HTTP                                                     A. Backman, Ed.
Internet-Draft                                                    Amazon
Intended status: Standards Track                               J. Richer
Expires: 14 February 23 June 2022                                Bespoke Engineering
                                                               M. Sporny
                                                          Digital Bazaar
                                                          13 August
                                                        20 December 2021

                        HTTP Message Signatures
                draft-ietf-httpbis-message-signatures-06
                draft-ietf-httpbis-message-signatures-07

Abstract

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

Note

About This Document

   This note is to Readers

   _RFC EDITOR: please remove this section be removed before publication_ publishing as an RFC.

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

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

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 14 February 23 June 2022.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Discussion . . . . . . . . . . . . . . . . .   5
     1.2.  HTTP Message Transformations  . . . . . . . . . . . . . .   5   6
     1.3.  Safe Transformations  . . . . . . . . . . . . . . . . . .   6
     1.4.  Conventions and Terminology . . . . . . . . . . . . . . .   7
     1.5.  Application of HTTP Message Signatures  . . . . . . . . .   9
   2.  HTTP Message Components . . . . . . . . . . . . . . . . . . .  10
     2.1.  HTTP Fields . . . . . . . . . . . . . . . . . . . . . . .  11
       2.1.1.  Canonicalized Structured HTTP Fields  . . . . . . . .  11  12
       2.1.2.  Canonicalization  HTTP Field Examples . . . . . . . . . . . . . .  11
     2.2.  Dictionary Structured Field Members . . . . . . . . . . .  12
       2.2.1.  Canonicalization Examples . . . . .
       2.1.3.  Dictionary Structured Field Members . . . . . . . . .  12
     2.3.
     2.2.  Specialty Components  . . . . . . . . . . . . . . . . . .  13
       2.3.1.
       2.2.1.  Signature Parameters  . . . . . . . . . . . . . . . .  14
       2.3.2.
       2.2.2.  Method  . . . . . . . . . . . . . . . . . . . . . . .  15
       2.3.3.  16
       2.2.3.  Target URI  . . . . . . . . . . . . . . . . . . . . .  16
       2.3.4.
       2.2.4.  Authority . . . . . . . . . . . . . . . . . . . . . .  16
       2.3.5.  17
       2.2.5.  Scheme  . . . . . . . . . . . . . . . . . . . . . . .  17
       2.3.6.
       2.2.6.  Request Target  . . . . . . . . . . . . . . . . . . .  17
       2.3.7.  18
       2.2.7.  Path  . . . . . . . . . . . . . . . . . . . . . . . .  19
       2.3.8.
       2.2.8.  Query . . . . . . . . . . . . . . . . . . . . . . . .  19
       2.3.9.  20
       2.2.9.  Query Parameters  . . . . . . . . . . . . . . . . . .  20
       2.3.10.
       2.2.10. Status Code . . . . . . . . . . . . . . . . . . . . .  21
       2.3.11.
       2.2.11. Request-Response Signature Binding  . . . . . . . . .  21
     2.4.  22
     2.3.  Creating the Signature Input String . . . . . . . . . . .  23
   3.  HTTP Message Signatures . . . . . . . . . . . . . . . . . . .  25  26
     3.1.  Creating a Signature  . . . . . . . . . . . . . . . . . .  25  26
     3.2.  Verifying a Signature . . . . . . . . . . . . . . . . . .  27  28
       3.2.1.  Enforcing Application Requirements  . . . . . . . . .  29  30
     3.3.  Signature Algorithm Methods . . . . . . . . . . . . . . .  29  31
       3.3.1.  RSASSA-PSS using SHA-512  . . . . . . . . . . . . . .  30  32
       3.3.2.  RSASSA-PKCS1-v1_5 using SHA-256 . . . . . . . . . . .  31  32
       3.3.3.  HMAC using SHA-256  . . . . . . . . . . . . . . . . .  31  33
       3.3.4.  ECDSA using curve P-256 DSS and SHA-256 . . . . . . .  31  33
       3.3.5.  JSON Web Signature (JWS) algorithms . . . . . . . . .  32  34
   4.  Including a Message Signature in a Message  . . . . . . . . .  32  34
     4.1.  The 'Signature-Input' HTTP Field  . . . . . . . . . . . .  33  35
     4.2.  The 'Signature' HTTP Field  . . . . . . . . . . . . . . .  33  35
     4.3.  Multiple Signatures . . . . . . . . . . . . . . . . . . .  34  36
   5.  Requesting Signatures . . . . . . . . . . . . . . . . . . . .  36  38
     5.1.  The Accept-Signature Field  . . . . . . . . . . . . . . .  37  39
     5.2.  Processing an Accept-Signature  . . . . . . . . . . . . .  37  40
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38  40
     6.1.  HTTP Signature Algorithms Registry  . . . . . . . . . . .  38  41
       6.1.1.  Registration Template . . . . . . . . . . . . . . . .  39  41
       6.1.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  39  42
     6.2.  HTTP Signature Metadata Parameters Registry . . . . . . .  41  42
       6.2.1.  Registration Template . . . . . . . . . . . . . . . .  41  42
       6.2.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  41  43
     6.3.  HTTP Signature Specialty Component Identifiers
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  41  43
       6.3.1.  Registration Template . . . . . . . . . . . . . . . .  42  44
       6.3.2.  Initial Contents  . . . . . . . . . . . . . . . . . .  42  44
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  43
   8.  References  . .  45
     7.1.  Signature Verification Skipping . . . . . . . . . . . . .  46
     7.2.  Use of TLS  . . . . . . . . . .  44
     8.1.  Normative References . . . . . . . . . . . . .  46
     7.3.  Signature Replay  . . . . .  44
     8.2.  Informative References . . . . . . . . . . . . . . .  47
     7.4.  Insufficient Coverage . .  45
   Appendix A.  Detecting HTTP Message Signatures . . . . . . . . .  46
   Appendix B.  Examples . . . . . . .  47
     7.5.  Cryptography and Signature Collision  . . . . . . . . . .  48
     7.6.  Key Theft . . . . .  46
     B.1.  Example Keys . . . . . . . . . . . . . . . . . . .  48
     7.7.  Modification of Required Message Parameters . . .  46
       B.1.1.  Example Key RSA test . . . .  49
     7.8.  Mismatch of Signature Parameters from Message . . . . . .  49
     7.9.  Multiple Signature Confusion  . . . . . .  46
       B.1.2.  Example RSA PSS Key . . . . . . . .  49
     7.10. Signature Labels  . . . . . . . . .  47
       B.1.3.  Example ECC P-256 Test Key . . . . . . . . . . .  50
     7.11. Symmetric Cryptography  . .  48
       B.1.4.  Example Shared Secret . . . . . . . . . . . . . . .  50
     7.12. Canonicalization Attacks  .  49
     B.2.  Test Cases . . . . . . . . . . . . . . .  50
     7.13. Key Specification Mix-Up  . . . . . . . .  49
       B.2.1.  Minimal Signature Using rsa-pss-sha512 . . . . . . .  50
       B.2.2.  Selective Covered Components using rsa-pss-sha512 .  51
     7.14. HTTP Versions and Component Ambiguity .  50
       B.2.3.  Full Coverage using rsa-pss-sha512 . . . . . . . . .  51
       B.2.4.  Signing a Response using ecdsa-p256-sha256
     7.15. Key and Algorithm Specification Downgrades  . . . . . . .  52
       B.2.5.  Signing a Request using hmac-sha256
     7.16. Parsing Structured Field Values . . . . . . . . . . . . .  52
     7.17. Choosing Message Components . . . . . . . . . . . . . . .  53
     B.3.  TLS-Terminating Proxies
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  53
   Acknowledgements
     8.1.  Identification through Keys . . . . . . . . . . . . . . .  53
     8.2.  Signatures do not provide confidentiality . . . . . . . .  54
     8.3.  Oracles . . . . . . . . . . . . . . . . . . . . . . . . .  54
     8.4.  Required Content  . . . . . . . . . . . . . . . . . . . .  54
   9.  References  . . . . . . . . . . . . . . . . . . . .  55
   Document History . . . . .  54
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  54
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  56
   Authors' Addresses
   Appendix A.  Detecting HTTP Message Signatures  . . . . . . . . .  57
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  57
     B.1.  Example Keys  . . . . . . . . . . . . . . . . . . . . . .  57
       B.1.1.  Example Key RSA test  . . . . . . . . . . . . . . . .  57
       B.1.2.  Example RSA PSS Key . . . . . . . . . . . . . . . . .  58
       B.1.3.  Example ECC P-256 Test Key  . . . . . . . . . . . . .  59

