HTTP Unprompted Authentication
draft-ietf-httpbis-unprompted-auth-02

HTTPBIS                                                      D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                               D. Oliver
Expires: 14 September 2023                              Guardian Project
                                                              J. Hoyland
                                                         Cloudflare Inc.
                                                           13 March 2023

                     HTTP Unprompted Authentication
                 draft-ietf-httpbis-unprompted-auth-02

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.

   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 . . . . . . . . . . . . . . .   4
   2.  Computing the Authentication Proof  . . . . . . . . . . . . .   4
   3.  Header Field Definition . . . . . . . . . . . . . . . . . . .   5
   4.  Authentication Parameters . . . . . . . . . . . . . . . . . .   5
     4.1.  The k Parameter . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  The p Parameter . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  The s Parameter . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  The h Parameter . . . . . . . . . . . . . . . . . . . . .   5
   5.  Authentication Schemes  . . . . . . . . . . . . . . . . . . .   5
     5.1.  Signature . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  HMAC  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Other HTTP Authentication Schemes . . . . . . . . . . . .   6
   6.  Server Handling . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Intermediary Considerations . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Unprompted-Authentication Header Field  . . . . . . . . .   8
     9.2.  HTTP Authentication Schemes Registry  . . . . . . . . . .   8
     9.3.  TLS Keying Material Exporter Labels . . . . . . . . . . .   9

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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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.

   Unprompted Authentication serves use cases in which a site wants to
   offer a service or capability only to "those who know" while all
   others are given no indication the service or capability exists.  The
   conceptual model is that of a "speakeasy".  "Knowing" is via 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).  Unprompted
   Authentication is also useful for cases where a service provider
   wants to distribute user-provisioning information for its resources
   without exposing the provisioning location to non-users.

   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.

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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.

2.  Computing the Authentication Proof

   This document only defines the Signature and HMAC authentication
   schemes 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].

   The user agent leverages a TLS keying material exporter [KEY-EXPORT]
   to generate a nonce which can be signed using the chosen key.  The
   keying material exporter uses a label that starts with the characters
   "EXPORTER-HTTP-Unprompted-Authentication-" (see Section 5 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.

   Because the TLS keying material exporter is only secure for
   authentication when it is uniquely bound to the TLS session
   [RFC7627], the Signature and HMAC authentication schemes require
   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 greater or equal to 1.2 and the Extended
      Master Secret extension [RFC7627] has been negotiated.

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

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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 credentials object, as
   defined in Section 11.4 of [HTTP].  Credentials contain an
   authentication scheme followed by optional authentication parameters.

4.  Authentication Parameters

   This specification defines the following authentication parameters,
   they can be used by the authentication schemes defined in Section 5.

4.1.  The k Parameter

   The OPTIONAL "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 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 key ID.

4.3.  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>.

4.4.  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>.

5.  Authentication Schemes

   This document defines the "Signature" and "HMAC" HTTP authentication
   schemes.

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5.1.  Signature

   The "Signature" HTTP Authentication Scheme 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.  When using this scheme, the "k", "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 key ID "basement" authenticating using Ed25519
   [ED25519] could produce the following header field (lines are folded
   to fit):

   Unprompted-Authentication: Signature k=:YmFzZW1lbnQ=:;s=7;
   p=:SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
   aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ==:

5.2.  HMAC

   The "HMAC" HTTP Authentication Scheme uses symmetric cryptography.
   User agents possess a key ID and a secret key, and origin servers
   maintain a mapping of authorized key IDs to their associated secret
   key.  When using this scheme, the "k", "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 key ID "basement" authenticating using HMAC-SHA-512
   [SHA] could produce the following header field (lines are folded to
   fit):

   Unprompted-Authentication: HMAC k="YmFzZW1lbnQ=";h=6;
   p="SW5zZXJ0IEhNQUMgb2Ygbm9uY2UgaGVyZSB3aGljaCB0YWtl
   cyA1MTIgYml0cyBmb3IgU0hBLTUxMiEhISEhIQ=="

5.3.  Other HTTP Authentication Schemes

   The HTTP Authentication Scheme registry maintained by IANA at
   <https://www.iana.org/assignments/http-authschemes/http-
   authschemes.xhtml> contains entries not defined in this document.
   Those entries MAY be used with Unprompted Authentication.

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6.  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 Unprompted-Authentication field *
   failure to parse the Unprompted-Authentication field * use of
   Unprompted Authentication with an unknown key ID * failure to
   validate the signature or MAC.

   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 and HMAC HTTP Authentication Schemes leverage TLS
   keying material exporters, their 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 nonce (see Section 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.

8.  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.

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   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 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.

   Key material used for authentication in unprompted authentication,
   whether symmetric or asymmetric MUST NOT be reused in other
   protocols.  Doing so can undermine the security guarantees of the
   authentication.

   Origins offering Unprompted Authentication are able to link requests
   that use the same key for the Authentication Schemes provided.
   However, requests are not linkable across origins if the keys used
   are specific to the individual origins using Unprompted
   Authentication.

9.  IANA Considerations

9.1.  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

9.2.  HTTP Authentication Schemes Registry

   This document, if approved, requests IANA to add two new entries to
   the "HTTP Authentication Schemes" Registry maintained at
   <https://www.iana.org/assignments/http-authschemes>.  Both entries
   have the Reference set to this document, and the Notes empty.  The
   Authentication Scheme Name of the entries are:

   *  Signature

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   *  HMAC

9.3.  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

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>.

   [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-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>.

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>.

   [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>.

Acknowledgments

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

Authors' Addresses

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

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   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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