

   To sign using this algorithm, the signer applies the Ed25519
   algorithm Section 5.1.6 of [RFC8032] with the signer's private
   signing key and the signature base (Section 2.4).  The signature base
   is taken as the input message (M) with no pre-hash function.  The
   signature is a 64-octet concatenation of R and S as specified in
   Section 5.1.6 of [RFC8032], and this is taken as a byte array for the
   HTTP message signature output used in Section 3.1.

   To verify using this algorithm, the signer applies the Ed25519
   algorithm Section 5.1.7 of [RFC8032] using the public key portion of
   the verification key material (A) and the signature base re-created
   as described in Section 3.2.  The signature base is taken as the
   input message (M) with no pre-hash function.  The signature to be
   verified is processed as the 64-octet concatenation of R and S as
   specified in Section 5.1.7 of [RFC8032].  The results of the
   verification function indicate if the signature presented is valid.

   Use of this algorithm can be indicated at runtime using the ed25519
   value for the alg signature parameter.

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3.3.6.  JSON Web Signature (JWS) algorithms

   If the signing algorithm is a JOSE signing algorithm from the JSON
   Web Signature and Encryption Algorithms Registry established by
   [RFC7518], the JWS algorithm definition determines the signature and
   hashing algorithms to apply for both signing and verification.

   For both signing and verification, the HTTP messages signature base
   (Section 2.4) is used as the entire "JWS Signing Input".  The JOSE
   Header defined in [RFC7517] is not used, and the signature base is
   not first encoded in Base64 before applying the algorithm.  The
   output of the JWS signature is taken as a byte array prior to the
   Base64url encoding used in JOSE.

   The JWS algorithm MUST NOT be none and MUST NOT be any algorithm with
   a JOSE Implementation Requirement of Prohibited.

   JWA algorithm values from the JSON Web Signature and Encryption
   Algorithms Registry are not included as signature parameters.  In
   fact, the explicit alg signature parameter is not used at all when
   using JOSE signing algorithms, as the JWS algorithm can be signaled
   using JSON Web Keys or other mechanisms common to JOSE
   implementations.

4.  Including a Message Signature in a Message

   Message signatures can be included within an HTTP message via the
   Signature-Input and Signature fields, both defined within this
   specification.  When attached to a message, an HTTP message signature
   is identified by a label.  This label MUST be unique within a given
   HTTP message and MUST be used in both the Signature-Input and
   Signature fields.  The label is chosen by the signer, except where a
   specific label is dictated by protocol negotiations such as described
   in Section 5

   An HTTP message signature MUST use both fields and each field MUST
   contain the same labels.  The Signature-Input and Signature
   Dictionaries are parallel data structures of each other, and the
   presence of any key in one field but not in the other is an error.
   The Signature-Input field identifies the covered components and
   parameters that describe how the signature was generated, while the
   Signature field contains the signature value.  Each field MAY contain
   multiple labeled values.

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4.1.  The Signature-Input HTTP Field

   The Signature-Input field is a Dictionary structured field defined in
   Section 3.2 of [STRUCTURED-FIELDS] containing the metadata for one or
   more message signatures generated from components within the HTTP
   message.  Each member describes a single message signature.  The
   member's key is an identifier that uniquely identifies the message
   signature within the context of the HTTP message.  The member's value
   is the serialization of the covered components including all
   signature metadata parameters, using the Inner List serialization
   process defined in Section 2.2.1.

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("@method" "@target-uri" "@authority" \
     "content-digest" "cache-control");\
     created=1618884475;keyid="test-key-rsa-pss"

   To facilitate signature validation, the Signature-Input field value
   MUST contain the same serialized value used in generating the
   signature base's @signature-params value.  Note that in a structured
   field value, parameter order has to be preserved.

   The signer MAY include the Signature-Input field as a trailer to
   facilitate signing a message after its content has been processed by
   the signer.  However, since intermediaries are allowed to drop
   trailers as per [HTTP], it is RECOMMENDED that the Signature-Input
   field be included only as a header to avoid signatures being
   inadvertently stripped from a message.

   Multiple Signature-Input fields MAY be included in a single HTTP
   message.  The signature labels MUST be unique across all field
   values.

4.2.  The Signature HTTP Field

   The Signature field is a Dictionary structured field defined in
   Section 3.2 of [STRUCTURED-FIELDS] containing one or more message
   signatures generated from components and context of the HTTP message.
   The member's key is an identifier that uniquely identifies the
   message signature within the context of the HTTP message.  The
   member's value is a Byte Sequence containing the signature value for
   the message signature identified by the member name.

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   NOTE: '\' line wrapping per RFC 8792

   Signature: sig1=:P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo\
     1RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCiHz\
     C87qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW84jS8\
     gyarxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53r58Rmp\
     Z+J9eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCVRj05NrxA\
     BNFv3r5S9IXf2fYJK+eyW4AiGVMvMcOg==:

   The signer MAY include the Signature field as a trailer to facilitate
   signing a message after its content has been processed by the signer.
   However, since intermediaries are allowed to drop trailers as per
   [HTTP], it is RECOMMENDED that the Signature field be included only
   as a header to avoid signatures being inadvertently stripped from a
   message.

   Multiple Signature fields MAY be included in a single HTTP message.
   The signature labels MUST be unique across all field values.

4.3.  Multiple Signatures

   Multiple distinct signatures MAY be included in a single message.
   Each distinct signature MUST have a unique label.  These multiple
   signatures could be added all by the same signer or could come from
   several different signers.  For example, a signer may include
   multiple signatures signing the same message components with
   different keys or algorithms to support verifiers with different
   capabilities, or a reverse proxy may include information about the
   client in fields when forwarding the request to a service host,
   including a signature over the client's original signature values.

   The following non-normative example starts with a signed request from
   the client.  The proxy takes this request validates the client's
   signature.

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   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature:  sig1=:LAH8BjcfcOcLojiuOBFWn0P5keD3xAOuJRGziCLuD8r5MW9S0\
     RoXXLzLSRfGY/3SF8kVIkHjE13SEFdTo4Af/fJ/Pu9wheqoLVdwXyY/UkBIS1M8Br\
     c8IODsn5DFIrG0IrburbLi0uCc+E2ZIIb6HbUJ+o+jP58JelMTe0QE3IpWINTEzpx\
     jqDf5/Df+InHCAkQCTuKsamjWXUpyOT1Wkxi7YPVNOjW4MfNuTZ9HdbD2Tr65+BXe\
     TG9ZS/9SWuXAc+BZ8WyPz0QRz//ec3uWXd7bYYODSjRAxHqX+S1ag3LZElYyUKaAI\
     jZ8MGOt4gXEwCSLDv/zqxZeWLj/PDkn6w==:

   {"hello": "world"}

   The proxy then alters the message before forwarding it on to the
   origin server, changing the target host and adding the Forwarded
   header field defined in [RFC7239].

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: origin.host.internal.example
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18
   Forwarded: for=192.0.2.123
   Signature-Input: sig1=("@method" "@authority" "@path" \
     "content-digest" "content-length" "content-type")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature:  sig1=:LAH8BjcfcOcLojiuOBFWn0P5keD3xAOuJRGziCLuD8r5MW9S0\
     RoXXLzLSRfGY/3SF8kVIkHjE13SEFdTo4Af/fJ/Pu9wheqoLVdwXyY/UkBIS1M8Br\
     c8IODsn5DFIrG0IrburbLi0uCc+E2ZIIb6HbUJ+o+jP58JelMTe0QE3IpWINTEzpx\
     jqDf5/Df+InHCAkQCTuKsamjWXUpyOT1Wkxi7YPVNOjW4MfNuTZ9HdbD2Tr65+BXe\
     TG9ZS/9SWuXAc+BZ8WyPz0QRz//ec3uWXd7bYYODSjRAxHqX+S1ag3LZElYyUKaAI\
     jZ8MGOt4gXEwCSLDv/zqxZeWLj/PDkn6w==:

   {"hello": "world"}

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   The proxy includes the client's signature value under the label sig1,
   which the proxy signs in addition to the Forwarded field.  Note that
   since the client's signature already covers the client's Signature-
   Input value for sig1, this value is transitively covered by the
   proxy's signature and need not be added explicitly.  The proxy
   identifies its own key and algorithm and, in this example, includes
   an expiration for the signature to indicate to downstream systems
   that the proxy will not vouch for this signed message past this short
   time window.  This results in a signature base of:

   NOTE: '\' line wrapping per RFC 8792

   "signature";key="sig1": :LAH8BjcfcOcLojiuOBFWn0P5keD3xAOuJRGziCLuD8\
     r5MW9S0RoXXLzLSRfGY/3SF8kVIkHjE13SEFdTo4Af/fJ/Pu9wheqoLVdwXyY/UkB\
     IS1M8Brc8IODsn5DFIrG0IrburbLi0uCc+E2ZIIb6HbUJ+o+jP58JelMTe0QE3IpW\
     INTEzpxjqDf5/Df+InHCAkQCTuKsamjWXUpyOT1Wkxi7YPVNOjW4MfNuTZ9HdbD2T\
     r65+BXeTG9ZS/9SWuXAc+BZ8WyPz0QRz//ec3uWXd7bYYODSjRAxHqX+S1ag3LZEl\
     YyUKaAIjZ8MGOt4gXEwCSLDv/zqxZeWLj/PDkn6w==:
   "forwarded": for=192.0.2.123
   "@signature-params": ("signature";key="sig1" "forwarded")\
     ;created=1618884480;expires=1618884540;keyid="test-key-rsa"\
     ;alg="rsa-v1_5-sha256"

   And a signature output value of:

   NOTE: '\' line wrapping per RFC 8792

   G1WLTL4/9PGSKEQbSAMypZNk+I2dpLJ6qvl2JISahlP31OO/QEUd8/HdO2O7vYLi5k3\
   JIiAK3UPK4U+kvJZyIUidsiXlzRI+Y2se3SGo0D8dLfhG95bKr6ukYXl60QHpsGRTfS\
   iwdtvYKXGpKNrMlISJYd+oGrGRyI9gbCy0aFhc6I/okIMLeK4g9PgzpC3YTwhUQ98KI\
   BNLWHgREfBgJxjPbxFlsgJ9ykPviLj8GKJ81HwsK3XM9P7WaS7fMGOt8h1kSqgkZQB9\
   YqiIo+WhHvJa7iPy8QrYFKzx9BBEY6AwfStZAsXXz3LobZseyxsYcLJLs8rY0wVA9NP\
   sxKrHGA==

   These values are added to the HTTP request message by the proxy.  The
   original signature is included under the identifier sig1, and the
   reverse proxy's signature is included under the label proxy_sig.  The
   proxy uses the key test-key-rsa to create its signature using the
   rsa-v1_5-sha256 signature algorithm, while the client's original
   signature was made using the key id of test-key-rsa-pss and an RSA
   PSS signature algorithm.