1.  Introduction
       B.1.4.  Example Shared Secret . . . . . . . . . . . . . . . .  60
     B.2.  Test Cases  . . . . . . . . . . . . . . . . . . . . . . .  60
       B.2.1.  Minimal Signature Using rsa-pss-sha512  . . . . . . .  61
       B.2.2.  Selective Covered Components using rsa-pss-sha512 . .  61
       B.2.3.  Full Coverage using rsa-pss-sha512  . . . . . . . . .  62
       B.2.4.  Signing a Response using ecdsa-p256-sha256  . . . . .  63
       B.2.5.  Signing a Request using hmac-sha256 . . . . . . . . .  63
     B.3.  TLS-Terminating Proxies . . . . . . . . . . . . . . . . .  64
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  66
   Document History  . . . . . . . . . . . . . . . . . . . . . . . .  67
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  70

1.  Introduction

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

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

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

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

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

   *  A mechanism for attaching a signature and related metadata to an
      HTTP message. message, and for parsing attached signatures and metadata
      from HTTP messages.

   This document also provides a mechanism for one party a potential verifier to
   signal to
   another party a potential signer that a signature is desired in one or
   more subsequent messages.  This optional negotiation mechanism can be
   used along with opportunistic or application-driven message
   signatures by either party.

1.1.  Requirements Discussion

   HTTP permits and sometimes requires intermediaries to transform
   messages in a variety of ways.  This may result in a recipient
   receiving a message that is not bitwise equivalent to the message
   that was originally sent.  In such a case, the recipient will be
   unable to verify a signature over the raw bytes of the sender's HTTP
   message, as verifying digital signatures or MACs requires both signer
   and verifier to have the exact same signature input.  Since the exact
   raw bytes of the message cannot be relied upon as a reliable source
   of signature input, the signer and verifier must derive the signature
   input from their respective versions of the message, via a mechanism
   that is resilient to safe changes that do not alter the meaning of
   the message.

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

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

1.2.  HTTP Message Transformations

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

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

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

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

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

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

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

1.3.  Safe Transformations

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

   *  Combination of header fields with the same field name.

   *  Reordering of header fields with different names.

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

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

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

   *  Addition or removal of "obs-folds". obs-folds.

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

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

1.4.  Conventions and Terminology

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

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

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

   For brevity, the term "signature" on its own is used in this document
   to refer to both digital signatures (which use asymmetric
   cryptography) and keyed MACs. MACs (which use symmetric cryptography).
   Similarly, the verb "sign" refers to the generation of either a
   digital signature or keyed MAC over a given input string.  The
   qualified term "digital signature" refers specifically to the output
   of an asymmetric cryptographic signing operation.

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

   HTTP Message Signature:

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

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

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

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

   HTTP Message Component Identifier:
      A value that uniquely identifies a specific HTTP Message Component
      in respect to a particular HTTP Message Signature and the HTTP
      Message it applies to.

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

   Covered Components:
      An ordered set of HTTP message component identifiers for fields
      (Section 2.1) and specialty components (Section 2.3) 2.2) that
      indicates the set of message components covered by the signature,
      not including the "@signature-params" @signature-params specialty identifier itself.
      The order of this set is preserved and communicated between the
      signer and verifier to facilitate reconstruction of the signature
      input.

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

   HTTP Message Signature Algorithm:

      A cryptographic algorithm that describes the signing and
      verification process for the signature.  When expressed
      explicitly, the value maps to a string signature, defined in terms of the HTTP
      Signature Algorithms Registry defined
      HTTP_SIGN and HTTP_VERIFY primitives described in this document. Section 3.3.

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

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

   Expiration Time:
      A timestamp representing the point in time at after which the
      signature
      expires, should no longer be accepted by the verifier, as
      asserted by the signer.  A signature's expiration time
      could be undefined, indicating that the signature does not expire
      from the perspective of the signer.

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

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

1.5.  Application of HTTP Message Signatures

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

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

   *  A means of retrieving the key material used to verify the
      signature.  An application will usually use the "keyid" keyid parameter of
      the signature parameters (Section 2.3.1) 2.2.1) and define rules for
      resolving a key from there, though the appropriate key could be
      known from other means.

   *  A means of determining the signature algorithm used to verify the
      signature is appropriate for the key material.  For example, the
      process could use the "alg" alg parameter of the signature parameters
      (Section 2.3.1) 2.2.1) to state the algorithm explicitly, derive the
      algorithm from the key material, or use some pre-configured
      algorithm agreed upon by the signer and verifier.

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

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

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

2.  HTTP Message Components

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

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

   Component identifiers are serialized using the production grammar
   defined by RFC8941, [RFC8941], Section 4 [RFC8941]. 4.  The component identifier itself is
   an "sf-string" sf-string value and MAY define parameters which are included using
   the "parameters" parameters rule.

   component-identifier = sf-string parameters

   Note that this means the value serialization of the component identifier
   itself is encased in double quotes, with parameters following as a semicolon-
   separated
   semicolon-separated list, such as ""cache-control"", ""date"", "cache-control", "date", or ""@signature-
   params"".
   "@signature-params".

   Component identifiers including their parameters MUST NOT be repeated
   within a single list of covered components.

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

   The following sections define component identifier types, their
   parameters, their associated values, and the canonicalization rules
   for their values.  The method for combining component identifiers
   into the signature input is defined in Section 2.4. 2.3.

2.1.  HTTP Fields

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

   Unless overridden by additional parameters and rules, the HTTP field
   value MUST be canonicalized with the following steps:

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

   2.  Strip leading and trailing whitespace from each item in the list.

   3.  Concatenate the list items together, with a single comma "," and
       space " " between each item.

   The resulting string is the canonicalized component value.

2.1.1.  Canonicalized Structured HTTP Fields

   If value of the the HTTP field in question is a structured field
   ([RFC8941]), the component identifier MAY include the "sf" sf parameter.
   If this parameter is included, the HTTP field value MUST be
   canonicalized using the rules specified in Section 4 of RFC8941 [RFC8941].
   For example, this process will replace any optional internal
   whitespace with a single space character.

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

2.1.2.  Canonicalization  HTTP Field Examples

   This section contains

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

   Host: www.example.com
   Date: Tue, 07 Jun 2014 20:51:35 GMT
   X-OWS-Header:   Leading and trailing whitespace.
   X-Obs-Fold-Header: Obsolete
       line folding.
   X-Empty-Header:
   Cache-Control: max-age=60
   Cache-Control:    must-revalidate
   X-Dictionary:  a=1,    b=2;x=1;y=2,   c=(a   b   c)

   The following table shows example shows canonicalized values for these example
   header fields, given that message:

        +=====================+==================================+
        | Header Field        | Canonicalized Value              |
        +=====================+==================================+
        | "cache-control"     | presented using the signature input string format
   discussed in Section 2.3:

   "cache-control": max-age=60, must-revalidate      |
        +---------------------+----------------------------------+
        | "date"              | must-revalidate|
   "date": Tue, 07 Jun 2014 20:51:35 GMT    |
        +---------------------+----------------------------------+
        | "host"              | www.example.com                  |
        +---------------------+----------------------------------+
        | "x-empty-header"    |                                  |
        +---------------------+----------------------------------+
        | "x-obs-fold-header" | GMT|
   "host": www.example.com|
   "x-empty-header":
   "x-obs-fold-header": Obsolete line folding.           |
        +---------------------+----------------------------------+
        | "x-ows-header"      | Leading
   "x-ows-header":Leading and trailing whitespace. |
        +---------------------+----------------------------------+
        | "x-dictionary"      |
   "x-dictionary": a=1,    b=2;x=1;y=2,   c=(a   b   c)      |
        +---------------------+----------------------------------+
        | "x-dictionary";sf   |
   "x-dictionary";sf: a=1, b=2;x=1;y=2, c=(a b c)      |
        +---------------------+----------------------------------+

             Table 1: Non-normative examples of header field
                            canonicalization.

