The Signature HTTP Authentication Scheme
draft-ietf-httpbis-unprompted-auth-03

HTTPBIS                                                      D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                               D. Oliver
Expires: 30 December 2023                               Guardian Project
                                                              J. Hoyland
                                                         Cloudflare Inc.
                                                            28 June 2023

                The Signature HTTP Authentication Scheme
                 draft-ietf-httpbis-unprompted-auth-03

Abstract

   Existing HTTP authentication schemes are probeable in the sense that
   it is possible for an unauthenticated client to probe whether an
   origin serves resources that require authentication.  It is possible
   for an origin to hide the fact that it requires authentication by not
   generating Unauthorized status codes, however that only works with
   non-cryptographic authentication schemes: cryptographic signatures
   require a fresh nonce to be signed, and there is no existing way for
   the origin to share such a nonce without exposing the fact that it
   serves resources that require authentication.  This document proposes
   a new non-probeable cryptographic authentication scheme.

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-unprompted-auth/.

   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/unprompted-auth.

Status of This Memo

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  The Signature Authentication Scheme . . . . . . . . . . . . .   4
   3.  Computing the Authentication Proof  . . . . . . . . . . . . .   4
     3.1.  Key Exporter Context  . . . . . . . . . . . . . . . . . .   4
     3.2.  Key Exporter Output . . . . . . . . . . . . . . . . . . .   6
     3.3.  Signature Computation . . . . . . . . . . . . . . . . . .   6
   4.  Authentication Parameters . . . . . . . . . . . . . . . . . .   7
     4.1.  The k Parameter . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  The p Parameter . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  The s Parameter . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  The v Parameter . . . . . . . . . . . . . . . . . . . . .   7
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Non-Probeable Server Handling . . . . . . . . . . . . . . . .   8
   7.  Intermediary Considerations . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  HTTP Authentication Schemes Registry  . . . . . . . . . .   9
     9.2.  TLS Keying Material Exporter Labels . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10

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     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   HTTP authentication schemes (see Section 11 of [HTTP]) allow origins
   to restrict access for some resources to only authenticated requests.
   While these schemes commonly involve a challenge where the origin
   asks the client to provide authentication information, it is possible
   for clients to send such information unprompted.  This is
   particularly useful in cases where an origin wants to offer a service
   or capability only to "those who know" while all others are given no
   indication the service or capability exists.  Such designs rely on an
   externally-defined mechanism by which keys are distributed.  For
   example, a company might offer remote employee access to company
   services directly via its website using their employee credentials,
   or offer access to limited special capabilities for specific
   employees, while making discovering (probing for) such capabilities
   difficult.  Members of less well-defined communities might use more
   ephemeral keys to acquire access to geography- or capability-specific
   resources, as issued by an entity whose user base is larger than the
   available resources can support (by having that entity metering the
   availability of keys temporally or geographically).

   While digital-signature-based HTTP authentication schemes already
   exist ([HOBA]), they rely on the origin explicitly sending a fresh
   challenge to the client, to ensure that the signature input is fresh.
   That makes the origin probeable as it send the challenge to
   unauthenticated clients.  This document defines a new signature-based
   authentication scheme that is not probeable.

1.1.  Conventions and Definitions

   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.

   This document uses the following terminology from Section 3 of
   [STRUCTURED-FIELDS] to specify syntax and parsing: Integer and Byte
   Sequence.  This document uses the notation from Section 1.3 of
   [QUIC].

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2.  The Signature Authentication Scheme

   This document defines the "Signature" HTTP authentication scheme.  It
   uses asymmetric cryptography.  User agents possess a key ID and a
   public/private key pair, and origin servers maintain a mapping of
   authorized key IDs to their associated public keys.

   This authentication scheme is only defined for uses of HTTP with TLS
   [TLS].  This includes any use of HTTP over TLS as typically used for
   HTTP/2 [HTTP/2], or HTTP/3 [HTTP/3] where the transport protocol uses
   TLS as its authentication and key exchange mechanism [QUIC-TLS].