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   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: origin.host.internal.example
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18
   Forwarded: for=192.0.2.123
   Signature-Input: sig1=("@method" "@authority" "@path" \
       "content-digest" "content-length" "content-type")\
       ;created=1618884475;keyid="test-key-rsa-pss", \
     proxy_sig=("signature";key="sig1" "forwarded")\
       ;created=1618884480;expires=1618884540;keyid="test-key-rsa"\
       ;alg="rsa-v1_5-sha256"
   Signature:  sig1=:LAH8BjcfcOcLojiuOBFWn0P5keD3xAOuJRGziCLuD8r5MW9S0\
       RoXXLzLSRfGY/3SF8kVIkHjE13SEFdTo4Af/fJ/Pu9wheqoLVdwXyY/UkBIS1M8\
       Brc8IODsn5DFIrG0IrburbLi0uCc+E2ZIIb6HbUJ+o+jP58JelMTe0QE3IpWINT\
       EzpxjqDf5/Df+InHCAkQCTuKsamjWXUpyOT1Wkxi7YPVNOjW4MfNuTZ9HdbD2Tr\
       65+BXeTG9ZS/9SWuXAc+BZ8WyPz0QRz//ec3uWXd7bYYODSjRAxHqX+S1ag3LZE\
       lYyUKaAIjZ8MGOt4gXEwCSLDv/zqxZeWLj/PDkn6w==:, \
     proxy_sig=:G1WLTL4/9PGSKEQbSAMypZNk+I2dpLJ6qvl2JISahlP31OO/QEUd8/\
       HdO2O7vYLi5k3JIiAK3UPK4U+kvJZyIUidsiXlzRI+Y2se3SGo0D8dLfhG95bKr\
       6ukYXl60QHpsGRTfSiwdtvYKXGpKNrMlISJYd+oGrGRyI9gbCy0aFhc6I/okIML\
       eK4g9PgzpC3YTwhUQ98KIBNLWHgREfBgJxjPbxFlsgJ9ykPviLj8GKJ81HwsK3X\
       M9P7WaS7fMGOt8h1kSqgkZQB9YqiIo+WhHvJa7iPy8QrYFKzx9BBEY6AwfStZAs\
       XXz3LobZseyxsYcLJLs8rY0wVA9NPsxKrHGA==:

   {"hello": "world"}

   The proxy's signature and the client's original signature can be
   verified independently for the same message, based on the needs of
   the application.  Since the proxy's signature covers the client
   signature, the backend service fronted by the proxy can trust that
   the proxy has validated the incoming signature.

5.  Requesting Signatures

   While a signer is free to attach a signature to a request or response
   without prompting, it is often desirable for a potential verifier to
   signal that it expects a signature from a potential signer using the
   Accept-Signature field.

   The message to which the requested signature is applied is known as
   the "target message".  When the Accept-Signature field is sent in an
   HTTP request message, the field indicates that the client desires the
   server to sign the response using the identified parameters, and the

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   target message is the response to this request.  All responses from
   resources that support such signature negotiation SHOULD either be
   uncacheable or contain a Vary header field that lists Accept-
   Signature, in order to prevent a cache from returning a response with
   a signature intended for a different request.

   When the Accept-Signature field is used in an HTTP response message,
   the field indicates that the server desires the client to sign its
   next request to the server with the identified parameters, and the
   target message is the client's next request.  The client can choose
   to also continue signing future requests to the same server in the
   same way.

   The target message of an Accept-Signature field MUST include all
   labeled signatures indicated in the Accept-Header signature, each
   covering the same identified components of the Accept-Signature
   field.

   The sender of an Accept-Signature field MUST include identifiers that
   are appropriate for the type of the target message.  For example, if
   the target message is a request, the component identifiers can not
   include the @status component identifier.

5.1.  The Accept-Signature Field

   The Accept-Signature field is a Dictionary structured field defined
   in Section 3.2 of [STRUCTURED-FIELDS] containing the metadata for one
   or more requested message signatures to be generated from message
   components of the target HTTP message.  Each member describes a
   single message signature.  The member's name is an identifier that
   uniquely identifies the requested message signature within the
   context of the target HTTP message.  The member's value is the
   serialization of the desired covered components of the target
   message, including any allowed signature metadata parameters, using
   the serialization process defined in Section 2.2.1.

   NOTE: '\' line wrapping per RFC 8792

   Accept-Signature: sig1=("@method" "@target-uri" "@authority" \
     "content-digest" "cache-control");\
     keyid="test-key-rsa-pss"

   The requested signature MAY include parameters, such as a desired
   algorithm or key identifier.  These parameters MUST NOT include
   parameters that the signer is expected to generate, including the
   created and nonce parameters.

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5.2.  Processing an Accept-Signature

   The receiver of an Accept-Signature field fulfills that header as
   follows:

   1.  Parse the field value as a Dictionary

   2.  For each member of the dictionary:

       1.  The name of the member is the label of the output signature
           as specified in Section 4.1

       2.  Parse the value of the member to obtain the set of covered
           component identifiers

       3.  Process the requested parameters, such as the signing
           algorithm and key material.  If any requested parameters
           cannot be fulfilled, or if the requested parameters conflict
           with those deemed appropriate to the target message, the
           process fails and returns an error.

       4.  Select any additional parameters necessary for completing the
           signature

       5.  Create the Signature-Input and Signature header values and
           associate them with the label

   3.  Optionally create any additional Signature-Input and Signature
       values, with unique labels not found in the Accept-Signature
       field

   4.  Combine all labeled Signature-Input and Signature values and
       attach both fields to the target message

   Note that by this process, a signature applied to a target message
   MUST have the same label, MUST have the same set of covered
   component, and MAY have additional parameters.  Also note that the
   target message MAY include additional signatures not specified by the
   Accept-Signature field.

6.  IANA Considerations

   IANA is requested to create three registries and to populate those
   registries with initial values as described in this section.

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6.1.  HTTP Signature Algorithms Registry

   This document defines HTTP Signature Algorithms, for which IANA is
   asked to create and maintain a new registry titled "HTTP Signature
   Algorithms".  Initial values for this registry are given in
   Section 6.1.2.  Future assignments and modifications to existing
   assignment are to be made through the Expert Review registration
   policy [RFC8126] and shall follow the template presented in
   Section 6.1.1.

   Algorithms referenced by algorithm identifiers have to be fully
   defined with all parameters fixed.  Algorithm identifiers in this
   registry are to be interpreted as whole string values and not as a
   combination of parts.  That is to say, it is expected that
   implementors understand rsa-pss-sha512 as referring to one specific
   algorithm with its hash, mask, and salt values set as defined here.
   Implementors do not parse out the rsa, pss, and sha512 portions of
   the identifier to determine parameters of the signing algorithm from
   the string.

   Algorithms added to this registry MUST NOT be aliases for other
   entries in the registry.

6.1.1.  Registration Template

   Algorithm Name:
      An identifier for the HTTP Signature Algorithm.  The name MUST be
      an ASCII string consisting only of lower-case characters ("a" -
      "z"), digits ("0" - "9"), and hyphens ("-"), and SHOULD NOT exceed
      20 characters in length.  The identifier MUST be unique within the
      context of the registry.

   Description:
      A brief description of the algorithm used to sign the signature
      base.

   Status:
      A brief text description of the status of the algorithm.  The
      description MUST begin with one of "Active" or "Deprecated", and
      MAY provide further context or explanation as to the reason for
      the status.

   Specification document(s):
      Reference to the document(s) that specify the algorithm,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

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6.1.2.  Initial Contents

    +===================+===================+========+===============+
    | Algorithm Name    | Description       | Status | Specification |
    |                   |                   |        | document(s)   |
    +===================+===================+========+===============+
    | rsa-pss-sha512    | RSASSA-PSS using  | Active | Section 3.3.1 |
    |                   | SHA-512           |        | of RFC nnnn   |
    +-------------------+-------------------+--------+---------------+
    | rsa-v1_5-sha256   | RSASSA-PKCS1-v1_5 | Active | Section 3.3.2 |
    |                   | using SHA-256     |        | of RFC nnnn   |
    +-------------------+-------------------+--------+---------------+
    | hmac-sha256       | HMAC using        | Active | Section 3.3.3 |
    |                   | SHA-256           |        | of RFC nnnn   |
    +-------------------+-------------------+--------+---------------+
    | ecdsa-p256-sha256 | ECDSA using curve | Active | Section 3.3.4 |
    |                   | P-256 DSS and     |        | of RFC nnnn   |
    |                   | SHA-256           |        |               |
    +-------------------+-------------------+--------+---------------+
    | ed25519           | Edwards Curve DSA | Active | Section 3.3.5 |
    |                   | using curve       |        | of RFC nnnn   |
    |                   | edwards25519      |        |               |
    +-------------------+-------------------+--------+---------------+

        Table 1: Initial contents of the HTTP Signature Algorithms
                                Registry.

6.2.  HTTP Signature Metadata Parameters Registry

   This document defines the signature parameters structure, the values
   of which may have parameters containing metadata about a message
   signature.  IANA is asked to create and maintain a new registry
   titled "HTTP Signature Metadata Parameters" to record and maintain
   the set of parameters defined for use with member values in the
   signature parameters structure.  Initial values for this registry are
   given in Section 6.2.2.  Future assignments and modifications to
   existing assignments are to be made through the Expert Review
   registration policy [RFC8126] and shall follow the template presented
   in Section 6.2.1.

6.2.1.  Registration Template

   Name:
      An identifier for the HTTP signature metadata parameter.  The name
      MUST be an ASCII string consisting only of lower-case characters
      ("a" - "z"), digits ("0" - "9"), and hyphens ("-"), and SHOULD NOT
      exceed 20 characters in length.  The identifier MUST be unique
      within the context of the registry.

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   Description:
      A brief description of the metadata parameter and what it
      represents.

   Specification document(s):
      Reference to the document(s) that specify the parameter,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

6.2.2.  Initial Contents

   The table below contains the initial contents of the HTTP Signature
   Metadata Parameters Registry.  Each row in the table represents a
   distinct entry in the registry.

