



Internet Engineering Task Force (IETF)                       D. Schinazi
Request for Comments: 9729                                    Google LLC
Category: Standards Track                                      D. Oliver
ISSN: 2070-1721                                         Guardian Project
                                                              J. Hoyland
                                                         Cloudflare Inc.
                                                           February 2025


                The Concealed HTTP Authentication Scheme

Abstract

   Most 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.  Prior to this document, there
   was no existing way for the origin to share such a nonce without
   exposing the fact that it serves resources that require
   authentication.  This document defines a new non-probeable
   cryptographic authentication scheme.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9729.

Copyright Notice

   Copyright (c) 2025 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
     1.1.  Conventions and Definitions
   2.  The Concealed HTTP Authentication Scheme
   3.  Client Handling
     3.1.  Key Exporter Context
       3.1.1.  Public Key Encoding
     3.2.  Key Exporter Output
     3.3.  Signature Computation
   4.  Authentication Parameters
     4.1.  The k Parameter
     4.2.  The a Parameter
     4.3.  The p Parameter
     4.4.  The s Parameter
     4.5.  The v Parameter
   5.  Example
   6.  Server Handling
     6.1.  Frontend Handling
     6.2.  Communication Between Frontend and Backend
     6.3.  Backend Handling
     6.4.  Non-Probeable Server Handling
   7.  Requirements on TLS Usage
   8.  Security Considerations
   9.  IANA Considerations
     9.1.  HTTP Authentication Schemes Registry
     9.2.  TLS Keying Material Exporter Labels
     9.3.  HTTP Field Name
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

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 (or probing for) such capabilities
   difficult.  As another example, 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 (e.g., [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 sends 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 notation from Section 1.3 of [QUIC].

   Various examples in this document contain long lines that may be
   folded, as described in [RFC8792].

2.  The Concealed HTTP Authentication Scheme

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

   The client uses a TLS keying material exporter to generate data to be
   signed (see Section 3) then sends the signature using the
   Authorization (or Proxy-Authorization) header field (see Section 11
   of [HTTP]).  The signature and additional information are exchanged
   using authentication parameters (see Section 4).  Once the server
   receives these, it can check whether the signature validates against
   an entry in its database of known keys.  The server can then use the
   validation result to influence its response to the client, for
   example, by restricting access to certain resources.

3.  Client Handling

   When a client wishes to use the Concealed HTTP authentication scheme
   with a request, it SHALL compute the authentication proof using a TLS
   keying material exporter with the following parameters:

   *  The label is set to "EXPORTER-HTTP-Concealed-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).

   Note that TLS 1.3 keying material exporters are defined in
   Section 7.5 of [TLS], while TLS 1.2 keying material exporters are
   defined in [KEY-EXPORT].

3.1.  Key Exporter Context

   The TLS key exporter context is described in Figure 1, using the
   notation from Section 1.3 of [QUIC]:

     Signature Algorithm (16),
     Key ID Length (i),
     Key ID (..),
     Public Key Length (i),
     Public Key (..),
     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 s Parameter
      (see Section 4.4).

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

   Public Key:  The public key used by the server to validate the
      signature provided by the client.  Its encoding is described in
      Section 3.1.1.

   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, encoded in network byte order.
      Note that the port is either included in the URI or is the default
      port for the scheme in use; see Section 3.2.3 of [URI].

   Realm:  The realm of authentication that is sent in the realm
      authentication parameter (see 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, Public Key,
   Scheme, Host, and Realm 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.

3.1.1.  Public Key Encoding

   Both the "Public Key" field of the TLS key exporter context (see
   above) and the a Parameter (see Section 4.2) carry the same public
   key.  The encoding of the public key is determined by the signature
   algorithm in use as follows:

   RSASSA-PSS algorithms:  The public key is an RSAPublicKey structure
      [PKCS1] encoded in DER [X.690].  BER encodings that are not DER
      MUST be rejected.

   ECDSA algorithms:  The public key is an
      UncompressedPointRepresentation structure defined in
      Section 4.2.8.2 of [TLS], using the curve specified by the
      SignatureScheme.

   EdDSA algorithms:  The public key is the byte string encoding defined
      in [EdDSA].

   This document does not define the public key encodings for other
   algorithms.  In order for a SignatureScheme to be usable with the
   Concealed HTTP authentication scheme, its public key encoding needs
   to be defined in a corresponding document.

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, using the notation from Section 1.3 of [QUIC]:

     Signature Input (256),
     Verification (128),

                    Figure 2: Key Exporter Output Format

   The key exporter output 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 4.5).

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.  The signature is computed over the concatenation of:

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

   *  The context string "HTTP Concealed Authentication"

   *  A single 0 byte that 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

               Figure 3: Example Content Covered by Signature

   The purpose of this static prefix is to mitigate issues that could
   arise if authentication asymmetric keys were accidentally reused
   across protocols (even though this is forbidden, see Section 8).
   This construction 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 4.3).

4.  Authentication Parameters

   This specification defines the following authentication parameters.

