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HTTP Unprompted Authentication
draft-schinazi-httpbis-transport-auth-08

Document Type Replaced Internet-Draft (individual)
Authors David Schinazi , David Oliver , Jonathan Hoyland
Last updated 2022-10-13
Replaced by draft-schinazi-httpbis-unprompted-auth
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draft-schinazi-httpbis-transport-auth-08
HTTPBIS                                                      D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Experimental                                  D. Oliver
Expires: 16 April 2023                                  Guardian Project
                                                              J. Hoyland
                                                         Cloudflare Inc.
                                                         13 October 2022

                     HTTP Unprompted Authentication
                draft-schinazi-httpbis-transport-auth-08

Abstract

   Existing HTTP authentication mechanisms 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 schemes (such
   as signatures or message authentication codes) 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.

   The latest revision of this draft can be found at
   https://DavidSchinazi.github.io/draft-schinazi-httpbis-transport-
   auth/draft-schinazi-httpbis-transport-auth.html.  Status information
   for this document may be found at https://datatracker.ietf.org/doc/
   draft-schinazi-httpbis-transport-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/.

   Source for this draft and an issue tracker can be found at
   https://github.com/DavidSchinazi/draft-schinazi-httpbis-transport-
   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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   This Internet-Draft will expire on 16 April 2023.

Copyright Notice

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

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Computing the Authentication Proof  . . . . . . . . . . . . .   3
   3.  Header Field Definition . . . . . . . . . . . . . . . . . . .   4
     3.1.  The u Parameter . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  The p Parameter . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  The s Parameter . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  The h Parameter . . . . . . . . . . . . . . . . . . . . .   4
   4.  Unprompted Authentication Schemes . . . . . . . . . . . . . .   5
     4.1.  Signature . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  HMAC  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Intermediary Considerations . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Unprompted-Authentication Header Field  . . . . . . . . .   6
     7.2.  Unprompted Authentication Schemes Registry  . . . . . . .   7
     7.3.  TLS Keying Material Exporter Labels . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8

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   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Existing HTTP authentication mechanisms (see Section 11 of [HTTP])
   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 schemes (such as signatures or message
   authentication codes) 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.

   There are scenarios where servers may want to expose the fact that
   authentication is required for access to specific resources.  This is
   left for future work.

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, Token and
   Byte Sequence.

2.  Computing the Authentication Proof

   This document only defines Unprompted Authentication 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].

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   The user agent leverages a TLS keying material exporter [KEY-EXPORT]
   to generate a nonce which can be signed using the user's key.  The
   keying material exporter uses a label that starts with the characters
   "EXPORTER-HTTP-Unprompted-Authentication-" (see Section 4 for the
   labels and contexts used by each scheme).  The TLS keying material
   exporter is used to generate a 32-byte key which is then used as a
   nonce.

3.  Header Field Definition

   The "Unprompted-Authentication" header field allows a user agent to
   authenticate with an origin server.  The authentication is scoped to
   the HTTP request associated with this header field.  The value of the
   Unprompted-Authentication header field is a token which represents
   the Unpromted Authentication Scheme; see Section 4.  This header
   field supports parameters.

3.1.  The u Parameter

   The OPTIONAL "u" (user ID) parameter is a byte sequence that
   specifies the user ID that the user agent wishes to authenticate.

3.2.  The p Parameter

   The OPTIONAL "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 user ID.

3.3.  The s Parameter

   The OPTIONAL "s" (signature) parameter is an integer that specifies
   the signature algorithm used to compute the proof transmitted in the
   "p" directive.  Its value is an integer between 0 and 255 inclusive
   from the IANA "TLS SignatureAlgorithm" registry maintained at
   <https://www.iana.org/assignments/tls-parameters#tls-parameters-16>.

3.4.  The h Parameter

   The OPTIONAL "h" (hash) parameter is an integer that specifies the
   hash algorithm used to compute the proof transmitted in the "p"
   directive.  Its value is an integer between 0 and 255 inclusive from
   the IANA "TLS HashAlgorithm" registry maintained at
   <https://www.iana.org/assignments/tls-parameters#tls-parameters-18>.