        +=========+===============================+===============+
        | Name    | Description                   | Specification |
        |         |                               | document(s)   |
        +=========+===============================+===============+
        | alg     | Explicitly declared signature | Section 2.2.1 |
        |         | algorithm                     | of RFC nnnn   |
        +---------+-------------------------------+---------------+
        | created | Timestamp of signature        | Section 2.2.1 |
        |         | creation                      | of RFC nnnn   |
        +---------+-------------------------------+---------------+
        | expires | Timestamp of proposed         | Section 2.2.1 |
        |         | signature expiration          | of RFC nnnn   |
        +---------+-------------------------------+---------------+
        | keyid   | Key identifier for the        | Section 2.2.1 |
        |         | signing and verification keys | of RFC nnnn   |
        |         | used to create this signature |               |
        +---------+-------------------------------+---------------+
        | nonce   | A single-use nonce value      | Section 2.2.1 |
        |         |                               | of RFC nnnn   |
        +---------+-------------------------------+---------------+

              Table 2: Initial contents of the HTTP Signature
                       Metadata Parameters Registry.

6.3.  HTTP Signature Derived Component Names Registry

   This document defines a method for canonicalizing HTTP message
   components, including components that can be derived from the context
   of the HTTP message outside of the HTTP fields.  These derived
   components are identified by a unique string, known as the component
   name.  Component names for derived components always start with the
   "@" (at) symbol to distinguish them from HTTP header fields.  IANA is

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   asked to create and maintain a new registry typed "HTTP Signature
   Derived Component Names" to record and maintain the set of non-field
   component names and the methods to produce their associated component
   values.  Initial values for this registry are given in Section 6.3.2.
   Future assignments and modifications to existing assignments are to
   be made through the Expert Review registration policy [RFC8126] and
   shall follow the template presented in Section 6.3.1.

6.3.1.  Registration Template

   Name:
      A name for the HTTP derived component.  The name MUST begin with
      the "@" character followed by an ASCII string consisting only of
      lower-case characters ("a" - "z"), digits ("0" - "9"), and hyphens
      ("-"), and SHOULD NOT exceed 20 characters in length.  The name
      MUST be unique within the context of the registry.

   Description:
      A description of the derived component.

   Status:
      A brief text description of the status of the algorithm.  The
      description MUST begin with one of "Active" or "Deprecated", and
      MAY provide further context or explanation as to the reason for
      the status.

   Target:
      The valid message targets for the derived parameter.  MUST be one
      of the values "Request", "Response", or "Request, Response".  The
      semantics of these are defined in Section 2.2.

   Specification document(s):
      Reference to the document(s) that specify the derived component,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

6.3.2.  Initial Contents

   The table below contains the initial contents of the HTTP Signature
   Derived Component Names Registry.

   +===========+============+=========+========+===============+=======+
   |Name       |Description |Status   |Target  |Specification  |       |
   |           |            |         |        |document(s)    |       |
   +===========+============+=========+========+===============+=======+
   |@signature-|Signature   |Active   |Request,|Section 2.2.1  |       |
   |params     |parameters, |         |Response|of RFC nnnn    |       |

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   |           |including   |         |        |               |       |
   |           |covered     |         |        |               |       |
   |           |content list|         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@method    |The HTTP    |Active   |Request |Section 2.2.2  |       |
   |           |request     |         |        |of RFC nnnn    |       |
   |           |method      |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@authority |The HTTP    |Active   |Request |Section 2.2.4  |       |
   |           |authority,  |         |        |of RFC nnnn    |       |
   |           |or target   |         |        |               |       |
   |           |host        |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@scheme    |The URI     |Active   |Request |Section 2.2.5  |       |
   |           |scheme of   |         |        |of RFC nnnn    |       |
   |           |the request |         |        |               |       |
   |           |URI         |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@target-uri|The full    |Active   |Request |Section 2.2.3  |       |
   |           |target URI  |         |        |of RFC nnnn    |       |
   |           |of the      |         |        |               |       |
   |           |request     |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@request-  |The request |Active   |Request |Section 2.2.6  |       |
   |target     |target of   |         |        |of RFC nnnn    |       |
   |           |the request |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@path      |The full    |Active   |Request |Section 2.2.7  |       |
   |           |path of the |         |        |of RFC nnnn    |       |
   |           |request URI |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@query     |The full    |Active   |Request |Section 2.2.8  |       |
   |           |query of the|         |        |of RFC nnnn    |       |
   |           |request URI |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+
   |@query-    |            |A single |Active  |Request        |Section|
   |param      |            |named    |        |               |2.2.9  |
   |           |            |query    |        |               |of RFC |
   |           |            |parameter|        |               |nnnn   |
   +-----------+------------+---------+--------+---------------+-------+
   |@status    |The status  |Active   |Response|Section 2.2.10 |       |
   |           |code of the |         |        |of RFC nnnn    |       |
   |           |response    |         |        |               |       |
   +-----------+------------+---------+--------+---------------+-------+

     Table 3: Initial contents of the HTTP Signature Derived Component
                              Names Registry.

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6.4.  HTTP Signature Component Parameters Registry

   This document defines several kinds of component identifiers, some of
   which can be parameterized in specific circumstances to provide
   unique modified behavior.  IANA is asked to create and maintain a new
   registry typed "HTTP Signature Component Parameters" to record and
   maintain the set of parameters names, the component identifiers they
   are associated with, and the modifications these parameters make to
   the component value.  Initial values for this registry are given in
   Section 6.4.2.  Future assignments and modifications to existing
   assignments are to be made through the Expert Review registration
   policy [RFC8126] and shall follow the template presented in
   Section 6.4.1.

6.4.1.  Registration Template

   Name:
      A name for the parameter.  The name MUST be an ASCII string that
      conforms to the key ABNF rule defined in [STRUCTURED-FIELDS] and
      SHOULD NOT exceed 20 characters in length.  The name MUST be
      unique within the context of the registry.

   Description:
      A description of the parameter's function.

   Status:
      A brief text description of the status of the parameter.  The
      description MUST begin with one of "Active" or "Deprecated", and
      MAY provide further context or explanation as to the reason for
      the status.

   Target:
      The applicable component identifiers for the parmeter.  Can be a
      combination of one or more derived component identifiers as
      described in Section 2.2, one or more specific HTTP field names,
      the special value "Field Value" meaning all HTTP fields, or a
      separate human-readable description of the target.

   Specification document(s):
      Reference to the document(s) that specify the derived component,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

6.4.2.  Initial Contents

   The table below contains the initial contents of the HTTP Signature
   Derived Component Names Registry.

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    +======+===============+========+=================+===============+
    | Name | Description   | Status | Target          | Specification |
    |      |               |        |                 | document(s)   |
    +======+===============+========+=================+===============+
    | key  | Dictionary    | Active | Section 2.1.2   |               |
    |      | structured    |        | of RFC nnnn     |               |
    |      | fields        |        |                 |               |
    +------+---------------+--------+-----------------+---------------+
    | name | Named query   | Active | @query-param    | Section 2.2.9 |
    |      | parameters    |        |                 | of RFC nnnn   |
    +------+---------------+--------+-----------------+---------------+
    | sf   | Strict        | Active | Structured      | Section 2.1.1 |
    |      | structured    |        | fields          | of RFC nnnn   |
    |      | field         |        |                 |               |
    |      | serialization |        |                 |               |
    +------+---------------+--------+-----------------+---------------+
    | req  | Related       | Active | Derived content | Section 2.2.5 |
    |      | request       |        | identifiers     | of RFC nnnn   |
    |      | indicator     |        | with a target   |               |
    |      |               |        | of Request or   |               |
    |      |               |        | any field       |               |
    +------+---------------+--------+-----------------+---------------+

         Table 4: Initial contents of the HTTP Signature Component
                            Parameters Registry.

7.  Security Considerations

   In order for an HTTP message to be considered covered by a signature,
   all of the following conditions have to be true:

   *  a signature is expected or allowed on the message by the verifier

   *  the signature exists on the message

   *  the signature is verified against the identified key material and
      algorithm

   *  the key material and algorithm are appropriate for the context of
      the message

   *  the signature is within expected time boundaries

   *  the signature covers the expected content, including any critical
      components

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7.1.  Signature Verification Skipping

   HTTP Message Signatures only provide security if the signature is
   verified by the verifier.  Since the message to which the signature
   is attached remains a valid HTTP message without the signature
   fields, it is possible for a verifier to ignore the output of the
   verification function and still process the message.  Common reasons
   for this could be relaxed requirements in a development environment
   or a temporary suspension of enforcing verification during debugging
   an overall system.  Such temporary suspensions are difficult to
   detect under positive-example testing since a good signature will
   always trigger a valid response whether or not it has been checked.

   To detect this, verifiers should be tested using both valid and
   invalid signatures, ensuring that the invalid signature fails as
   expected.

7.2.  Use of TLS

   The use of HTTP Message Signatures does not negate the need for TLS
   or its equivalent to protect information in transit.  Message
   signatures provide message integrity over the covered message
   components but do not provide any confidentiality for the
   communication between parties.

   TLS provides such confidentiality between the TLS endpoints.  As part
   of this, TLS also protects the signature data itself from being
   captured by an attacker, which is an important step in preventing
   signature replay (Section 7.3).

   When TLS is used, it needs to be deployed according to the
   recommendations in [BCP195].

7.3.  Signature Replay

   Since HTTP Message Signatures allows sub-portions of the HTTP message
   to be signed, it is possible for two different HTTP messages to
   validate against the same signature.  The most extreme form of this
   would be a signature over no message components.  If such a signature
   were intercepted, it could be replayed at will by an attacker,
   attached to any HTTP message.  Even with sufficient component
   coverage, a given signature could be applied to two similar HTTP
   messages, allowing a message to be replayed by an attacker with the
   signature intact.

   To counteract these kinds of attacks, it's first important for the
   signer to cover sufficient portions of the message to differentiate
   it from other messages.  In addition, the signature can use the nonce

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   signature parameter to provide a per-message unique value to allow
   the verifier to detect replay of the signature itself if a nonce
   value is repeated.  Furthermore, the signer can provide a timestamp
   for when the signature was created and a time at which the signer
   considers the signature to be invalid, limiting the utility of a
   captured signature value.

   If a verifier wants to trigger a new signature from a signer, it can
   send the Accept-Signature header field with a new nonce parameter.
   An attacker that is simply replaying a signature would not be able to
   generate a new signature with the chosen nonce value.

7.4.  Insufficient Coverage

   Any portions of the message not covered by the signature are
   susceptible to modification by an attacker without affecting the
   signature.  An attacker can take advantage of this by introducing a
   header field or other message component that will change the
   processing of the message but will not be covered by the signature.
   Such an altered message would still pass signature verification, but
   when the verifier processes the message as a whole, the unsigned
   content injected by the attacker would subvert the trust conveyed by
   the valid signature and change the outcome of processing the message.