   All of the byte sequences below are encoded using base64url (see
   Section 5 of [BASE64]) without quotes and without padding.  In other
   words, the values of these byte-sequence authentication parameters
   MUST NOT include any characters other than ASCII letters, digits,
   dash, and underscore.

   The integer below is encoded without a minus and without leading
   zeroes.  In other words, the value of this integer authentication
   parameter MUST NOT include any characters other than digits and MUST
   NOT start with a zero unless the full value is "0".

   Using the syntax from [ABNF]:

   concealed-byte-sequence-param-value = *( ALPHA / DIGIT / "-" / "_" )
   concealed-integer-param-value =  %x31-39 1*4( DIGIT ) / "0"

               Figure 4: Authentication Parameter Value ABNF

4.1.  The k Parameter

   The REQUIRED "k" (key ID) Parameter is a byte sequence that
   identifies which key the client wishes to use to authenticate.  This
   is used by the backend to point to an entry in a server-side database
   of known keys (see Section 6.3).

4.2.  The a Parameter

   The REQUIRED "a" (public key) Parameter is a byte sequence that
   specifies the public key used by the server to validate the signature
   provided by the client.  This avoids key confusion issues (see
   [SEEMS-LEGIT]).  The encoding of the public key is described in
   Section 3.1.1.

4.3.  The p Parameter

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

4.4.  The s Parameter

   The REQUIRED "s" (signature scheme) Parameter is an integer that
   specifies the signature scheme used to compute the proof transmitted
   in the p Parameter.  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>.

4.5.  The v Parameter

   The REQUIRED "v" (verification) Parameter is a byte sequence that
   specifies the verification that the client provides to attest to
   possessing the key exporter output (see Section 3.2 for details).
   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, a client using the key ID "basement" and the signature
   algorithm Ed25519 [ED25519] could produce the following header field:

   NOTE: '\' line wrapping per RFC 8792

   Authorization: Concealed \
     k=YmFzZW1lbnQ, \
     a=VGhpcyBpcyBh-HB1YmxpYyBrZXkgaW4gdXNl_GhlcmU, \
     s=2055, \
     v=dmVyaWZpY2F0aW9u_zE2Qg, \
     p=QzpcV2luZG93c_xTeXN0ZW0zMlxkcml2ZXJz-ENyb3dkU\
       3RyaWtlXEMtMDAwMDAwMDAyOTEtMD-wMC0w_DAwLnN5cw

                       Figure 5: Example Header Field

6.  Server Handling

   In this section, we subdivide the server role in two:

   *  The "frontend" runs in the HTTP server that terminates the TLS or
      QUIC connection created by the client.

   *  The "backend" runs in the HTTP server that has access to the
      database of accepted key identifiers and public keys.

   In most deployments, we expect both the frontend and backend roles to
   be implemented in a single HTTP origin server (as defined in
   Section 3.6 of [HTTP]).  However, these roles can be split such that
   the frontend is an HTTP gateway (as defined in Section 3.7 of [HTTP])
   and the backend is an HTTP origin server.

6.1.  Frontend Handling

   If a frontend is configured to check the Concealed HTTP
   authentication scheme, it will parse the Authorization (or Proxy-
   Authorization) header field.  If the authentication scheme is set to
   "Concealed", the frontend MUST validate that all the required
   authentication parameters are present and can be parsed correctly as
   defined in Section 4.  If any parameter is missing or fails to parse,
   the frontend MUST ignore the entire Authorization (or Proxy-
   Authorization) header field.

   The frontend then uses the data from these authentication parameters
   to compute the key exporter output, as defined in Section 3.2.  The
   frontend then shares the header field and the key exporter output
   with the backend.

6.2.  Communication Between Frontend and Backend

   If the frontend and backend roles are implemented in the same
   machine, this can be handled by a simple function call.

   If the roles are split between two separate HTTP servers, then the
   backend won't be able to directly access the TLS keying material
   exporter from the TLS connection between the client and frontend, so
   the frontend needs to explicitly send it.  This document defines the
   "Concealed-Auth-Export" request header field for this purpose.  The
   Concealed-Auth-Export header field's value is a Structured Field Byte
   Sequence (see Section 3.3.5 of [STRUCTURED-FIELDS]) that contains the
   48-byte key exporter output (see Section 3.2), without any
   parameters.  Note that Structured Field Byte Sequences are encoded
   using the non-URL-safe variant of base64.  For example:

   NOTE: '\' line wrapping per RFC 8792

   Concealed-Auth-Export: :VGhpc+BleGFtcGxlIFRMU/BleHBvcn\
     Rlc+BvdXRwdXQ/aXMgNDggYnl0ZXMgI/+h:

            Figure 6: Example Concealed-Auth-Export Header Field

   The frontend SHALL forward the HTTP request to the backend, including
   the original unmodified Authorization (or Proxy-Authorization) header
   field and the newly added Concealed-Auth-Export header field.