2.2.

2.1.3.  Dictionary Structured Field Members

   An individual member in the value of a Dictionary Structured Field is
   identified by using the parameter "key" on the component identifier
   for the field.  The value of this parameter is a the key being
   identified, without any parameters present on that key in to indicate the
   original dictionary. member key as
   an sf-string value.

   An individual member in the value of a Dictionary Structured Field is
   canonicalized by applying the serialization algorithm described in
   Section 4.1.2 of RFC8941 [RFC8941] on a Dictionary containing only that item.

2.2.1.  Canonicalization Examples

   This section contains non-normative examples of canonicalized values

   Each parameterized key for a given field MUST NOT appear more than
   once in the signature input.  Parameterized keys MAY appear in any
   order.

   Following are non-normative examples of canonicalized values for
   Dictionary Structured Field Members given the following example
   header field, whose value is known to be a Dictionary:

   X-Dictionary:  a=1, b=2;x=1;y=2, c=(a b c)

   The following table shows example shows canonicalized values for different
   component identifiers, given that field:

                +======================+=================+
                | Component Identifier | Component Value |
                +======================+=================+
                | "x-dictionary";key=a | identifiers of this field, presented using the signature
   input string format discussed in Section 2.3:

   "x-dictionary";key="a": 1               |
                +----------------------+-----------------+
                | "x-dictionary";key=b |
   "x-dictionary";key="b": 2;x=1;y=2       |
                +----------------------+-----------------+
                | "x-dictionary";key=c |
   "x-dictionary";key="c": (a, b, c)       |
                +----------------------+-----------------+

                    Table 2: Non-normative examples of
                   Dictionary member canonicalization.

2.3.

2.2.  Specialty Components

   Message components not found in an HTTP field can be included in the
   signature input by defining a component identifier and the
   canonicalization method for its component value.

   To differentiate specialty component identifiers from HTTP fields,
   specialty component identifiers MUST start with the "at" "@" @ character.
   This specification defines the following specialty component
   identifiers:

   @signature-params  The signature metadata parameters for this
      signature.  (Section 2.3.1) 2.2.1)

   @method  The method used for a request.  (Section 2.3.2) 2.2.2)

   @target-uri  The full target URI for a request.  (Section 2.3.3) 2.2.3)

   @authority  The authority of the target URI for a request.
      (Section 2.3.4) 2.2.4)

   @scheme  The scheme of the target URI for a request.  (Section 2.3.5) 2.2.5)

   @request-target  The request target.  (Section 2.3.6) 2.2.6)

   @path  The absolute path portion of the target URI for a request.

      (Section 2.3.7) 2.2.7)

   @query  The query portion of the target URI for a request.
      (Section 2.3.8) 2.2.8)

   @query-params  The parsed query parameters of the target URI for a
      request.  (Section 2.3.9) 2.2.9)

   @status  The status code for a response.  (Section 2.3.10). 2.2.10).

   @request-response  A signature from a request message that resulted
      in this response message.  (Section 2.3.11) 2.2.11)

   Additional specialty component identifiers MAY be defined and
   registered in the HTTP Signatures Specialty Component Identifier
   Registry.  (Section 6.3)

2.3.1.

   Specialty components can be applied in one or more of three targets:

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

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

   related-response:  Values derived from an HTTP request message and
      results applied to the HTTP response message that is responding to
      that specific request.

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

2.2.1.  Signature Parameters

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

   The signature parameters component identifier is "@signature-params". @signature-params.
   This message component's value is REQUIRED as part of the signature
   input string (Section 2.3) but the component identifier MUST NOT be
   enumerated within the set of covered components itself.

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

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

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

   *  "nonce":  nonce: A random unique value generated for this signature. signature as an
      sf-string value.

   *  "alg":  alg: The HTTP message signature algorithm from the HTTP Message
      Signature Algorithm Registry, as an "sf-string" sf-string value.

   *  "keyid":  keyid: The identifier for the key material as an "sf-string" sf-string value.

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

   The signature parameters component value is serialized as a
   parameterized inner list using the rules in Section 4 of RFC8941 [RFC8941] as
   follows:

   1.  Let the output be an empty string.

   2.  Determine an order for the component identifiers of the covered
       components.
       components, not including the @signature-params component
       identifier itself.  Once this order is chosen, it cannot be
       changed.  This order MUST be the same order as used in creating
       the signature input (Section 2.4). 2.3).

   3.  Serialize the component identifiers of the covered components,
       including all parameters, as an ordered "inner-list" inner-list according to
       Section 4.1.1.1 of RFC8941 [RFC8941] and append this to the output.

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

   5.  Append the parameters to the "inner-list" inner-list in the chosen order
       according to Section 4.1.1.2 of RFC8941 [RFC8941], skipping parameters
       that are not available or not used for this message signature.

   6.  The output contains the signature parameters component value.

   Note that the "inner-list" inner-list serialization is used for the covered
   component value instead of the "sf-list" sf-list serialization in order to
   facilitate this value's inclusion in message fields such as the
   "Signature-Input"
   Signature-Input field's dictionary, as discussed in Section 4.1.

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

   NOTE: '\' line wrapping per RFC 8792

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

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

2.3.2.

2.2.2.  Method

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

   For example, the following request message:

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

   Would result in the following "@method" @method value:

   "@method": POST

   If used in a response message, related-response, the "@method" @method component identifier
   refers to the associated component value of the request that
   triggered the response message being signed.

2.3.3.

2.2.3.  Target URI

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

   For example, the following message sent over HTTPS:

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

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

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

   If used in a response message, related-response, the "@target-uri" @target-uri component identifier
   refers to the associated component value of the request that
   triggered the response message being signed.

2.3.4.

2.2.4.  Authority

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

   For example, the following request message:

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

   Would result in the following "@authority" @authority component value:

   "@authority": www.example.com

   If used in a response message, related-response, the "@authority" @authority component identifier
   refers to the associated component value of the request that
   triggered the response message being signed.

2.3.5.

2.2.5.  Scheme

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

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

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

   Would result in the following "@scheme" @scheme value:

   "@scheme": http

   If used in a response message, related-response, the "@scheme" @scheme component identifier
   refers to the associated component value of the request that
   triggered the response message being signed.

2.3.6.

2.2.6.  Request Target

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   "@request-target": *

   If used in a response message, related-response, the "@request-target" @request-target component
   identifier refers to the associated component value of the request
   that triggered the response message being signed.

2.3.7.

2.2.7.  Path

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

   For example, the following request message:

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

   Would result in the following "@path" @path value:

   "@path": /path

   If used in a response message, related-response, the "@path" @path identifier refers to the
   associated component value of the request that triggered the response
   message being signed.

2.3.8.

2.2.8.  Query

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

   For example, the following request message:

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

   Would result in the following "@query" @query value:

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

   The following request message:

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

   Would result in the following "@query" @query value:

   "@query": ?queryString

   If used in a response message, related-response, the "@query" @query component identifier refers
   to the associated component value of the request that triggered the
   response message being signed.

2.3.9.

2.2.9.  Query Parameters

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

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

   If a parameter name occurs multiple times in a request, all parameter
   values of that name MUST be included in separate signature input
   lines in the order in which the parameters occur in the target URI.

   For example for the following request:

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

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

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

   If used in a response message, related-response, the "@query-params" @query-params component identifier
   refers to the associated component value of the request that
   triggered the response message being signed.

2.3.10.

2.2.10.  Status Code

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

   For example, the following response message:

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

   Would result in the following "@status" @status value:

   "@status": 200

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

2.3.11.

2.2.11.  Request-Response Signature Binding

   When a signed request message results in a signed response message,
   the "@request-response" @request-response component identifier can be used to
   cryptographically link the request and the response to each other by
   including the identified request signature value in the response's
   signature input without copying the value of the request's signature
   to the response directly.  This component identifier has a single
   REQUIRED parameter:

   "key"

   key  Identifies which signature from the response to sign.

   The component value is the "sf-binary" sf-binary representation of the signature
   value of the referenced request identified by the "key" key parameter.