   To combat this, an application of this specification should require
   as much of the message as possible to be signed, within the limits of
   the application and deployment.  The verifier should only trust
   message components that have been signed.  Verifiers could also strip
   out any sensitive unsigned portions of the message before processing
   of the message continues.

7.5.  Cryptography and Signature Collision

   The HTTP Message Signatures specification does not define any of its
   own cryptographic primitives, and instead relies on other
   specifications to define such elements.  If the signature algorithm
   or key used to process the signature base is vulnerable to any
   attacks, the resulting signature will also be susceptible to these
   same attacks.

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   A common attack against signature systems is to force a signature
   collision, where the same signature value successfully verifies
   against multiple different inputs.  Since this specification relies
   on reconstruction of the input string based on an HTTP message, and
   the list of components signed is fixed in the signature, it is
   difficult but not impossible for an attacker to effect such a
   collision.  An attacker would need to manipulate the HTTP message and
   its covered message components in order to make the collision
   effective.

   To counter this, only vetted keys and signature algorithms should be
   used to sign HTTP messages.  The HTTP Message Signatures Algorithm
   Registry is one source of potential trusted algorithms.

   While it is possible for an attacker to substitute the signature
   parameters value or the signature value separately, the signature
   base generation algorithm (Section 2.4) always covers the signature
   parameters as the final value in the input string using a
   deterministic serialization method.  This step strongly binds the
   signature base with the signature value in a way that makes it much
   more difficult for an attacker to perform a partial substitution on
   the signature bases.

7.6.  Key Theft

   A foundational assumption of signature-based cryptographic systems is
   that the signing key is not compromised by an attacker.  If the keys
   used to sign the message are exfiltrated or stolen, the attacker will
   be able to generate their own signatures using those keys.  As a
   consequence, signers have to protect any signing key material from
   exfiltration, capture, and use by an attacker.

   To combat this, signers can rotate keys over time to limit the amount
   of time stolen keys are useful.  Signers can also use key escrow and
   storage systems to limit the attack surface against keys.
   Furthermore, the use of asymmetric signing algorithms exposes key
   material less than the use of symmetric signing algorithms
   (Section 7.11).

7.7.  Modification of Required Message Parameters

   An attacker could effectively deny a service by modifying an
   otherwise benign signature parameter or signed message component.
   While rejecting a modified message is the desired behavior,
   consistently failing signatures could lead to the verifier turning
   off signature checking in order to make systems work again (see
   Section 7.1).

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   If such failures are common within an application, the signer and
   verifier should compare their generated signature bases with each
   other to determine which part of the message is being modified.
   However, the signer and verifier should not remove the requirement to
   sign the modified component when it is suspected an attacker is
   modifying the component.

7.8.  Mismatch of Signature Parameters from Message

   The verifier needs to make sure that the signed message components
   match those in the message itself.  This specification encourages
   this by requiring the verifier to derive these values from the
   message, but lazy cacheing or conveyance of the signature base to a
   processing system could lead to downstream verifiers accepting a
   message that does not match the presented signature.

7.9.  Multiple Signature Confusion

   Since multiple signatures can be applied to one message
   (Section 4.3), it is possible for an attacker to attach their own
   signature to a captured message without modifying existing
   signatures.  This new signature could be completely valid based on
   the attacker's key, or it could be an invalid signature for any
   number of reasons.  Each of these situations need to be accounted
   for.

   A verifier processing a set of valid signatures needs to account for
   all of the signers, identified by the signing keys.  Only signatures
   from expected signers should be accepted, regardless of the
   cryptographic validity of the signature itself.

   A verifier processing a set of signatures on a message also needs to
   determine what to do when one or more of the signatures are not
   valid.  If a message is accepted when at least one signature is
   valid, then a verifier could drop all invalid signatures from the
   request before processing the message further.  Alternatively, if the
   verifier rejects a message for a single invalid signature, an
   attacker could use this to deny service to otherwise valid messages
   by injecting invalid signatures alongside the valid ones.

7.10.  Signature Labels

   HTTP Message Signature values are identified in the Signature and
   Signature-Input field values by unique labels.  These labels are
   chosen only when attaching the signature values to the message and
   are not accounted for in the signing process.  An intermediary adding
   its own signature is allowed to re-label an existing signature when
   processing the message.

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   Therefore, applications should not rely on specific labels being
   present, and applications should not put semantic meaning on the
   labels themselves.  Instead, additional signature parmeters can be
   used to convey whatever additional meaning is required to be attached
   to and covered by the signature.

7.11.  Symmetric Cryptography

   The HTTP Message Signatures specification allows for both asymmetric
   and symmetric cryptography to be applied to HTTP messages.  By its
   nature, symmetric cryptographic methods require the same key material
   to be known by both the signer and verifier.  This effectively means
   that a verifier is capable of generating a valid signature, since
   they have access to the same key material.  An attacker that is able
   to compromise a verifier would be able to then impersonate a signer.

   Where possible, asymmetric methods or secure key agreement mechanisms
   should be used in order to avoid this type of attack.  When symmetric
   methods are used, distribution of the key material needs to be
   protected by the overall system.  One technique for this is the use
   of separate cryptographic modules that separate the verification
   process (and therefore the key material) from other code, minimizing
   the vulnerable attack surface.  Another technique is the use of key
   derivation functions that allow the signer and verifier to agree on
   unique keys for each message without having to share the key values
   directly.

   Additionally, if symmetric algorithms are allowed within a system,
   special care must be taken to avoid key downgrade attacks
   (Section 7.15).

7.12.  Canonicalization Attacks

   Any ambiguity in the generation of the signature base could provide
   an attacker with leverage to substitute or break a signature on a
   message.  Some message component values, particularly HTTP field
   values, are potentially susceptible to broken implementations that
   could lead to unexpected and insecure behavior.  Naive
   implementations of this specification might implement HTTP field
   processing by taking the single value of a field and using it as the
   direct component value without processing it appropriately.

   For example, if the handling of obs-fold field values does not remove
   the internal line folding and whitespace, additional newlines could
   be introduced into the signature base by the signer, providing a
   potential place for an attacker to mount a signature collision
   (Section 7.5) attack.  Alternatively, if header fields that appear
   multiple times are not joined into a single string value, as is

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   required by this specification, similar attacks can be mounted as a
   signed component value would show up in the input string more than
   once and could be substituted or otherwise attacked in this way.

   To counter this, the entire field processing algorithm needs to be
   implemented by all implementations of signers and verifiers.

7.13.  Key Specification Mix-Up

   The existence of a valid signature on an HTTP message is not
   sufficient to prove that the message has been signed by the
   appropriate party.  It is up to the verifier to ensure that a given
   key and algorithm are appropriate for the message in question.  If
   the verifier does not perform such a step, an attacker could
   substitute their own signature using their own key on a message and
   force a verifier to accept and process it.  To combat this, the
   verifier needs to ensure that not only does the signature validate
   for a message, but that the key and algorithm used are appropriate.

7.14.  HTTP Versions and Component Ambiguity

   Some message components are expressed in different ways across HTTP
   versions.  For example, the authority of the request target is sent
   using the Host header field in HTTP 1.1 but with the :authority
   pseudo-header in HTTP 2.  If a signer sends an HTTP 1.1 message and
   signs the Host field, but the message is translated to HTTP 2 before
   it reaches the verifier, the signature will not validate as the Host
   header field could be dropped.

   It is for this reason that HTTP Message Signatures defines a set of
   derived components that define a single way to get value in question,
   such as the @authority derived component (Section 2.2.4) in lieu of
   the Host header field.  Applications should therefore prefer derived
   components for such options where possible.

7.15.  Key and Algorithm Specification Downgrades

   Applications of this specification need to protect against key
   specification downgrade attacks.  For example, the same RSA key can
   be used for both RSA-PSS and RSA v1.5 signatures.  If an application
   expects a key to only be used with RSA-PSS, it needs to reject
   signatures for that key using the weaker RSA 1.5 specification.

   Another example of a downgrade attack occurs when an asymmetric
   algorithm is expected, such as RSA-PSS, but an attacker substitutes a
   signature using symmetric algorithm, such as HMAC.  A naive verifier
   implementation could use the value of the public RSA key as the input
   to the HMAC verification function.  Since the public key is known to

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   the attacker, this would allow the attacker to create a valid HMAC
   signature against this known key.  To prevent this, the verifier
   needs to ensure that both the key material and the algorithm are
   appropriate for the usage in question.  Additionally, while this
   specification does allow runtime specification of the algorithm using
   the alg signature parameter, applications are encouraged to use other
   mechanisms such as static configuration or higher protocol-level
   algorithm specification instead.

7.16.  Parsing Structured Field Values

   Several parts of this specification rely on the parsing of structured
   field values [STRUCTURED-FIELDS].  In particular, normalization of
   HTTP structured field values (Section 2.1.1), referencing members of
   a dictionary structured field (Section 2.1.2), and processing the
   @signature-input value when verifying a signature (Section 3.2).
   While structured field values are designed to be relatively simple to
   parse, a naive or broken implementation of such a parser could lead
   to subtle attack surfaces being exposed in the implementation.

   For example, if a buggy parser of the @signature-input value does not
   enforce proper closing of quotes around string values within the list
   of component identifiers, an attacker could take advantage of this
   and inject additional content into the signature base through
   manipulating the Signature-Input field value on a message.

   To counteract this, implementations should use fully compliant and
   trusted parsers for all structured field processing, both on the
   signer and verifier side.

7.17.  Choosing Message Components

   Applications of HTTP Message Signatures need to decide which message
   components will be covered by the signature.  Depending on the
   application, some components could be expected to be changed by
   intermediaries prior to the signature's verification.  If these
   components are covered, such changes would, by design, break the
   signature.

   However, the HTTP Message Signature standard allows for flexibility
   in determining which components are signed precisely so that a given
   application can choose the appropriate portions of the message that
   need to be signed, avoiding problematic components.  For example, a
   web application framework that relies on rewriting query parameters
   might avoid use of the @query derived component in favor of sub-
   indexing the query value using @query-param derived components
   instead.

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   Some components are expected to be changed by intermediaries and
   ought not to be signed under most circumstance.  The Via and
   Forwarded header fields, for example, are expected to be manipulated
   by proxies and other middle-boxes, including replacing or entirely
   dropping existing values.  These fields should not be covered by the
   signature except in very limited and tightly-coupled scenarios.

   Additional considerations for choosing signature aspects are
   discussed in Section 1.4.