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4.  Unprompted Authentication Schemes

   The Unprompted Authentication Framework allows defining Unprompted
   Authentication Schemes, which specify how to authenticate user IDs.
   This documents defined the "Signature" and "HMAC" schemes.

4.1.  Signature

   The "Signature" Unprompted Authentication Scheme uses asymmetric
   cyptography.  User agents possess a user ID and a public/private key
   pair, and origin servers maintain a mapping of authorized user IDs to
   their associated public keys.  When using this scheme, the "u", "p",
   and "s" parameters are REQUIRED.  The TLS keying material export
   label for this scheme is "EXPORTER-HTTP-Unprompted-Authentication-
   Signature" and the associated context is empty.  The nonce is then
   signed using the selected asymmetric signature algorithm and
   transmitted as the proof directive.

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

   Unprompted-Authentication: Signature u=:am9obi5kb2U=:;s=7;
   p=:SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
   aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ==:

4.2.  HMAC

   The "HMAC" Unprompted Authentication Scheme uses symmetric
   cyptography.  User agents possess a user ID and a secret key, and
   origin servers maintain a mapping of authorized user IDs to their
   associated secret key.  When using this scheme, the "u", "p", and "h"
   parameters are REQUIRED.  The TLS keying material export label for
   this scheme is "EXPORTER-HTTP-Unprompted-Authentication-HMAC" and the
   associated context is empty.  The nonce is then HMACed using the
   selected HMAC algorithm and transmitted as the proof directive.

   For example, the user ID "john.doe" authenticating using HMAC-SHA-512
   [SHA] could produce the following header field (lines are folded to
   fit):

   Unprompted-Authentication: HMAC u="am9obi5kb2U=";h=6;
   p="SW5zZXJ0IEhNQUMgb2Ygbm9uY2UgaGVyZSB3aGljaCB0YWtl
   cyA1MTIgYml0cyBmb3IgU0hBLTUxMiEhISEhIQ=="

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5.  Intermediary Considerations

   Since Unprompted Authentication leverages TLS keying material
   exporters, it cannot be transparently forwarded by HTTP
   intermediaries.  HTTP intermediaries that support this specification
   will validate the authentication received from the client themselves,
   then inform the upstream HTTP server of the presence of valid
   authentication using some other mechanism.

6.  Security Considerations

   Unprompted Authentication 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 same user sends an authentication
   proof on multiple requests 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 unprompted authentication 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.

7.  IANA Considerations

7.1.  Unprompted-Authentication Header Field

   This document will request IANA to register the following entry in
   the "HTTP Field Name" registry maintained at
   <https://www.iana.org/assignments/http-fields>:

   Field Name:  Unprompted-Authentication
   Template:  None
   Status:  provisional (permanent if this document is approved)
   Reference:  This document
   Comments:  None

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7.2.  Unprompted Authentication Schemes Registry

   This document, if approved, requests IANA to create a new "HTTP
   Unprompted Authentication Schemes" Registry.  This new registry
   contains strings and is covered by the First Come First Served policy
   from Section 4.4 of [IANA-POLICY].  Each entry contains an optional
   "Reference" field.

   It initially contains the following entries:

   *  Signature

   *  HMAC

   The reference for both is this document.

7.3.  TLS Keying Material Exporter Labels

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

   *  EXPORTER-HTTP-Unprompted-Authentication-Signature

   *  EXPORTER-HTTP-Unprompted-Authentication-HMAC

   Both of these entries are listed with the following qualifiers:

   DTLS-OK:  N
   Recommended:  Y
   Reference:  This document

8.  References

8.1.  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>.

   [IANA-POLICY]
              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>.

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   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

8.2.  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>.

   [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>.

   [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>.

   [SHA]      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>.

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Acknowledgments

   The authors would like to thank many members of the IETF community,
   as this document is the fruit of many hallway conversations.

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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