   Note that, since the security of this mechanism requires the key
   exporter output to be correct, backends need to trust frontends to
   send it truthfully.  This trust relationship is common because the
   frontend already needs access to the TLS certificate private key in
   order to respond to requests.  HTTP servers that parse the Concealed-
   Auth-Export header field MUST ignore it unless they have already
   established that they trust the sender.  Similarly, frontends that
   send the Concealed-Auth-Export header field MUST ensure that they do
   not forward any Concealed-Auth-Export header field received from the
   client.

6.3.  Backend Handling

   Once the backend receives the Authorization (or Proxy-Authorization)
   header field and the key exporter output, it looks up the key ID in
   its database of public keys.  The backend SHALL then perform the
   following checks:

   *  validate that all the required authentication parameters are
      present and can be parsed correctly as defined in Section 4

   *  ensure the key ID is present in the backend's database and maps to
      a corresponding public key

   *  validate that the public key from the database is equal to the one
      in the Authorization (or Proxy-Authorization) header field

   *  validate that the verification field from the Authorization (or
      Proxy-Authorization) header field matches the one extracted from
      the key exporter output

   *  verify the cryptographic signature as defined in Section 3.3

   If all of these checks succeed, the backend can consider the request
   to be properly authenticated and can reply accordingly (the backend
   can also forward the request to another HTTP server).

   If any of the above checks fail, the backend MUST treat it as if the
   Authorization (or Proxy-Authorization) header field was missing.

6.4.  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 nonexistent resources.  For
   example, this can mean using HTTP status code 404 (Not Found) instead
   of 401 (Unauthorized).

   The authentication checks described above can take time to compute,
   and an attacker could detect use of this mechanism if that time is
   observable by comparing the timing of a request for a known
   nonexistent resource to the timing of a request for a potentially
   authenticated resource.  Servers can mitigate this observability by
   slightly delaying responses to some nonexistent resources such that
   the timing of the authentication verification is not observable.
   This delay needs to be carefully considered to avoid having the delay
   itself leak the fact that this origin uses this mechanism at all.

   Non-probeable resources also need to be non-discoverable for
   unauthenticated users.  For example, if a server operator wishes to
   hide an authenticated resource by pretending it does not exist to
   unauthenticated users, then the server operator needs to ensure there
   are no unauthenticated pages with links to that resource and no other
   out-of-band ways for unauthenticated users to discover this resource.

7.  Requirements on TLS Usage

   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 Concealed authentication scheme requires either one of
   the following properties:

   *  The TLS version in use is greater than 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 Concealed HTTP 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 if the authentication were not present.

8.  Security Considerations

   The Concealed HTTP authentication scheme allows a client to
   authenticate to an origin server while guaranteeing freshness and
   without the need for the server to transmit a nonce to the client.
   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 client leaking
   the presence of authentication to observers due to cleartext TLS
   Client Hello extensions.

   Since the freshness described above is provided by a TLS key
   exporter, it can be as old as the underlying TLS connection.  Servers
   can require better freshness by forcing clients to create new
   connections using mechanisms such as the GOAWAY frame (see
   Section 5.2 of [HTTP/3]).

   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 it 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.

   Authentication asymmetric keys used for the Concealed HTTP
   authentication scheme MUST NOT be reused in other protocols.  Even
   though we attempt to mitigate these issues by adding a static prefix
   to the signed data (see Section 3.3), reusing keys could 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

   IANA has registered the following entry in the "HTTP Authentication
   Schemes" registry maintained at <https://www.iana.org/assignments/
   http-authschemes>:

   Authentication Scheme Name:  Concealed
   Reference:  RFC 9729
   Notes:  None

9.2.  TLS Keying Material Exporter Labels

   IANA has registered the following entry in the "TLS Exporter Labels"
   registry maintained at <https://www.iana.org/assignments/tls-
   parameters>:

   Value:  EXPORTER-HTTP-Concealed-Authentication
   DTLS-OK:  N
   Recommended:  Y
   Reference:  RFC 9729

9.3.  HTTP Field Name

   IANA has registered the following entry in the "Hypertext Transfer
   Protocol (HTTP) Field Name Registry" maintained at
   <https://www.iana.org/assignments/http-fields>:

   Field Name:  Concealed-Auth-Export
   Status:  permanent
   Structured Type:  Item
   Reference:  RFC 9729
   Comments:  None

10.  References

10.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

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

   [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/info/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/info/rfc5705>.

   [PKCS1]    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/info/rfc8017>.

   [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/info/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/info/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/info/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/info/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/info/rfc8792>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
              <https://www.rfc-editor.org/info/rfc9651>.

   [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/info/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/info/rfc3986>.

   [X.690]    ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X690, ISO/IEC 8825-1:2021,
              February 2021, <https://www.itu.int/rec/T-REC-X.690>.

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/info/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/info/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/info/rfc9113>.

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/info/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/info/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
              the 2019 ACM SIGSAC Conference on Computer and
              Communications Security, pp. 2165-2180,
              DOI 10.1145/3319535.3339813, November 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 David Benjamin, Reese
   Enghardt, Nick Harper, Dennis Jackson, Ilari Liusvaara, François
   Michel, Lucas Pardue, Justin Richer, Ben Schwartz, Martin Thomson,
   and Chris A. 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