7.18.  Confusing HTTP Field Names for Derived Component Names

   The definition of HTTP field names does not allow for the use of the
   @ character anywhere in the name.  As such, since all derived
   component names start with the @ character, these namespaces should
   be completely separate.  However, some HTTP implementations are not
   sufficiently strict about the characters accepted in HTTP headers.
   In such implementations, a sender (or attacker) could inject a header
   field starting with an @ character and have it passed through to the
   application code.  These invalid header fields could be used to
   override a portion of the derived message content and substitute an
   arbitrary value, providing a potential place for an attacker to mount
   a signature collision (Section 7.5) attack.

   To combat this, when selecting values for a message component, if the
   component name starts with the @ character, it needs to be processed
   as a derived component and never taken as a fields.  Only if the
   component name does not start with the @ character can it be taken
   from the fields of the message.  The algorithm discussed in
   Section 2.4 provides a safe order of operations.

7.19.  Non-deterministic Signature Primitives

   Some cryptographic primitives such as RSA PSS and ECDSA have non-
   deterministic outputs, which include some amount of entropy within
   the algorithm.  For such algorithms, multiple signatures generated in
   succession will not match.  A lazy implementation of a verifier could
   ignore this distinction and simply check for the same value being
   created by re-signing the signature base.  Such an implementation
   would work for deterministic algorithms such as HMAC and EdDSA but
   fail to verify valid signatures made using non-deterministic
   algorithms.  It is therefore important that a verifier always use the
   correctly-defined verification function for the algorithm in question
   and not do a simple comparison.

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7.20.  Choosing Signature Parameters and Derived Components over HTTP
       Fields

   Some HTTP fields have values and interpretations that are similar to
   HTTP signature parameters or derived components.  In most cases, it
   is more desirable to sign the non-field alternative.  In particular,
   the following fields should usually not be included in the signature
   unless the application specifically requires it:

   "date"  The "date" field value represents the timestamp of the HTTP
      message.  However, the creation time of the signature itself is
      encoded in the created signature parameter.  These two values can
      be different, depending on how the signature and the HTTP message
      are created and serialized.  Applications processing signatures
      for valid time windows should use the created signature parameter
      for such calculations.  An application could also put limits on
      how much skew there is between the "date" field and the created
      signature parameter, in order to limit the application of a
      generated signature to different HTTP messages.  See also
      Section 7.3 and Section 7.4.

   "host"  The "host" header field is specific to HTTP 1.1, and its
      functionality is subsumed by the "@authority" derived component,
      defined in Section 2.2.4.  In order to preserve the value across
      different HTTP versions, applications should always use the
      "@authority" derived component.

7.21.  Semantically Equivalent Field Values

   The signature base generation algorithm (Section 2.4) uses the value
   of an HTTP field as its component value.  In the common case, this
   amounts to taking the actual bytes of the field value as the
   component value for both the signer and verifier.  However, some
   field values allow for transformation of the values in semantically
   equivalent ways that alter the bytes used in the value itself.  For
   example, a field definition can declare some or all of its value to
   be case-insensitive, or to have special handling of internal
   whitespace characters.  Other fields have expected transformations
   from intermediaries, such as the removal of comments in the Via
   header field.  In such cases, a verifier could be tripped up by using
   the equivalent transformed field value, which would differ from the
   byte value used by the signer.  The verifier would have have a
   difficult time finding this class of errors since the value of the
   field is still acceptable for the application, but the actual bytes
   required by the signature base would not match.

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   When processing such fields, the signer and verifier have to agree
   how to handle such transformations, if at all.  One option is to not
   sign problematic fields, but care must be taken to ensure that there
   is still sufficient signature coverage (Section 7.4) for the
   application.  Another option is to define an application-specific
   canonicalization value for the field before it is added to the HTTP
   message, such as to always remove internal comments before signing,
   or to always transform values to lowercase.  Since these
   transformations are applied prior to the field being used as input to
   the signature base generation algorithm, the signature base will
   still simply contain the byte value of the field as it appears within
   the message.  If the transformations were to be applied after the
   value is extracted from the message but before it is added to the
   signature base, different attack surfaces such as value substitution
   attacks could be launched against the application.  All application-
   specific additional rules are outside the scope of this
   specification, and by their very nature these transformations would
   harm interoperability of the implementation outside of this specific
   application.  It is recommended that applications avoid the use of
   such additional rules wherever possible.

7.22.  Message Content

   On its own, this specification does not provide coverage for the
   content of an HTTP message under the signature, either in request or
   response.  However, [DIGEST] defines a set of fields that allow a
   cryptographic digest of the content to be represented in a field.
   Once this field is created, it can be included just like any other
   field as defined in Section 2.1.

   For example, in the following response message:

   HTTP/1.1 200 OK
   Content-Type: application/json

   {"hello": "world"}

   The digest of the content can be added to the Content-Digest field as
   follows:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Digest: \
     sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:

   {"hello": "world"}

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   This field can be included in a signature base just like any other
   field along with the basic signature parameters:

   "@status": 200
   "content-digest": \
     sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
   "@signature-input": ("@status" "content-digest")

   From here, the signing process proceeds as usual.

   Upon verification, it is important that the verifier validate not
   only the signature but also the value of the Content-Digest field
   itself against the actual received content.  Unless the verifier
   performs this step, it would be possible for an attacker to
   substitute the message content but leave the Content-Digest field
   value untouched.  Since only the field value is covered by the
   signature directly, checking only the signature is not sufficient
   protection against such a substitution attack.

8.  Privacy Considerations

8.1.  Identification through Keys

   If a signer uses the same key with multiple verifiers, or uses the
   same key over time with a single verifier, the ongoing use of that
   key can be used to track the signer throughout the set of verifiers
   that messages are sent to.  Since cryptographic keys are meant to be
   functionally unique, the use of the same key over time is a strong
   indicator that it is the same party signing multiple messages.

   In many applications, this is a desirable trait, and it allows HTTP
   Message Signatures to be used as part of authenticating the signer to
   the verifier.  However, it could be unintentional tracking that a
   signer might not be aware of.  To counter this kind of tracking, a
   signer can use a different key for each verifier that it is in
   communication with.  Sometimes, a signer could also rotate their key
   when sending messages to a given verifier.  These approaches do not
   negate the need for other anti-tracking techniques to be applied as
   necessary.

8.2.  Signatures do not provide confidentiality

   HTTP Message Signatures do not provide confidentiality of any of the
   information protected by the signature.  The content of the HTTP
   message, including the value of all fields and the value of the
   signature itself, is presented in plaintext to any party with access
   to the message.

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   To provide confidentiality at the transport level, TLS or its
   equivalent can be used as discussed in Section 7.2.

8.3.  Oracles

   It is important to balance the need for providing useful feedback to
   developers on error conditions without providing additional
   information to an attacker.  For example, a naive but helpful server
   implementation might try to indicate the required key identifier
   needed for requesting a resource.  If someone knows who controls that
   key, a correlation can be made between the resource's existence and
   the party identified by the key.  Access to such information could be
   used by an attacker as a means to target the legitimate owner of the
   resource for further attacks.

8.4.  Required Content

   A core design tenet of this specification is that all message
   components covered by the signature need to be available to the
   verifier in order to recreate the signature base and verify the
   signature.  As a consequence, if an application of this specification
   requires that a particular field be signed, the verifier will need
   access to the value of that field.

   For example, in some complex systems with intermediary processors
   this could cause the surprising behavior of an intermediary not being
   able to remove privacy-sensitive information from a message before
   forwarding it on for processing, for fear of breaking the signature.
   A possible mitigation for this specific situation would be for the
   intermediary to verify the signature itself, then modifying the
   message to remove the privacy-sensitive information.  The
   intermediary can add its own signature at this point to signal to the
   next destination that the incoming signature was validated, as is
   shown in the example in Section 4.3.

9.  References

9.1.  Normative References

   [FIPS186-4]
              "Digital Signature Standard (DSS)", 2013,
              <https://csrc.nist.gov/publications/detail/fips/186/4/
              final>.

   [HTMLURL]  "URL (Living Standard)", 2021,
              <https://url.spec.whatwg.org/>.

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   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [HTTP1]    Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              messaging-19>.

   [POSIX.1]  "The Open Group Base Specifications Issue 7, 2018
              edition", 2018,
              <https://pubs.opengroup.org/onlinepubs/9699919799/>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/rfc/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/rfc/rfc6234>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7517>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7518>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/rfc/rfc8017>.

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   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/rfc/rfc8792>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

9.2.  Informative References

   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, May 2015.

              Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, March 2021.

              <https://www.rfc-editor.org/info/bcp195>

   [CLIENT-CERT]
              Campbell, B. and M. Bishop, "Client-Cert HTTP Header
              Field", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-client-cert-field-02, 25 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              client-cert-field-02>.

   [DIGEST]   Polli, R. and L. Pardue, "Digest Fields", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-digest-
              headers-09, 25 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              digest-headers-09>.

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7239>.

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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

Appendix A.  Detecting HTTP Message Signatures

   There have been many attempts to create signed HTTP messages in the
   past, including other non-standardized definitions of the Signature
   field, which is used within this specification.  It is recommended
   that developers wishing to support both this specification and other
   historical drafts do so carefully and deliberately, as
   incompatibilities between this specification and various versions of
   other drafts could lead to unexpected problems.

   It is recommended that implementers first detect and validate the
   Signature-Input field defined in this specification to detect that
   this standard is in use and not an alternative.  If the Signature-
   Input field is present, all Signature fields can be parsed and
   interpreted in the context of this draft.

Appendix B.  Examples

   The following non-normative examples are provided as a means of
   testing implementations of HTTP Message Signatures.  The signed
   messages given can be used to create the signature base with the
   stated parameters, creating signatures using the stated algorithms
   and keys.

   The private keys given can be used to generate signatures, though
   since several of the demonstrated algorithms are nondeterministic,
   the results of a signature are expected to be different from the
   exact bytes of the examples.  The public keys given can be used to
   validate all signed examples.

B.1.  Example Keys

   This section provides cryptographic keys that are referenced in
   example signatures throughout this document.  These keys MUST NOT be
   used for any purpose other than testing.

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   The key identifiers for each key are used throughout the examples in
   this specification.  It is assumed for these examples that the signer
   and verifier can unambiguously dereference all key identifiers used
   here, and that the keys and algorithms used are appropriate for the
   context in which the signature is presented.

   The components for each private key in PEM format can be displayed by
   executing the following OpenSSL command:

   openssl pkey -text

   This command was tested with all the example keys on OpenSSL version
   1.1.1m.  Note that some systems cannot produce or use these keys
   directly, and may require additional processing.