   Because the TLS keying material exporter is only secure for
   authentication when it is uniquely bound to the TLS session
   [RFC7627], the Signature authentication scheme requires either one of
   the following properties:

   *  The TLS version in use is greater or equal to 1.3 [TLS].

   *  The TLS version in use is 1.2 and the Extended Master Secret
      extension [RFC7627] has been negotiated.

   Clients MUST NOT use the Signature authentication scheme on
   connections that do not meet one of the two properties above.  If a
   server receives a request that uses this authentication scheme on a
   connection that meets neither of the above properties, the server
   MUST treat the request as malformed.

3.  Computing the Authentication Proof

   The user agent computes the authentication proof using a TLS keying
   material exporter [KEY-EXPORT] with the following parameters:

   *  the label is set to "EXPORTER-HTTP-Signature-Authentication"

   *  the context is set to the structure described in Section 3.1

   *  the exporter output length is set to 48 bytes (see Section 3.2)

3.1.  Key Exporter Context

   The TLS key exporter context is described in Figure 1:

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     Signature Algorithm (16),
     Key ID Length (i),
     Key ID (..),
     Scheme Length (i),
     Scheme (..),
     Host Length (i),
     Host (..),
     Port (16),
     Realm Length (i),
     Realm (..),

                   Figure 1: Key Exporter Context Format

   The key exporter context contains the following fields:

   Signature Algorithm:  The signature scheme sent in the p Parameter
      (see Section 4.3).

   Key ID:  The key ID sent in the k Parameter (see Section 4.1).

   Scheme:  The scheme for this request, encoded using the format of the
      scheme portion of a URI as defined in Section 3.1 of [URI].

   Host:  The host for this request, encoded using the format of the
      host portion of a URI as defined in Section 3.2.2 of [URI].

   Port:  The port for this request.

   Realm:  The real of authentication that is sent in the realm
      authentication parameter (Section 11.5 of [HTTP]).  If the realm
      authentication parameter is not present, this SHALL be empty.
      This document does not define a means for the origin to
      communicate a realm to the client.  If a client is not configured
      to use a specific realm, it SHALL use an empty realm and SHALL NOT
      send the realm authentication parameter.

   The Signature Algorithm and Port fields are encoded as unsigned
   16-bit integers in network byte order.  The Key ID, Scheme, Host, and
   Real fields are length prefixed strings; they are preceded by a
   Length field that represents their length in bytes.  These length
   fields are encoded using the variable-length integer encoding from
   Section 16 of [QUIC] and MUST be encoded in the minimum number of
   bytes necessary.

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3.2.  Key Exporter Output

   The key exporter output is 48 bytes long.  Of those, the first 32
   bytes are part of the input to the signature and the next 16 bytes
   are sent alongside the signature.  This allows the recipient to
   confirm that the exporter produces the right values.  This is
   described in Figure 2:

     Signature Input (256),
     Verification (128),

                    Figure 2: Key Exporter Output Format

   The key exporter context contains the following fields:

   Signature Input:  This is part of the data signed using the client's
      chosen asymmetric private key (see Section 3.3).

   Verification:  The verification is transmitted to the server using
      the v Parameter (see Section 4.4).

3.3.  Signature Computation

   Once the Signature Input has been extracted from the key exporter
   output (see Section 3.2), it is prefixed with static data before
   being signed to mitigate issues caused by key reuse.  The signature
   is computed over the concatenation of:

   *  A string that consists of octet 32 (0x20) repeated 64 times

   *  The context string "HTTP Signature Authentication"

   *  A single 0 byte which serves as a separator

   *  The Signature Input extracted from the key exporter output (see
      Section 3.2)

   For example, if the Signature Input has all its 32 bytes set to 01,
   the content covered by the signature (in hexadecimal format) would
   be:

   2020202020202020202020202020202020202020202020202020202020202020
   2020202020202020202020202020202020202020202020202020202020202020
   48545450205369676E61747572652041757468656E7469636174696F6E
   00
   0101010101010101010101010101010101010101010101010101010101010101

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   This constructions mirrors that of the TLS 1.3 CertificateVerify
   message defined in Section 4.4.3 of [TLS].