   For example, when serving this signed request:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?param=value&pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Signature-Input: sig1=("@authority" "content-type")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature: sig1=:KuhJjsOKCiISnKHh2rln5ZNIrkRvue0DSu5rif3g7ckTbbX7C4\
     Jp3bcGmi8zZsFRURSQTcjbHdJtN8ZXlRptLOPGHkUa/3Qov79gBeqvHNUO4bhI27p\
     4WzD1bJDG9+6ml3gkrs7rOvMtROObPuc78A95fa4+skS/t2T7OjkfsHAm/enxf1fA\
     wkk15xj0n6kmriwZfgUlOqyff0XLwuH4XFvZ+ZTyxYNoo2+EfFg4NVfqtSJch2WDY\
     7n/qmhZOzMfyHlggWYFnDpyP27VrzQCQg8rM1Crp6MrwGLa94v6qP8pq0sQVq2DLt\
     4NJSoRRqXTvqlWIRnexmcKXjQFVz6YSA==:

   {"hello": "world"}

   This would result in the following unsigned response message:

   HTTP/1.1 200 OK
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62

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

   The server signs the response with its own key and includes the
   signature of "sig1" sig1 from the request in the covered components of the
   response.  The signature input string for this example is:

   NOTE: '\' line wrapping per RFC 8792

   "content-type": application/json
   "content-length": 62
   "@status": 200
   "@request-response";key="sig1": :KuhJjsOKCiISnKHh2rln5ZNIrkRvue0DSu\
     5rif3g7ckTbbX7C4Jp3bcGmi8zZsFRURSQTcjbHdJtN8ZXlRptLOPGHkUa/3Qov79\
     gBeqvHNUO4bhI27p4WzD1bJDG9+6ml3gkrs7rOvMtROObPuc78A95fa4+skS/t2T7\
     OjkfsHAm/enxf1fAwkk15xj0n6kmriwZfgUlOqyff0XLwuH4XFvZ+ZTyxYNoo2+Ef\
     Fg4NVfqtSJch2WDY7n/qmhZOzMfyHlggWYFnDpyP27VrzQCQg8rM1Crp6MrwGLa94\
     v6qP8pq0sQVq2DLt4NJSoRRqXTvqlWIRnexmcKXjQFVz6YSA==:
   "@signature-params": ("content-type" "content-length" "@status" \
     "@request-response";key="sig1");created=1618884475\
     ;keyid="test-key-ecc-p256"

   The signed response message is:

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Content-Length: 62
   Signature-Input: sig1=("content-type" "content-length" "@status" \
     "@request-response";key="sig1");created=1618884475\
     ;keyid="test-key-ecc-p256"
   Signature: sig1=:crVqK54rxvdx0j7qnt2RL1oQSf+o21S/6Uk2hyFpoIfOT0q+Hv\
     msYAXUXzo0Wn8NFWh/OjWQOXHAQdVnTk87Pw==:

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

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

   The "@request-response" @request-response component identifier MUST NOT be used in a
   request message.

2.4.

2.3.  Creating the Signature Input String

   The signature input is a US-ASCII string containing the canonicalized
   HTTP message components covered by the signature.  The input to the
   signature creation algorithm is the list of covered component
   identifiers and their associated values, along with an additional
   signature parameters.  To create the signature input string, the
   signer or verifier concatenates together entries for each identifier
   in the signature's covered components (including their parameters)
   using the following algorithm:

   1.  Let the output be an empty string.

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

       1.  Append the component identifier for the covered component
           serialized according to the "component-identifier" component-identifier rule.

       2.  Append a single colon "":"" :

       3.  Append a single space "" "" " "

       4.  Append the covered component's canonicalized component value,
           as defined by the HTTP message component type.  (Section 2.1
           and Section 2.3) 2.2)

       5.  Append a single newline ""\\n"" \n

   3.  Append the signature parameters component (Section 2.3.1) 2.2.1) as
       follows:

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

       2.  Append a single colon "":"" :

       3.  Append a single space "" "" " "

       4.  Append the signature parameters' canonicalized component
           value as defined in Section 2.3.1 2.2.1

   4.  Return the output string.

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

   *  The signer or verifier does not understand the component
      identifier.

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

   *  The component identifier indicates that a structured field
      serialization is used, but the field in question is known to not
      be a Dictionary structured field or the type of structured field is not known
      to the verifier.

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

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

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

   GET /foo HTTP/1.1
   Host: example.org
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   X-Example: Example header
           with some whitespace.
   X-Empty-Header:
   Cache-Control: max-age=60
   Cache-Control: must-revalidate

   The covered components consist of the "@method", "@path", @method, @path, and
   "@authority" @authority
   specialty component identifiers followed by the "Cache-
   Control", "X-Empty-Header", "X-Example" Cache-Control, X-
   Empty-Header, X-Example HTTP headers, in order.  The signature
   parameters consist of a creation timestamp is "1618884475" 1618884475 and the key
   identifier is "test-key-rsa-pss". test-key-rsa-pss.  The signature input string for this
   message with these parameters is:

   NOTE: '\' line wrapping per RFC 8792

   "@method": GET
   "@path": /foo
   "@authority": example.org
   "cache-control": max-age=60, must-revalidate
   "x-empty-header":
   "x-example": Example header with some whitespace.
   "@signature-params": ("@method" "@path" "@authority" \
     "cache-control" "x-empty-header" "x-example");created=1618884475\
     ;keyid="test-key-rsa-pss"

              Figure 1: Non-normative example Signature Input

3.  HTTP Message Signatures

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

3.1.  Creating a Signature

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

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

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

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

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

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

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

       *  Each covered component identifier MUST be either an HTTP field
          in the message Section 2.1 or a specialty component identifier
          listed in Section 2.3 2.2 or its associated registry.

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

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

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

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

   5.  The signer creates the signature input string based on these
       signature parameters.  (Section 2.4) 2.3)

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

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

   For example, given the HTTP message and signature parameters in the
   example in Section 2.4, 2.3, the example signature input string when is signed
   with the "test-key-rsa-pss" test-key-rsa-pss key in Appendix B.1.2 gives and the RSA PSS
   algorithm described in Section 3.3.1, giving the following message
   signature output value, encoded in Base64:

   NOTE: '\' line wrapping per RFC 8792

   P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo1RSHi+oEF1FuX6O29\
   d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCiHzC87qmSQjvu1CFyFuWSj\
   dGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW84jS8gyarxAiWI97mPXU+OVM64\
   +HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53r58RmpZ+J9eKR2CD6IJQvacn5A4Ix\
   5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCVRj05NrxABNFv3r5S9IXf2fYJK+eyW4AiG\
   VMvMcOg==

              Figure 2: Non-normative example signature value

3.2.  Verifying a Signature

   A verifier processes a signature and its associated

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

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

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

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

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

   2.  Parse the values of the chosen "Signature-Input" Signature-Input field as a
       parameterized structured field inner list item (inner-list) to
       get the signature parameters for the signature to be verified.

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

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

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

   6.  Determine the algorithm to apply for verification:

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

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

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

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

   7.  Use the received HTTP message and the signature's metadata to
       recreate the signature input, using the process described in
       Section 2.4. 2.3.  The value of the "@signature-params" @signature-params input is the
       value of the "SignatureInput" Signature-Input field for this signature serialized
       according to the rules described in Section 2.3.1, 2.2.1, not including
       the signature's label from the "Signature-Input" Signature-Input field.

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

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

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

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

   NOTE: '\' line wrapping per RFC 8792

   GET /foo HTTP/1.1
   Host: example.org
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   X-Example: Example header
           with some whitespace.
   X-Empty-Header:
   Cache-Control: max-age=60
   Cache-Control: must-revalidate
   Signature-Input: sig1=("@method" "@path" "@authority" \
     "cache-control" "x-empty-header" "x-example");created=1618884475\
     ;keyid="test-key-rsa-pss"
   Signature: sig1=:P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo1\
     RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCiHzC8\
     7qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW84jS8gya\
     rxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53r58RmpZ+J9\
     eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCVRj05NrxABNFv3\
     r5S9IXf2fYJK+eyW4AiGVMvMcOg==:

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

3.2.1.  Enforcing Application Requirements

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

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

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

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

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

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

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

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

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

   *  Enforcing uniqueness of a "nonce" nonce value.

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

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

3.3.  Signature Algorithm Methods

   HTTP Message signatures MAY use any cryptographic digital signature
   or MAC method that is appropriate for the key material, environment,
   and needs of the signer and verifier.  All signatures are generated
   from and verified against the byte values of the signature input
   string defined in Section 2.4. 2.3.