B.1.1.  Example Key RSA test

   The following key is a 2048-bit RSA public and private key pair,
   referred to in this document as test-key-rsa.  This key is encoded in
   PEM Format, with no encryption.

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   -----BEGIN RSA PUBLIC KEY-----
   MIIBCgKCAQEAhAKYdtoeoy8zcAcR874L8cnZxKzAGwd7v36APp7Pv6Q2jdsPBRrw
   WEBnez6d0UDKDwGbc6nxfEXAy5mbhgajzrw3MOEt8uA5txSKobBpKDeBLOsdJKFq
   MGmXCQvEG7YemcxDTRPxAleIAgYYRjTSd/QBwVW9OwNFhekro3RtlinV0a75jfZg
   kne/YiktSvLG34lw2zqXBDTC5NHROUqGTlML4PlNZS5Ri2U4aCNx2rUPRcKIlE0P
   uKxI4T+HIaFpv8+rdV6eUgOrB2xeI1dSFFn/nnv5OoZJEIB+VmuKn3DCUcCZSFlQ
   PSXSfBDiUGhwOw76WuSSsf1D4b/vLoJ10wIDAQAB
   -----END RSA PUBLIC KEY-----

   -----BEGIN RSA PRIVATE KEY-----
   MIIEqAIBAAKCAQEAhAKYdtoeoy8zcAcR874L8cnZxKzAGwd7v36APp7Pv6Q2jdsP
   BRrwWEBnez6d0UDKDwGbc6nxfEXAy5mbhgajzrw3MOEt8uA5txSKobBpKDeBLOsd
   JKFqMGmXCQvEG7YemcxDTRPxAleIAgYYRjTSd/QBwVW9OwNFhekro3RtlinV0a75
   jfZgkne/YiktSvLG34lw2zqXBDTC5NHROUqGTlML4PlNZS5Ri2U4aCNx2rUPRcKI
   lE0PuKxI4T+HIaFpv8+rdV6eUgOrB2xeI1dSFFn/nnv5OoZJEIB+VmuKn3DCUcCZ
   SFlQPSXSfBDiUGhwOw76WuSSsf1D4b/vLoJ10wIDAQABAoIBAG/JZuSWdoVHbi56
   vjgCgkjg3lkO1KrO3nrdm6nrgA9P9qaPjxuKoWaKO1cBQlE1pSWp/cKncYgD5WxE
   CpAnRUXG2pG4zdkzCYzAh1i+c34L6oZoHsirK6oNcEnHveydfzJL5934egm6p8DW
   +m1RQ70yUt4uRc0YSor+q1LGJvGQHReF0WmJBZHrhz5e63Pq7lE0gIwuBqL8SMaA
   yRXtK+JGxZpImTq+NHvEWWCu09SCq0r838ceQI55SvzmTkwqtC+8AT2zFviMZkKR
   Qo6SPsrqItxZWRty2izawTF0Bf5S2VAx7O+6t3wBsQ1sLptoSgX3QblELY5asI0J
   YFz7LJECgYkAsqeUJmqXE3LP8tYoIjMIAKiTm9o6psPlc8CrLI9CH0UbuaA2JCOM
   cCNq8SyYbTqgnWlB9ZfcAm/cFpA8tYci9m5vYK8HNxQr+8FS3Qo8N9RJ8d0U5Csw
   DzMYfRghAfUGwmlWj5hp1pQzAuhwbOXFtxKHVsMPhz1IBtF9Y8jvgqgYHLbmyiu1
   mwJ5AL0pYF0G7x81prlARURwHo0Yf52kEw1dxpx+JXER7hQRWQki5/NsUEtv+8RT
   qn2m6qte5DXLyn83b1qRscSdnCCwKtKWUug5q2ZbwVOCJCtmRwmnP131lWRYfj67
   B/xJ1ZA6X3GEf4sNReNAtaucPEelgR2nsN0gKQKBiGoqHWbK1qYvBxX2X3kbPDkv
   9C+celgZd2PW7aGYLCHq7nPbmfDV0yHcWjOhXZ8jRMjmANVR/eLQ2EfsRLdW69bn
   f3ZD7JS1fwGnO3exGmHO3HZG+6AvberKYVYNHahNFEw5TsAcQWDLRpkGybBcxqZo
   81YCqlqidwfeO5YtlO7etx1xLyqa2NsCeG9A86UjG+aeNnXEIDk1PDK+EuiThIUa
   /2IxKzJKWl1BKr2d4xAfR0ZnEYuRrbeDQYgTImOlfW6/GuYIxKYgEKCFHFqJATAG
   IxHrq1PDOiSwXd2GmVVYyEmhZnbcp8CxaEMQoevxAta0ssMK3w6UsDtvUvYvF22m
   qQKBiD5GwESzsFPy3Ga0MvZpn3D6EJQLgsnrtUPZx+z2Ep2x0xc5orneB5fGyF1P
   WtP+fG5Q6Dpdz3LRfm+KwBCWFKQjg7uTxcjerhBWEYPmEMKYwTJF5PBG9/ddvHLQ
   EQeNC8fHGg4UXU8mhHnSBt3EA10qQJfRDs15M38eG2cYwB1PZpDHScDnDA0=
   -----END RSA PRIVATE KEY-----

B.1.2.  Example RSA PSS Key

   The following key is a 2048-bit RSA public and private key pair,
   referred to in this document as test-key-rsa-pss.  This key is PCKS#8
   encoded in PEM format, with no encryption.

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   -----BEGIN PUBLIC KEY-----
   MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAr4tmm3r20Wd/PbqvP1s2
   +QEtvpuRaV8Yq40gjUR8y2Rjxa6dpG2GXHbPfvMs8ct+Lh1GH45x28Rw3Ry53mm+
   oAXjyQ86OnDkZ5N8lYbggD4O3w6M6pAvLkhk95AndTrifbIFPNU8PPMO7OyrFAHq
   gDsznjPFmTOtCEcN2Z1FpWgchwuYLPL+Wokqltd11nqqzi+bJ9cvSKADYdUAAN5W
   Utzdpiy6LbTgSxP7ociU4Tn0g5I6aDZJ7A8Lzo0KSyZYoA485mqcO0GVAdVw9lq4
   aOT9v6d+nb4bnNkQVklLQ3fVAvJm+xdDOp9LCNCN48V2pnDOkFV6+U9nV5oyc6XI
   2wIDAQAB
   -----END PUBLIC KEY-----

   -----BEGIN PRIVATE KEY-----
   MIIEvgIBADALBgkqhkiG9w0BAQoEggSqMIIEpgIBAAKCAQEAr4tmm3r20Wd/Pbqv
   P1s2+QEtvpuRaV8Yq40gjUR8y2Rjxa6dpG2GXHbPfvMs8ct+Lh1GH45x28Rw3Ry5
   3mm+oAXjyQ86OnDkZ5N8lYbggD4O3w6M6pAvLkhk95AndTrifbIFPNU8PPMO7Oyr
   FAHqgDsznjPFmTOtCEcN2Z1FpWgchwuYLPL+Wokqltd11nqqzi+bJ9cvSKADYdUA
   AN5WUtzdpiy6LbTgSxP7ociU4Tn0g5I6aDZJ7A8Lzo0KSyZYoA485mqcO0GVAdVw
   9lq4aOT9v6d+nb4bnNkQVklLQ3fVAvJm+xdDOp9LCNCN48V2pnDOkFV6+U9nV5oy
   c6XI2wIDAQABAoIBAQCUB8ip+kJiiZVKF8AqfB/aUP0jTAqOQewK1kKJ/iQCXBCq
   pbo360gvdt05H5VZ/RDVkEgO2k73VSsbulqezKs8RFs2tEmU+JgTI9MeQJPWcP6X
   aKy6LIYs0E2cWgp8GADgoBs8llBq0UhX0KffglIeek3n7Z6Gt4YFge2TAcW2WbN4
   XfK7lupFyo6HHyWRiYHMMARQXLJeOSdTn5aMBP0PO4bQyk5ORxTUSeOciPJUFktQ
   HkvGbym7KryEfwH8Tks0L7WhzyP60PL3xS9FNOJi9m+zztwYIXGDQuKM2GDsITeD
   2mI2oHoPMyAD0wdI7BwSVW18p1h+jgfc4dlexKYRAoGBAOVfuiEiOchGghV5vn5N
   RDNscAFnpHj1QgMr6/UG05RTgmcLfVsI1I4bSkbrIuVKviGGf7atlkROALOG/xRx
   DLadgBEeNyHL5lz6ihQaFJLVQ0u3U4SB67J0YtVO3R6lXcIjBDHuY8SjYJ7Ci6Z6
   vuDcoaEujnlrtUhaMxvSfcUJAoGBAMPsCHXte1uWNAqYad2WdLjPDlKtQJK1diCm
   rqmB2g8QE99hDOHItjDBEdpyFBKOIP+NpVtM2KLhRajjcL9Ph8jrID6XUqikQuVi
   4J9FV2m42jXMuioTT13idAILanYg8D3idvy/3isDVkON0X3UAVKrgMEne0hJpkPL
   FYqgetvDAoGBAKLQ6JZMbSe0pPIJkSamQhsehgL5Rs51iX4m1z7+sYFAJfhvN3Q/
   OGIHDRp6HjMUcxHpHw7U+S1TETxePwKLnLKj6hw8jnX2/nZRgWHzgVcY+sPsReRx
   NJVf+Cfh6yOtznfX00p+JWOXdSY8glSSHJwRAMog+hFGW1AYdt7w80XBAoGBAImR
   NUugqapgaEA8TrFxkJmngXYaAqpA0iYRA7kv3S4QavPBUGtFJHBNULzitydkNtVZ
   3w6hgce0h9YThTo/nKc+OZDZbgfN9s7cQ75x0PQCAO4fx2P91Q+mDzDUVTeG30mE
   t2m3S0dGe47JiJxifV9P3wNBNrZGSIF3mrORBVNDAoGBAI0QKn2Iv7Sgo4T/XjND
   dl2kZTXqGAk8dOhpUiw/HdM3OGWbhHj2NdCzBliOmPyQtAr770GITWvbAI+IRYyF
   S7Fnk6ZVVVHsxjtaHy1uJGFlaZzKR4AGNaUTOJMs6NadzCmGPAxNQQOCqoUjn4XR
   rOjr9w349JooGXhOxbu8nOxX
   -----END PRIVATE KEY-----

B.1.3.  Example ECC P-256 Test Key

   The following key is a public and private elliptical curve key pair
   over the curve P-256, referred to in this document as `test-key-ecc-
   p256.  This key is encoded in PEM format, with no encryption.