   The resulting signature is then transmitted to the server using the p
   Parameter (see Section 4.2).

4.  Authentication Parameters

   This specification defines the following authentication parameters.
   These parameters use structured fields ([STRUCTURED-FIELDS]) in their
   definition, even though the Authorization field itself does not use
   structured fields.

4.1.  The k Parameter

   The REQUIRED "k" (key ID) parameter is a byte sequence that
   identifies which key the user agent wishes to use to authenticate.
   This can for example be used to point to an entry into a server-side
   database of known keys.

4.2.  The p Parameter

   The REQUIRED "p" (proof) parameter is a byte sequence that specifies
   the proof that the user agent provides to attest to possessing the
   credential that matches its key ID.

4.3.  The s Parameter

   The REQUIRED "s" (signature) parameter is an integer that specifies
   the signature scheme used to compute the proof transmitted in the "p"
   directive.  Its value is an integer between 0 and 65535 inclusive
   from the IANA "TLS SignatureScheme" registry maintained at
   <https://www.iana.org/assignments/tls-parameters/tls-
   parameters.xhtml#tls-signaturescheme>.

4.4.  The v Parameter

   The REQUIRED "v" (verification) parameter is a byte sequence that
   specifies the verification that the user agent provides to attest to
   possessing the key exporter output.  This avoids issues with
   signature schemes where certain keys can generate signatures that are
   valid for multiple inputs (see [SEEMS-LEGIT]).

5.  Example

   For example, the key ID "basement" authenticating using Ed25519
   [ED25519] could produce the following header field (lines are folded
   to fit):

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   Authorization: Signature k=:YmFzZW1lbnQ=:;s=2055;
   v=:dmVyaWZpY2F0aW9uXzE2Qg==:;
   p=:SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
   aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ==:

6.  Non-Probeable Server Handling

   Servers that wish to introduce resources whose existence cannot be
   probed need to ensure that they do not reveal any information about
   those resources to unauthenticated clients.  In particular, such
   servers MUST respond to authentication failures with the exact same
   response that they would have used for non-existent resources.  For
   example, this can mean using HTTP status code 404 (Not Found) instead
   of 401 (Unauthorized).  Such authentication failures can be caused
   for example by: * absence of the Authorization field * failure to
   parse the Authorization field * use of the Signature authentication
   scheme with an unknown key ID * failure to validate the verification
   parameter * failure to validate the signature.

   Such servers MUST also ensure that the timing of their request
   handling does not leak any information.  This can be accomplished by
   delaying responses to all non-existent resources such that the timing
   of the authentication verification is not observable.

7.  Intermediary Considerations

   Since the Signature HTTP authentication scheme leverages TLS keying
   material exporters, its output cannot be transparently forwarded by
   HTTP intermediaries.  HTTP intermediaries that support this
   specification have two options:

   *  The intermediary can validate the authentication received from the
      client, then inform the upstream HTTP server of the presence of
      valid authentication.

   *  The intermediary can export the Signature Input and Verification
      (see Section 3.2}), and forward it to the upstream HTTP server,
      then the upstream server performs the validation.

   The mechanism for the intermediary to communicate this information to
   the upstream HTTP server is out of scope for this document.

   Note that both of these mechanisms require the upstream HTTP server
   to trust the intermediary.  This is usually the case because the
   intermediary already needs access to the TLS certificate private key
   in order to respond to requests.

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8.  Security Considerations

   The Signature HTTP authentication scheme allows a user agent to
   authenticate to an origin server while guaranteeing freshness and
   without the need for the server to transmit a nonce to the user
   agent.  This allows the server to accept authenticated clients
   without revealing that it supports or expects authentication for some
   resources.  It also allows authentication without the user agent
   leaking the presence of authentication to observers due to clear-text
   TLS Client Hello extensions.