   Each signature algorithm method takes as its input the signature
   input string as a set of byte values ("I"), (I), the signing key material
   ("Ks"),
   (Ks), and outputs the signature output as a set of byte values
   ("S"): (S):

   HTTP_SIGN (I, Ks)  ->  S

   Each verification algorithm method takes as its input the
   recalculated signature input string as a set of byte values ("I"), (I), the
   verification key material ("Kv"), (Kv), and the presented signature to be
   verified as a set of byte values ("S") (S) and outputs the verification
   result ("V") (V) as a boolean:

   HTTP_VERIFY (I, Kv, S) -> V
   This section contains several common algorithm methods.  The method
   to use can be communicated through the algorithm signature parameter
   defined in Section 2.3.1, 2.2.1, by reference to the key material, or
   through mutual agreement between the signer and verifier.

3.3.1.  RSASSA-PSS using SHA-512

   To sign using this algorithm, the signer applies the "RSASSA-PSS-SIGN RSASSA-PSS-SIGN
   (K, M)" M) function [RFC8017] with the signer's private signing key
   ("K") (K)
   and the signature input string ("M") (M) (Section 2.4). 2.3).  The mask
   generation function is "MGF1" MGF1 as specified in [RFC8017] with a hash
   function of SHA-512 [RFC6234].  The salt length ("sLen") (sLen) is 64 bytes.
   The hash function ("Hash") (Hash) SHA-512 [RFC6234] is applied to the
   signature input string to create the digest content to which the
   digital signature is applied.  The resulting signed content byte
   array ("S") (S) is the HTTP message signature output used in Section 3.1.

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

3.3.2.  RSASSA-PKCS1-v1_5 using SHA-256

   To sign using

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

3.3.2.  RSASSA-PKCS1-v1_5 using SHA-256

   To sign using this algorithm, the signer applies the "RSASSA- RSASSA-
   PKCS1-V1_5-SIGN (K, M)" M) function [RFC8017] with the signer's private
   signing key ("K") (K) and the signature input string ("M") (M) (Section 2.4). 2.3).
   The hash SHA-256 [RFC6234] is applied to the signature input string
   to create the digest content to which the digital signature is
   applied.  The resulting signed content byte array ("S") (S) is the HTTP
   message signature output used in Section 3.1.

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

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

3.3.3.  HMAC using SHA-256

   To sign and verify using this algorithm, the signer applies the
   "HMAC" HMAC
   function [RFC2104] with the shared signing key ("K") (K) and the signature
   input string ("text") (text) (Section 2.4). 2.3).  The hash function SHA-256
   [RFC6234] is applied to the signature input string to create the
   digest content to which the HMAC is applied, giving the signature
   result.

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

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

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

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

   To sign using this algorithm, the signer applies the "ECDSA" ECDSA algorithm
   [FIPS186-4] using curve P-256 with the signer's private signing key
   and the signature input string (Section 2.4). 2.3).  The hash SHA-256
   [RFC6234] is applied to the signature input string to create the
   digest content to which the digital signature is applied.  The
   resulting signed content byte array is the HTTP message signature
   output used in Section 3.1.

   To verify using this algorithm, the verifier applies the "ECDSA" ECDSA
   algorithm [FIPS186-4] using the public key portion of the
   verification key material and the signature input string re-created
   as described in Section 3.2.  The hash function SHA-256 [RFC6234] is
   applied to the signature input string to create the digest content to
   which the verification function is applied.  The verifier extracts
   the HTTP message signature to be verified ("S") (S) as described in
   Section 3.2.  The results of the verification function are compared
   to the http message signature to determine if the signature presented
   is valid.

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

3.3.5.  JSON Web Signature (JWS) algorithms

   If the signing algorithm is a JOSE signing algorithm from the JSON
   Web Signature and Encryption Algorithms Registry established by
   [RFC7518], the JWS algorithm definition determines the signature and
   hashing algorithms to apply for both signing and verification.  There
   is no use of the explicit "alg" signature parameter when using JOSE
   signing algorithms.

   For both signing and verification, the HTTP messages signature input
   string (Section 2.4) 2.3) is used as the entire "JWS Signing Input".  The
   JOSE Header defined in [RFC7517] is not used, and the signature input
   string is not first encoded in Base64 before applying the algorithm.
   The output of the JWS signature is taken as a byte array prior to the
   Base64url encoding used in JOSE.

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

   There is no use of the explicit alg signature parameter when using
   JOSE signing algorithms, as they can be signaled using JSON Web Keys
   or other mechanisms.

4.  Including a Message Signature in a Message

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

   An HTTP message signature MUST use both fields containing the same
   labels: the "Signature" Signature HTTP field contains the signature value, while
   the "Signature-Input" Signature-Input HTTP field identifies the covered components and
   parameters that describe how the signature was generated.  Each field
   contains labeled values and MAY contain multiple labeled values,
   where the labels determine the correlation between the "Signature" Signature and "Signature-Input"
   Signature-Input fields.

4.1.  The 'Signature-Input' HTTP Field

   The "Signature-Input" Signature-Input HTTP field is a Dictionary Structured Field
   [RFC8941] containing the metadata for one or more message signatures
   generated from components within the HTTP message.  Each member
   describes a single message signature.  The member's name is an
   identifier that uniquely identifies the message signature within the
   context of the HTTP message.  The member's value is the serialization
   of the covered components including all signature metadata
   parameters, using the serialization process defined in Section 2.3.1. 2.2.1.

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("@method" "@target-uri" "host" "date" \
     "cache-control" "x-empty-header" "x-example");created=1618884475\
     ;keyid="test-key-rsa-pss"

   To facilitate signature validation, the "Signature-Input" Signature-Input field value
   MUST contain the same serialized value used in generating the
   signature input string's "@signature-params" @signature-params value.

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

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

4.2.  The 'Signature' HTTP Field

   The "Signature" Signature HTTP field is a Dictionary Structured field [RFC8941]
   containing one or more message signatures generated from components
   within the HTTP message.  Each member's name is a signature
   identifier that is present as a member name in the "Signature-Input" Signature-Input
   Structured field within the HTTP message.  Each member's value is a
   Byte Sequence containing the signature value for the message
   signature identified by the member name.  Any member in the
   "Signature" Signature
   HTTP field that does not have a corresponding member in the HTTP
   message's "Signature-Input" Signature-Input HTTP field MUST be ignored.

   NOTE: '\' line wrapping per RFC 8792

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

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

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

4.3.  Multiple Signatures

   Multiple distinct signatures MAY be included in a single message.
   Each distinct signature MUST have a unique label.  Since "Signature-Input" Signature-
   Input and "Signature" Signature are both defined as Dictionary Structured fields,
   they can be used to include multiple signatures within the same HTTP
   message by using distinct signature labels.  These multiple
   signatures could be added all by the same signer or could come from
   several different signers.  For example, a signer may include
   multiple signatures signing the same message components with
   different keys or algorithms to support verifiers with different
   capabilities, or a reverse proxy may include information about the
   client in fields when forwarding the request to a service host,
   including a signature over the client's original signature values.

   The following is a non-normative example of header fields a reverse
   proxy sets in addition to the examples in the previous sections.