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   -----BEGIN PUBLIC KEY-----
   MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEqIVYZVLCrPZHGHjP17CTW0/+D9Lf
   w0EkjqF7xB4FivAxzic30tMM4GF+hR6Dxh71Z50VGGdldkkDXZCnTNnoXQ==
   -----END PUBLIC KEY-----

   -----BEGIN EC PRIVATE KEY-----
   MHcCAQEEIFKbhfNZfpDsW43+0+JjUr9K+bTeuxopu653+hBaXGA7oAoGCCqGSM49
   AwEHoUQDQgAEqIVYZVLCrPZHGHjP17CTW0/+D9Lfw0EkjqF7xB4FivAxzic30tMM
   4GF+hR6Dxh71Z50VGGdldkkDXZCnTNnoXQ==
   -----END EC PRIVATE KEY-----

B.1.4.  Example Ed25519 Test Key

   The following key is an elliptical curve key over the Edwards curve
   ed25519, referred to in this document as test-key-ed25519.  This key
   is PCKS#8 encoded in PEM format, with no encryption.

   -----BEGIN PUBLIC KEY-----
   MCowBQYDK2VwAyEAJrQLj5P/89iXES9+vFgrIy29clF9CC/oPPsw3c5D0bs=
   -----END PUBLIC KEY-----

   -----BEGIN PRIVATE KEY-----
   MC4CAQAwBQYDK2VwBCIEIJ+DYvh6SEqVTm50DFtMDoQikTmiCqirVv9mWG9qfSnF
   -----END PRIVATE KEY-----

B.1.5.  Example Shared Secret

   The following shared secret is 64 randomly-generated bytes encoded in
   Base64, referred to in this document as test-shared-secret.

   NOTE: '\' line wrapping per RFC 8792

   uzvJfB4u3N0Jy4T7NZ75MDVcr8zSTInedJtkgcu46YW4XByzNJjxBdtjUkdJPBt\
     bmHhIDi6pcl8jsasjlTMtDQ==

B.2.  Test Cases

   This section provides non-normative examples that may be used as test
   cases to validate implementation correctness.  These examples are
   based on the following HTTP messages:

   For requests, this test-request message is used:

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   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX+T\
     aPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   Content-Length: 18

   {"hello": "world"}

   For responses, this test-response message is used:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Digest: sha-512=:JlEy2bfUz7WrWIjc1qV6KVLpdr/7L5/L4h7Sxvh6sN\
     HpDQWDCL+GauFQWcZBvVDhiyOnAQsxzZFYwi0wDH+1pw==:
   Content-Length: 23

   {"message": "good dog"}

B.2.1.  Minimal Signature Using rsa-pss-sha512

   This example presents a minimal signature using the rsa-pss-sha512
   algorithm over test-request, covering none of the components of the
   HTTP message, but providing a timestamped signature proof of
   possession of the key with a signer-provided nonce.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "@signature-params": ();created=1618884473;keyid="test-key-rsa-pss"\
     ;nonce="b3k2pp5k7z-50gnwp.yemd"

   This results in the following Signature-Input and Signature headers
   being added to the message under the signature label sig-b21:

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   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b21=();created=1618884473\
     ;keyid="test-key-rsa-pss";nonce="b3k2pp5k7z-50gnwp.yemd"
   Signature: sig-b21=:d2pmTvmbncD3xQm8E9ZV2828BjQWGgiwAaw5bAkgibUopem\
     LJcWDy/lkbbHAve4cRAtx31Iq786U7it++wgGxbtRxf8Udx7zFZsckzXaJMkA7ChG\
     52eSkFxykJeNqsrWH5S+oxNFlD4dzVuwe8DhTSja8xxbR/Z2cOGdCbzR72rgFWhzx\
     2VjBqJzsPLMIQKhO4DGezXehhWwE56YCE+O6c0mKZsfxVrogUvA4HELjVKWmAvtl6\
     UnCh8jYzuVG5WSb/QEVPnP5TmcAnLH1g+s++v6d4s8m0gCw1fV5/SITLq9mhho8K3\
     +7EPYTU8IU1bLhdxO5Nyt8C8ssinQ98Xw9Q==:

   Note that since the covered components list is empty, this signature
   could be applied by an attacker to an unrelated HTTP message.  In
   this example, the nonce parameter is included to prevent the same
   signature from being replayed more than once, but if an attacker
   intercepts the signature and prevents its delivery to the verifier,
   the attacker could apply this signature to another message.
   Therefore, use of an empty covered components set is discouraged.
   See Section 7.4 for more discussion.

   Note that the RSA PSS algorithm in use here is non-deterministic,
   meaning a different signature value will be created every time the
   algorithm is run.  The signature value provided here can be validated
   against the given keys, but newly-generated signature values are not
   expected to match the example.  See Section 7.19.

B.2.2.  Selective Covered Components using rsa-pss-sha512

   This example covers additional components in test-request using the
   rsa-pss-sha512 algorithm.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "@authority": example.com
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "@signature-params": ("@authority" "content-digest")\
     ;created=1618884473;keyid="test-key-rsa-pss"

   This results in the following Signature-Input and Signature headers
   being added to the message under the label sig-b22:

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   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b22=("@authority" "content-digest")\
     ;created=1618884473;keyid="test-key-rsa-pss"
   Signature: sig-b22=:Fee1uy9YGZq5UUwwYU6vz4dZNvfw3GYrFl1L6YlVIyUMuWs\
     wWDNSvql4dVtSeidYjYZUm7SBCENIb5KYy2ByoC3bI+7gydd2i4OAT5lyDtmeapnA\
     a8uP/b9xUpg+VSPElbBs6JWBIQsd+nMdHDe+ls/IwVMwXktC37SqsnbNyhNp6kcvc\
     WpevjzFcD2VqdZleUz4jN7P+W5A3wHiMGfIjIWn36KXNB+RKyrlGnIS8yaBBrom5r\
     cZWLrLbtg6VlrH1+/07RV+kgTh/l10h8qgpl9zQHu7mWbDKTq0tJ8K4ywcPoC4s2I\
     4rU88jzDKDGdTTQFZoTVZxZmuTM1FvHfzIw==:

   Note that the RSA PSS algorithm in use here is non-deterministic,
   meaning a different signature value will be created every time the
   algorithm is run.  The signature value provided here can be validated
   against the given keys, but newly-generated signature values are not
   expected to match the example.  See Section 7.19.

B.2.3.  Full Coverage using rsa-pss-sha512

   This example covers all applicable message components in test-request
   (including the content type and length) plus many derived components,
   again using the rsa-pss-sha512 algorithm.  Note that the Host header
   field is not covered because the @authority derived component is
   included instead.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "@method": POST
   "@path": /foo
   "@query": ?param=Value&Pet=dog
   "@authority": example.com
   "content-type": application/json
   "content-digest": sha-512=:WZDPaVn/7XgHaAy8pmojAkGWoRx2UFChF41A2svX\
     +TaPm+AbwAgBWnrIiYllu7BNNyealdVLvRwEmTHWXvJwew==:
   "content-length": 18
   "@signature-params": ("date" "@method" "@path" "@query" \
     "@authority" "content-type" "content-digest" "content-length")\
     ;created=1618884473;keyid="test-key-rsa-pss"

   This results in the following Signature-Input and Signature headers
   being added to the message under the label sig-b23:

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   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b23=("date" "@method" "@path" "@query" \
     "@authority" "content-type" "content-digest" "content-length")\
     ;created=1618884473;keyid="test-key-rsa-pss"
   Signature: sig-b23=:bbN8oArOxYoyylQQUU6QYwrTuaxLwjAC9fbY2F6SVWvh0yB\
     iMIRGOnMYwZ/5MR6fb0Kh1rIRASVxFkeGt683+qRpRRU5p2voTp768ZrCUb38K0fU\
     xN0O0iC59DzYx8DFll5GmydPxSmme9v6ULbMFkl+V5B1TP/yPViV7KsLNmvKiLJH1\
     pFkh/aYA2HXXZzNBXmIkoQoLd7YfW91kE9o/CCoC1xMy7JA1ipwvKvfrs65ldmlu9\
     bpG6A9BmzhuzF8Eim5f8ui9eH8LZH896+QIF61ka39VBrohr9iyMUJpvRX2Zbhl5Z\
     JzSRxpJyoEZAFL2FUo5fTIztsDZKEgM4cUA==:

   Note in this example that the value of the Date header and the value
   of the created signature parameter need not be the same.  This is due
   to the fact that the Date header is added when creating the HTTP
   Message and the created parameter is populated when creating the
   signature over that message, and these two times could vary.  If the
   Date header is covered by the signature, it is up to the verifier to
   determine whether its value has to match that of the created
   parameter or not.  See Section 7.20 for more discussion.

   Note that the RSA PSS algorithm in use here is non-deterministic,
   meaning a different signature value will be created every time the
   algorithm is run.  The signature value provided here can be validated
   against the given keys, but newly-generated signature values are not
   expected to match the example.  See Section 7.19.

B.2.4.  Signing a Response using ecdsa-p256-sha256

   This example covers portions of the test-response response message
   using the ecdsa-p256-sha256 algorithm and the key test-key-ecc-p256.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "@status": 200
   "content-type": application/json
   "content-digest": sha-512=:mEWXIS7MaLRuGgxOBdODa3xqM1XdEvxoYhvlCFJ4\
     1QJgJc4GTsPp29l5oGX69wWdXymyU0rjJuahq4l5aGgfLQ==:
   "content-length": 23
   "@signature-params": ("@status" "content-type" "content-digest" \
     "content-length");created=1618884473;keyid="test-key-ecc-p256"

   This results in the following Signature-Input and Signature headers
   being added to the message under the label sig-b24:

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   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b24=("@status" "content-type" \
     "content-digest" "content-length");created=1618884473\
     ;keyid="test-key-ecc-p256"
   Signature: sig-b24=:wNmSUAhwb5LxtOtOpNa6W5xj067m5hFrj0XQ4fvpaCLx0NK\
     ocgPquLgyahnzDnDAUy5eCdlYUEkLIj+32oiasw==:

   Note that the ECDSA algorithm in use here is non-deterministic,
   meaning a different signature value will be created every time the
   algorithm is run.  The signature value provided here can be validated
   against the given keys, but newly-generated signature values are not
   expected to match the example.  See Section 7.19.

B.2.5.  Signing a Request using hmac-sha256

   This example covers portions of the test-request using the hmac-
   sha256 algorithm and the secret test-shared-secret.