   The authentication proofs described in this document are not bound to
   individual HTTP requests; if the key is used for authentication
   proofs on multiple requests on the same connection, they will all be
   identical.  This allows for better compression when sending over the
   wire, but implies that client implementations that multiplex
   different security contexts over a single HTTP connection need to
   ensure that those contexts cannot read each other's header fields.
   Otherwise, one context would be able to replay the Authorization
   header field of another.  This constraint is met by modern Web
   browsers.  If an attacker were to compromise the browser such that it
   could access another context's memory, the attacker might also be
   able to access the corresponding key, so binding authentication to
   requests would not provide much benefit in practice.

   Key material used for the Signature HTTP authentication scheme MUST
   NOT be reused in other protocols.  Doing so can undermine the
   security guarantees of the authentication.

   Origins offering this scheme can link requests that use the same key.
   However, requests are not linkable across origins if the keys used
   are specific to the individual origins using this scheme.

9.  IANA Considerations

9.1.  HTTP Authentication Schemes Registry

   This document, if approved, requests IANA to register the following
   entry in the "HTTP Authentication Schemes" Registry maintained at
   <https://www.iana.org/assignments/http-authschemes>:

   Authentication Scheme Name:  Signature
   Reference:  This document
   Notes:  None

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9.2.  TLS Keying Material Exporter Labels

   This document, if approved, requests IANA to register the following
   entry in the "TLS Exporter Labels" registry maintained at
   <https://www.iana.org/assignments/tls-parameters#exporter-labels>:

   Value:  EXPORTER-HTTP-Signature-Authentication
   DTLS-OK:  N
   Recommended:  Y
   Reference:  This document

10.  References

10.1.  Normative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [KEY-EXPORT]
              Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/rfc/rfc5705>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [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>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7627>.

   [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>.

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

   [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>.

   [URI]      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>.

10.2.  Informative References

   [ED25519]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8410>.

   [HOBA]     Farrell, S., Hoffman, P., and M. Thomas, "HTTP Origin-
              Bound Authentication (HOBA)", RFC 7486,
              DOI 10.17487/RFC7486, March 2015,
              <https://www.rfc-editor.org/rfc/rfc7486>.

   [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9113>.

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

   [MASQUE-ORIGINAL]
              Schinazi, D., "The MASQUE Protocol", Work in Progress,
              Internet-Draft, draft-schinazi-masque-00, 28 February
              2019, <https://datatracker.ietf.org/doc/html/draft-
              schinazi-masque-00>.

   [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9001>.

   [SEEMS-LEGIT]
              Jackson, D., Cremers, C., Cohn-Gordon, K., and R. Sasse,
              "Seems Legit: Automated Analysis of Subtle Attacks on
              Protocols That Use Signatures", CCS '19: Proceedings of

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              the 2019 ACM SIGSAC Conference on Computer and
              Communications Security, pp. 2165–2180,
              DOI 10.1145/3319535.3339813, 2019,
              <https://doi.org/10.1145/3319535.3339813>.

Acknowledgments

   The authors would like to thank many members of the IETF community,
   as this document is the fruit of many hallway conversations.  In
   particular, the authors would like to thank Nick Harper, Dennis
   Jackson, Ilari Liusvaara, Justin Richer, Ben Schwartz, Martin
   Thomson, and Chris Wood for their reviews and contributions.  The
   mechanism described in this document was originally part of the first
   iteration of MASQUE [MASQUE-ORIGINAL].

Authors' Addresses

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America
   Email: dschinazi.ietf@gmail.com

   David M. Oliver
   Guardian Project
   Email: david@guardianproject.info
   URI:   https://guardianproject.info

   Jonathan Hoyland
   Cloudflare Inc.
   Email: jonathan.hoyland@gmail.com

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