   NOTE: '\' line wrapping per RFC 8792

   Forwarded: for=192.0.2.123
   Signature-Input: sig1=("@method" "@path" "@authority" \
       "cache-control" "x-empty-header" "x-example")\
       ;created=1618884475;keyid="test-key-rsa-pss"
   Signature: sig1=:P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo\
       1RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCi\
       HzC87qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW8\
       4jS8gyarxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53\
       r58RmpZ+J9eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCV\
       Rj05NrxABNFv3r5S9IXf2fYJK+eyW4AiGVMvMcOg==:

   The client's request includes a signature value under the label
   "sig1", sig1,
   which the proxy signs in addition to the "Forwarded" Forwarded header defined in
   [RFC7239].  Note that since the client's signature already covers the
   client's "Signature-Input" Signature-Input value for "sig1", sig1, this value is transitively
   covered by the proxy's signature and need not be added explicitly.
   This results in a signature input string of:

   NOTE: '\' line wrapping per RFC 8792

   "signature";key="sig1": :P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP\
     4uKwxyJo1RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9Gl\
     yntiCiHzC87qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyo\
     yZW84jS8gyarxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg\
     53r58RmpZ+J9eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCV\
     Rj05NrxABNFv3r5S9IXf2fYJK+eyW4AiGVMvMcOg==:
   "forwarded": for=192.0.2.123
   "@signature-params": ("signature";key="sig1" "forwarded")\
     ;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"

   And a signature output value of:

   NOTE: '\' line wrapping per RFC 8792

   cjGvZwbsq9JwexP9TIvdLiivxqLINwp/ybAc19KOSQuLvtmMt3EnZxNiE+797dXK2cj\
   PPUFqoZxO8WWx1SnKhAU9SiXBr99NTXRmA1qGBjqus/1Yxwr8keB8xzFt4inv3J3zP0\
   k6TlLkRJstkVnNjuhRIUA/ZQCo8jDYAl4zWJJjppy6Gd1XSg03iUa0sju1yj6rcKbMA\
   BBuzhUz4G0u1hZkIGbQprCnk/FOsqZHpwaWvY8P3hmcDHkNaavcokmq+3EBDCQTzgwL\
   qfDmV0vLCXtDda6CNO2Zyum/pMGboCnQn/VkQ+j8kSydKoFg6EbVuGbrQijth6I0dDX\
   2/HYcJg==
   These values are added to the HTTP request message by the proxy.  The
   original signature is included under the identifier "sig1", sig1, and the
   reverse proxy's signature is included under the label "proxy_sig". proxy_sig.  The
   proxy uses the key "test-key-rsa" test-key-rsa to create its signature using the "rsa-v1_5-sha256"
   rsa-v1_5-sha256 signature algorithm, while the client's original
   signature was made using the key id of "test-key-rsa-pss" test-key-rsa-pss and an RSA
   PSS signature algorithm.

   NOTE: '\' line wrapping per RFC 8792

   Forwarded: for=192.0.2.123
   Signature-Input: sig1=("@method" "@path" "@authority" \
       "cache-control" "x-empty-header" "x-example")\
       ;created=1618884475;keyid="test-key-rsa-pss", \
     proxy_sig=("signature";key="sig1" "forwarded")\
       ;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"
   Signature: sig1=:P0wLUszWQjoi54udOtydf9IWTfNhy+r53jGFj9XZuP4uKwxyJo\
       1RSHi+oEF1FuX6O29d+lbxwwBao1BAgadijW+7O/PyezlTnqAOVPWx9GlyntiCi\
       HzC87qmSQjvu1CFyFuWSjdGa3qLYYlNm7pVaJFalQiKWnUaqfT4LyttaXyoyZW8\
       4jS8gyarxAiWI97mPXU+OVM64+HVBHmnEsS+lTeIsEQo36T3NFf2CujWARPQg53\
       r58RmpZ+J9eKR2CD6IJQvacn5A4Ix5BUAVGqlyp8JYm+S/CWJi31PNUjRRCusCV\
       Rj05NrxABNFv3r5S9IXf2fYJK+eyW4AiGVMvMcOg==:, \
     proxy_sig=:cjGvZwbsq9JwexP9TIvdLiivxqLINwp/ybAc19KOSQuLvtmMt3EnZx\
       NiE+797dXK2cjPPUFqoZxO8WWx1SnKhAU9SiXBr99NTXRmA1qGBjqus/1Yxwr8k\
       eB8xzFt4inv3J3zP0k6TlLkRJstkVnNjuhRIUA/ZQCo8jDYAl4zWJJjppy6Gd1X\
       Sg03iUa0sju1yj6rcKbMABBuzhUz4G0u1hZkIGbQprCnk/FOsqZHpwaWvY8P3hm\
       cDHkNaavcokmq+3EBDCQTzgwLqfDmV0vLCXtDda6CNO2Zyum/pMGboCnQn/VkQ+\
       j8kSydKoFg6EbVuGbrQijth6I0dDX2/HYcJg==:

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

5.  Requesting Signatures

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

   The message to which the requested signature is applied is known as
   the "target message".  When the "Accept-Signature" Accept-Signature field is sent in an
   HTTP Request message, the field indicates that the client desires the
   server to sign the response using the identified parameters and the
   target message is the response to this request.  All responses from
   resources that support such signature negotiation SHOULD either be
   uncacheable or contain a "Vary" Vary header field that lists "Accept-
   Signature", Accept-
   Signature, in order to prevent a cache from returning a response with
   a signature intended for a different request.

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

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

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

5.1.  The Accept-Signature Field

   The "Accept-Signature" Accept-Signature HTTP header field is a Dictionary Structured
   field [RFC8941] containing the metadata for one or more requested
   message signatures to be generated from message components of the
   target HTTP message.  Each member describes a single message
   signature.  The member's name is an identifier that uniquely
   identifies the requested message signature within the context of the
   target HTTP message.  The member's value is the serialization of the
   desired covered components of the target message, including any
   allowed signature metadata parameters, using the serialization
   process defined in Section 2.3.1. 2.2.1.

   NOTE: '\' line wrapping per RFC 8792

   Accept-Signature: sig1=("@method" "@target-uri" "host" "date" \
     "cache-control" "x-empty-header" "x-example")\
     ;keyid="test-key-rsa-pss"

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

5.2.  Processing an Accept-Signature

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

   1.  Parse the field value as a Dictionary

   2.  For each member of the dictionary:

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

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

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

       4.  Select any additional parameters necessary for completing the
           signature

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

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

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

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

6.  IANA Considerations

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

6.1.  HTTP Signature Algorithms Registry

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

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

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

6.1.1.  Registration Template

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

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

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

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

6.1.2.  Initial Contents

6.1.2.1.  rsa-pss-sha512

    +===================+========+===================+===============+
    | Algorithm Name:
      "rsa-pss-sha512"

   Status: Name    | Status | Description       | Specification |
    |                   |        |                   | document(s)   |
    +===================+========+===================+===============+
    | rsa-pss-sha512    | Active

   Definition: | RSASSA-PSS using SHA-256

   Specification document(s):  | [[This        |
    |                   |        | SHA-512           | document]],   |
    |                   |        |                   | Section 3.3.1

6.1.2.2. |
    +-------------------+--------+-------------------+---------------+
    | rsa-v1_5-sha256

   Algorithm Name:
      "rsa-v1_5-sha256"

   Status:   | Active

   Description: | RSASSA-PKCS1-v1_5 | [[This        |
    |                   |        | using SHA-256

   Specification document(s):
      [[This     | document]],   |
    |                   |        |                   | Section 3.3.2

6.1.2.3. |
    +-------------------+--------+-------------------+---------------+
    | hmac-sha256

   Algorithm Name:
      "hmac-sha256"

   Status:       | Active

   Description: | HMAC using SHA-256

   Specification document(s):        | [[This        |
    |                   |        | SHA-256           | document]],   |
    |                   |        |                   | Section 3.3.3

6.1.2.4. |
    +-------------------+--------+-------------------+---------------+
    | ecdsa-p256-sha256

   Algorithm Name:
      "ecdsa-p256-sha256"

   Status: | Active

   Description: | ECDSA using curve | [[This        |
    |                   |        | P-256 DSS and SHA-256

   Specification document(s):
      [[This     | document]],   |
    |                   |        | SHA-256           | Section 3.3.4 |
    +-------------------+--------+-------------------+---------------+

                                 Table 1

6.2.  HTTP Signature Metadata Parameters Registry

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

6.2.1.  Registration Template

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

   Description:

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

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

6.2.2.  Initial Contents

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

           +=========+========+================================+

      +=========+===============================+==================+
      | Name    | Status Description                   | Specification    |
      |         |                               | Reference(s) document(s)      |
           +=========+========+================================+
      +=========+===============================+==================+
      | alg     | Active Explicitly declared signature | Section 2.3.1 2.2.1 of |
      |         | algorithm                     | this document    |
           +---------+--------+--------------------------------+
      +---------+-------------------------------+------------------+
      | created | Active Timestamp of signature        | Section 2.3.1 2.2.1 of |
      |         | creation                      | this document    |
           +---------+--------+--------------------------------+
      +---------+-------------------------------+------------------+
      | expires | Active Timestamp of proposed         | Section 2.3.1 2.2.1 of |
      |         | signature expiration          | this document    |
           +---------+--------+--------------------------------+
      +---------+-------------------------------+------------------+
      | keyid   | Active Key identifier for the        | Section 2.3.1 2.2.1 of |
      |         | signing and verification keys | this document    |
           +---------+--------+--------------------------------+
      |         | used to create this signature |                  |
      +---------+-------------------------------+------------------+
      | nonce   | Active A single-use nonce value      | Section 2.3.1 2.2.1 of |
      |         |                               | this document    |
           +---------+--------+--------------------------------+
      +---------+-------------------------------+------------------+

         Table 3: 2: Initial contents of the HTTP Signature Metadata
                           Parameters Registry.