   The corresponding signature base is:

   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "@authority": example.com
   "content-type": application/json
   "@signature-params": ("date" "@authority" "content-type")\
     ;created=1618884473;keyid="test-shared-secret"

   This results in the following Signature-Input and Signature headers
   being added to the message under the label sig-b25:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b25=("date" "@authority" "content-type")\
     ;created=1618884473;keyid="test-shared-secret"
   Signature: sig-b25=:pxcQw6G3AjtMBQjwo8XzkZf/bws5LelbaMk5rGIGtE8=:

   Before using symmetric signatures in practice, see the discussion of
   the security tradeoffs in Section 7.11.

B.2.6.  Signing a Request using ed25519

   This example covers portions of the test-request using the ed25519
   algorithm and the key test-key-ed25519.

   The corresponding signature base is:

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   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "@method": POST
   "@path": /foo
   "@authority": example.com
   "content-type": application/json
   "content-length": 18
   "@signature-params": ("date" "@method" "@path" "@authority" \
     "content-type" "content-length");created=1618884473\
     ;keyid="test-key-ed25519"

   This results in the following Signature-Input and Signature headers
   being added to the message under the label sig-b26:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig-b26=("date" "@method" "@path" "@authority" \
     "content-type" "content-length");created=1618884473\
     ;keyid="test-key-ed25519"
   Signature: sig-b26=:wqcAqbmYJ2ji2glfAMaRy4gruYYnx2nEFN2HN6jrnDnQCK1\
     u02Gb04v9EDgwUPiu4A0w6vuQv5lIp5WPpBKRCw==:

B.3.  TLS-Terminating Proxies

   In this example, there is a TLS-terminating reverse proxy sitting in
   front of the resource.  The client does not sign the request but
   instead uses mutual TLS to make its call.  The terminating proxy
   validates the TLS stream and injects a Client-Cert header according
   to [CLIENT-CERT], and then applies a signature to this field.  By
   signing this header field, a reverse proxy can not only attest to its
   own validation of the initial request's TLS parameters but also
   authenticate itself to the backend system independently of the
   client's actions.

   The client makes the following request to the TLS terminating proxy
   using mutual TLS:

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18

   {"hello": "world"}

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   The proxy processes the TLS connection and extracts the client's TLS
   certificate to a Client-Cert header field and passes it along to the
   internal service hosted at service.internal.example.  This results in
   the following unsigned request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: service.internal.example
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKD\
     BJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQT\
     AeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFk\
     wEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmck\
     C8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDV\
     R0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf\
     8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV\
     4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6\
     bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:

   {"hello": "world"}

   Without a signature, the internal service would need to trust that
   the incoming connection has the right information.  By signing the
   Client-Cert header and other portions of the internal request, the
   internal service can be assured that the correct party, the trusted
   proxy, has processed the request and presented it to the correct
   service.  The proxy's signature base consists of the following:

   NOTE: '\' line wrapping per RFC 8792

   "@path": /foo
   "@query": ?param=Value&Pet=dog
   "@method": POST
   "@authority": service.internal.example
   "client-cert": :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQ\
     KDBJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBD\
     QTAeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDM\
     FkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXm\
     ckC8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQY\
     DVR0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8B\
     Af8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQ\
     GV4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0\
     Q6bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
   "@signature-params": ("@path" "@query" "@method" "@authority" \
     "client-cert");created=1618884473;keyid="test-key-ecc-p256"

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   This results in the following signature:

   NOTE: '\' line wrapping per RFC 8792

   xVMHVpawaAC/0SbHrKRs9i8I3eOs5RtTMGCWXm/9nvZzoHsIg6Mce9315T6xoklyy0y\
   zhD9ah4JHRwMLOgmizw==

   Which results in the following signed request sent from the proxy to
   the internal service with the proxy's signature under the label ttrp:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=Value&Pet=dog HTTP/1.1
   Host: service.internal.example
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKD\
     BJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQT\
     AeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFk\
     wEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmck\
     C8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDV\
     R0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf\
     8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV\
     4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6\
     bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
   Signature-Input: ttrp=("@path" "@query" "@method" "@authority" \
     "client-cert");created=1618884473;keyid="test-key-ecc-p256"
   Signature: ttrp=:xVMHVpawaAC/0SbHrKRs9i8I3eOs5RtTMGCWXm/9nvZzoHsIg6\
     Mce9315T6xoklyy0yzhD9ah4JHRwMLOgmizw==:

   {"hello": "world"}

   The internal service can validate the proxy's signature and therefore
   be able to trust that the client's certificate has been appropriately
   processed.

Acknowledgements

   This specification was initially based on the draft-cavage-http-
   signatures internet draft.  The editors would like to thank the
   authors of that draft, Mark Cavage and Manu Sporny, for their work on
   that draft and their continuing contributions.  The specification
   also includes contributions from the draft-oauth-signed-http-request
   internet draft and other similar efforts.

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   The editors would also like to thank the following individuals for
   feedback, insight, and implementation of this draft and its
   predecessors (in alphabetical order): Mark Adamcin, Mark Allen, Paul
   Annesley, Karl Böhlmark, Stéphane Bortzmeyer, Sarven Capadisli, Liam
   Dennehy, Stephen Farrell, Phillip Hallam-Baker, Tyler Ham, Eric
   Holmes, Andrey Kislyuk, Adam Knight, Dave Lehn, Dave Longley, Ilari
   Liusvaara, James H.  Manger, Kathleen Moriarty, Mark Nottingham, Yoav
   Nir, Adrian Palmer, Lucas Pardue, Roberto Polli, Julian Reschke,
   Michael Richardson, Wojciech Rygielski, Rich Salz, Adam Scarr, Cory
   J.  Slep, Dirk Stein, Henry Story, Lukasz Szewc, Chris Webber, and
   Jeffrey Yasskin.

Document History

   _RFC EDITOR: please remove this section before publication_

   *  draft-ietf-httpbis-message-signatures

      -  -10

         o  Removed "related response" and "@request-response" in favor
            of generic "req" parameter.

         o  Editorial fixes to comply with HTTP extension style
            guidelines.

         o  Add security consideration on message content.

      -  -09

         o  Explained key formats better.

         o  Removed "host" and "date" from most examples.

         o  Fixed query component generation.

         o  Renamed "signature input" and "signature input string" to
            "signature base".

         o  Added consideration for semantically equivalent field
            values.

      -  -08

         o  Editorial fixes.

         o  Changed "specialty component" to "derived component".

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         o  Expanded signature input generation and ABNF rules.

         o  Added Ed25519 algorithm.

         o  Clarified encoding of ECDSA signature.

         o  Clarified use of non-deterministic algorithms.

      -  -07

         o  Added security and privacy considerations.

         o  Added pointers to algorithm values from definition sections.

         o  Expanded IANA registry sections.

         o  Clarified that the signing and verification algorithms take
            application requirements as inputs.

         o  Defined "signature targets" of request, response, and
            related-response for specialty components.

      -  -06

         o  Updated language for message components, including
            identifiers and values.

         o  Clarified that Signature-Input and Signature are fields
            which can be used as headers or trailers.

         o  Add "Accept-Signature" field and semantics for signature
            negotiation.

         o  Define new specialty content identifiers, re-defined
            request-target identifier.

         o  Added request-response binding.

      -  -05

         o  Remove list prefixes.

         o  Clarify signature algorithm parameters.

         o  Update and fix examples.

         o  Add examples for ECC and HMAC.

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      -  -04

         o  Moved signature component definitions up to intro.

         o  Created formal function definitions for algorithms to
            fulfill.

         o  Updated all examples.

         o  Added nonce parameter field.

      -  -03

         o  Clarified signing and verification processes.

         o  Updated algorithm and key selection method.

         o  Clearly defined core algorithm set.

         o  Defined JOSE signature mapping process.

         o  Removed legacy signature methods.

         o  Define signature parameters separately from "signature"
            object model.

         o  Define serialization values for signature-input header based
            on signature input.

      -  -02

         o  Removed editorial comments on document sources.

         o  Removed in-document issues list in favor of tracked issues.

         o  Replaced unstructured Signature header with Signature-Input
            and Signature Dictionary Structured Header Fields.

         o  Defined content identifiers for individual Dictionary
            members, e.g., "x-dictionary-field";key=member-name.

         o  Defined content identifiers for first N members of a List,
            e.g., "x-list-field":prefix=4.

         o  Fixed up examples.

         o  Updated introduction now that it's adopted.

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         o  Defined specialty content identifiers and a means to extend
            them.

         o  Required signature parameters to be included in signature.

         o  Added guidance on backwards compatibility, detection, and
            use of signature methods.

      -  -01

         o  Strengthened requirement for content identifiers for header
            fields to be lower-case (changed from SHOULD to MUST).

         o  Added real example values for Creation Time and Expiration
            Time.

         o  Minor editorial corrections and readability improvements.

      -  -00

         o  Initialized from draft-richanna-http-message-signatures-00,
            following adoption by the working group.

   *  draft-richanna-http-message-signatures

      -  -00

         o  Converted to xml2rfc v3 and reformatted to comply with RFC
            style guides.

         o  Removed Signature auth-scheme definition and related
            content.

         o  Removed conflicting normative requirements for use of
            algorithm parameter.  Now MUST NOT be relied upon.

         o  Removed Extensions appendix.

         o  Rewrote abstract and introduction to explain context and
            need, and challenges inherent in signing HTTP messages.

         o  Rewrote and heavily expanded algorithm definition, retaining
            normative requirements.

         o  Added definitions for key terms, referenced RFC 7230 for
            HTTP terms.

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         o  Added examples for canonicalization and signature generation
            steps.

         o  Rewrote Signature header definition, retaining normative
            requirements.

         o  Added default values for algorithm and expires parameters.

         o  Rewrote HTTP Signature Algorithms registry definition.
            Added change control policy and registry template.  Removed
            suggested URI.

         o  Added IANA HTTP Signature Parameter registry.

         o  Added additional normative and informative references.

         o  Added Topics for Working Group Discussion section, to be
            removed prior to publication as an RFC.

Authors' Addresses

   Annabelle Backman (editor)
   Amazon
   P.O. Box 81226
   Seattle, WA 98108-1226
   United States of America
   Email: richanna@amazon.com
   URI:   https://www.amazon.com/

   Justin Richer
   Bespoke Engineering
   Email: ietf@justin.richer.org
   URI:   https://bspk.io/

   Manu Sporny
   Digital Bazaar
   203 Roanoke Street W.
   Blacksburg, VA 24060
   United States of America
   Email: msporny@digitalbazaar.com
   URI:   https://manu.sporny.org/

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