6.3.  HTTP Signature Specialty Component Identifiers Registry

   This document defines a method for canonicalizing HTTP message
   components, including components that can be generated derived from the context
   of the HTTP message outside of the HTTP fields.  These components are
   identified by a unique string, known as the component identifier.  IANA is asked to create and
   Component identifiers for specialty components always start with the
   "@" (at) symbol to distinguish them from HTTP header fields.  IANA is
   asked to create and maintain a new registry typed "HTTP Signature
   Specialty Component Identifiers" to record and maintain the set of
   non-field component identifiers and the methods to produce their
   associated component values.  Initial values for this registry are
   given in Section 6.3.2.  Future assignments and modifications to
   existing assignments are to be made through the Expert Review
   registration policy [RFC8126] and shall follow the template presented
   in Section 6.3.1.

6.3.1.  Registration Template

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

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

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

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

6.3.2.  Initial Contents

   The table below contains the initial contents of the HTTP Signature
   Specialty Component Identifiers Registry.

   +===================+========+===================+==================+

   +===================+========+==================+==================+
   | Name Identifier        | Status | Target           | Reference Specification    |
   |                   |        |
   +===================+========+===================+==================+                  | document(s)      |
   +===================+========+==================+==================+
   | @signature-params | Active | Request,         | Section 2.3.1 2.2.1 of |
   |                   |        | Response         | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @method           | Active | Request,         | Section 2.3.2 2.2.2 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @authority        | Active | Request,         | Section 2.3.4 2.2.4 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @scheme           | Active | Request,         | Section 2.3.5 2.2.5 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @target-uri       | Active | Request,         | Section 2.3.3 2.2.3 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @request-target   | Active | Request,         | Section 2.3.6 2.2.6 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @path             | Active | Request,         | Section 2.3.7 2.2.7 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @query            | Active | Request,         | Section 2.3.8 2.2.8 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @query-params     | Active | Request,         | Section 2.3.9 2.2.9 of |
   |                   |        | Related-Response | this document    |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @status           | Active | Response         | Section 2.3.10 2.2.10   |
   |                   |        |                  | of this document |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+
   | @request-response | Active | Related-Response | Section 2.3.11 2.2.11   |
   |                   |        |                  | of this document |                  |
   +-------------------+--------+-------------------+------------------+
   +-------------------+--------+------------------+------------------+

   Table 4: 3: Initial contents of the HTTP Signature Specialty Component
                          Identifiers Registry.

7.  Security Considerations

   (( TODO:

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

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

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

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

   *  the signature is within expected time boundaries

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

7.1.  Signature Verification Skipping

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

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

7.2.  Use of TLS

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

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

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

7.3.  Signature Replay

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

   To counteract these kinds of attacks, it's first important for the
   signer to cover sufficient portions of the message to differentiate
   it from other messages.  In addition, the signature can use the nonce
   signature parameter to provide a per-message unique value to allow
   the verifier to detect replay of the signature itself if a nonce
   value is repeated.  Furthermore, the signer can provide a timestamp
   for when the signature was created and a time at which the signer
   considers the signature to be invalid, limiting the utility of a
   captured signature value.

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

7.4.  Insufficient Coverage

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

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

7.5.  Cryptography and Signature Collision

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

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

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

   While it is possible for an attacker to substitute the signature
   parameters value or the signature value separately, the signature
   input generation algorithm (Section 2.3) always covers the signature
   parameters as the final value in the input string using a
   deterministic serialization method.  This step strongly binds the
   signature input with the signature value in a way that makes it much
   more difficult for an attacker to perform a partial substitution on
   the signature inputs.

7.6.  Key Theft

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

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

7.7.  Modification of Required Message Parameters

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

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

7.8.  Mismatch of Signature Parameters from Message

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

7.9.  Multiple Signature Confusion

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

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

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

7.10.  Signature Labels

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

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

7.11.  Symmetric Cryptography

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

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

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

7.12.  Canonicalization Attacks

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

   For example, if the handling of obs-fold field values does not remove
   the internal line folding and whitespace, additional newlines could
   be introduced into the signature input string by the signer,
   providing a potential place for an attacker to mount a signature
   collision (Section 7.5) attack.  Alternatively, if header fields that
   appear multiple times are not joined into a single string value, as
   is required by this specification, similar attacks can be mounted as
   a signed component value would show up in the input string more than
   once and could be substituted or otherwise attacked in this way.

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

7.13.  Key Specification Mix-Up

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

7.14.  HTTP Versions and Component Ambiguity

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

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

7.15.  Key and Algorithm Specification Downgrades

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

   Another example of a downgrade attack occurs when an asymmetric
   algorithm is expected, such as RSA-PSS, but an attacker substitutes a
   signature using symmetric algorithm, such as HMAC.  A naive verifier
   implementation could use the value of the public RSA key as the input
   to the HMAC verification function.  Since the public key is known to
   the attacker, this would allow the attacker to create a valid HMAC
   signature against this known key.  To prevent this, the verifier
   needs to ensure that both the key material and the algorithm are
   appropriate for the usage in question.  Additionally, while this
   specification does allow runtime specification of the algorithm using
   the alg signature parameter, applications are encouraged to use other
   mechanisms such as static configuration or higher protocol-level
   algorithm specification instead.

7.16.  Parsing Structured Field Values

   Several parts of this specification rely on the parsing of structured
   field values [RFC8941].  In particular, normalization of HTTP
   structured field values (Section 2.1.1), referencing members of a
   dictionary structured field (Section 2.1.3), and processing the
   @signature-input value when verifying a signature (Section 3.2).
   While structured field values are designed to be relatively simple to
   parse, a naive or broken implementation of such a parser could lead
   to subtle attack surfaces being exposed in the implementation.

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

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

7.17.  Choosing Message Components

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

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

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

   Additional considerations for choosing signature aspects are
   discussed in Section 1.5.

8.  Privacy Considerations

8.1.  Identification through Keys

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

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

8.2.  Signatures do not provide confidentiality

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

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

8.3.  Oracles

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

8.4.  Required Content

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

   For example, in some complex systems with intermediary processors
   this section; not sure how much could cause the surprising behavior of
   what's referenced below is actually applicable, or if it covers
   everything we need an intermediary not being
   able to worry about. ))

   (( TODO: Should provide some recommendations remove privacy-sensitive information from a message before
   forwarding it on how to determine what
   components need to be signed for a given use case. ))
   There are a number processing, for fear of security considerations to take into account
   when implementing or utilizing this specification. breaking the signature.
   A thorough
   security analysis of possible mitigation for this protocol, including its strengths and
   weaknesses, can specific situation would be found for the
   intermediary to verify the signature itself, then modifying the
   message to remove the privacy-sensitive information.  The
   intermediary can add its own signature at this point to signal to the
   next destination that the incoming signature was validated, as is
   shown in [WP-HTTP-Sig-Audit].

8. the example in Section 4.3.

9.  References

8.1.

9.1.  Normative References

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

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

   [MESSAGING]
              Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-17, 25 July
              httpbis-messaging-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              messaging-17>.
              messaging-19>.

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

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

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

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

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

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

   [RFC8941]  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>.

   [SEMANTICS]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-17, 25 July
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-17>.

8.2.
              semantics-19>.

9.2.  Informative References

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

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

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

   [I-D.ietf-httpbis-client-cert-field]
              Campbell, B. and M. Bishop, "Client-Cert HTTP Header
              Field: Conveying Client Certificate Information from TLS
              Terminating Reverse Proxies to Origin Server
              Applications", Work in Progress, Internet-Draft, draft-
              ietf-httpbis-client-cert-field-00, 8 June 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              client-cert-field-00>.

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

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

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

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

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

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

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

   [WP-HTTP-Sig-Audit]
              "Security Considerations for HTTP Signatures", 2013,
              <https://web-payments.org/specs/source/http-signatures-
              audit/>.

Appendix A.  Detecting HTTP Message Signatures

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

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

Appendix B.  Examples

B.1.  Example Keys

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

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

B.1.1.  Example Key RSA test

   The following key is a 2048-bit RSA public and private key pair,
   referred to in this document as "test-key-rsa": test-key-rsa:

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

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

B.1.2.  Example RSA PSS Key

   The following key is a 2048-bit RSA public and private key pair,
   referred to in this document as "test-key-rsa-pss": test-key-rsa-pss:

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

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

B.1.3.  Example ECC P-256 Test Key

   The following key is an elliptical curve key over the curve P-256,
   referred to in this document as "test-key-ecc-p256". test-key-ecc-p256.

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

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

B.1.4.  Example Shared Secret

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

   NOTE: '\' line wrapping per RFC 8792

   uzvJfB4u3N0Jy4T7NZ75MDVcr8zSTInedJtkgcu46YW4XByzNJjxBdtjUkdJPBt\
     bmHhIDi6pcl8jsasjlTMtDQ==

B.2.  Test Cases

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

   For requests, this "test-request" test-request message is used:

   POST /foo?param=value&pet=dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   Content-Length: 18

   {"hello": "world"}

   For responses, this "test-response" test-response message is used:

   HTTP/1.1 200 OK
   Date: Tue, 20 Apr 2021 02:07:56 GMT
   Content-Type: application/json
   Digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   Content-Length: 18

   {"hello": "world"}

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

   This example presents a minimal "Signature-Input" Signature-Input and "Signature" Signature header
   for a signature using the "rsa-pss-sha512" rsa-pss-sha512 algorithm over
   "test-request", test-request,
   covering none of the components of the HTTP message request but
   providing a timestamped signature proof of possession of the key.

   The corresponding signature input is:

   NOTE: '\' line wrapping per RFC 8792

   "@signature-params": ();created=1618884475\
     ;keyid="test-key-rsa-pss";alg="rsa-pss-sha512"

   This results in the following "Signature-Input" Signature-Input and "Signature" Signature headers
   being added to the message:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=();created=1618884475\
     ;keyid="test-key-rsa-pss";alg="rsa-pss-sha512"
   Signature: sig1=:HWP69ZNiom9Obu1KIdqPPcu/C1a5ZUMBbqS/xwJECV8bhIQVmE\
     AAAzz8LQPvtP1iFSxxluDO1KE9b8L+O64LEOvhwYdDctV5+E39Jy1eJiD7nYREBgx\
     TpdUfzTO+Trath0vZdTylFlxK4H3l3s/cuFhnOCxmFYgEa+cw+StBRgY1JtafSFwN\
     cZgLxVwialuH5VnqJS4JN8PHD91XLfkjMscTo4jmVMpFd3iLVe0hqVFl7MDt6TMkw\
     IyVFnEZ7B/VIQofdShO+C/7MuupCSLVjQz5xA+Zs6Hw+W9ESD/6BuGs6LF1TcKLxW\
     +5K+2zvDY/Cia34HNpRW5io7Iv9/b7iQ==:

   Note that since the covered components list is empty, this signature
   could be applied by an attacker to an unrelated HTTP message.
   Therefore, use of an empty covered components set is discouraged.

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

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

   The corresponding signature input is:

   NOTE: '\' line wrapping per RFC 8792

   "@authority": example.com
   "content-type": application/json
   "@signature-params": ("@authority" "content-type")\
     ;created=1618884475;keyid="test-key-rsa-pss"

   This results in the following "Signature-Input" Signature-Input and "Signature" Signature headers
   being added to the message:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("@authority" "content-type")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature: sig1=:ik+OtGmM/kFqENDf9Plm8AmPtqtC7C9a+zYSaxr58b/E6h81gh\
     JS3PcH+m1asiMp8yvccnO/RfaexnqanVB3C72WRNZN7skPTJmUVmoIeqZncdP2mlf\
     xlLP6UbkrgYsk91NS6nwkKC6RRgLhBFqzP42oq8D2336OiQPDAo/04SxZt4Wx9nDG\
     uy2SfZJUhsJqZyEWRk4204x7YEB3VxDAAlVgGt8ewilWbIKKTOKp3ymUeQIwptqYw\
     v0l8mN404PPzRBTpB7+HpClyK4CNp+SVv46+6sHMfJU4taz10s/NoYRmYCGXyadzY\
     YDj0BYnFdERB6NblI/AOWFGl5Axhhmjg==:

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

   This example covers all headers in "test-request" test-request (including the
   message body "Digest") Digest) plus various elements of the control data, using
   the "rsa-pss-sha512" rsa-pss-sha512 algorithm.

   The corresponding signature input is:

   NOTE: '\' line wrapping per RFC 8792

   "date": Tue, 20 Apr 2021 02:07:56 GMT
   "@method": POST
   "@path": /foo
   "@query": ?param=value&pet=dog
   "@authority": example.com
   "content-type": application/json
   "digest": SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   "content-length": 18
   "@signature-params": ("date" "@method" "@path" "@query" \
     "@authority" "content-type" "digest" "content-length")\
     ;created=1618884475;keyid="test-key-rsa-pss"

   This results in the following "Signature-Input" Signature-Input and "Signature" Signature headers
   being added to the message:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("date" "@method" "@path" "@query" \
     "@authority" "content-type" "digest" "content-length")\
     ;created=1618884475;keyid="test-key-rsa-pss"
   Signature: sig1=:JuJnJMFGD4HMysAGsfOY6N5ZTZUknsQUdClNG51VezDgPUOW03\
     QMe74vbIdndKwW1BBrHOHR3NzKGYZJ7X3ur23FMCdANe4VmKb3Rc1Q/5YxOO8p7Ko\
     yfVa4uUcMk5jB9KAn1M1MbgBnqwZkRWsbv8ocCqrnD85Kavr73lx51k1/gU8w673W\
     T/oBtxPtAn1eFjUyIKyA+XD7kYph82I+ahvm0pSgDPagu917SlqUjeaQaNnlZzO03\
     Iy1RZ5XpgbNeDLCqSLuZFVID80EohC2CQ1cL5svjslrlCNstd2JCLmhjL7xV3NYXe\
     rLim4bqUQGRgDwNJRnqobpS6C1NBns/Q==:

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

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

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

   The corresponding signature input is:

   NOTE: '\' line wrapping per RFC 8792

   "content-type": application/json
   "digest": SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   "content-length": 18
   "@signature-params": ("content-type" "digest" "content-length")\
     ;created=1618884475;keyid="test-key-ecc-p256"

   This results in the following "Signature-Input" Signature-Input and "Signature" Signature headers
   being added to the message:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("content-type" "digest" "content-length")\
     ;created=1618884475;keyid="test-key-ecc-p256"
   Signature: sig1=:n8RKXkj0iseWDmC6PNSQ1GX2R9650v+lhbb6rTGoSrSSx18zmn\
     6fPOtBx48/WffYLO0n1RHHf9scvNGAgGq52Q==:

B.2.5.  Signing a Request using hmac-sha256

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

   The corresponding signature input is:

   NOTE: '\' line wrapping per RFC 8792

   "@authority": example.com
   "date": Tue, 20 Apr 2021 02:07:55 GMT
   "content-type": application/json
   "@signature-params": ("@authority" "date" "content-type")\
     ;created=1618884475;keyid="test-shared-secret"

   This results in the following "Signature-Input" Signature-Input and "Signature" Signature headers
   being added to the message:

   NOTE: '\' line wrapping per RFC 8792

   Signature-Input: sig1=("@authority" "date" "content-type")\
     ;created=1618884475;keyid="test-shared-secret"
   Signature: sig1=:fN3AMNGbx0V/cIEKkZOvLOoC3InI+lM2+gTv22x3ia8=:

B.3.  TLS-Terminating Proxies

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

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

   POST /foo?Param=value&pet=Dog HTTP/1.1
   Host: example.com
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18

   {"hello": "world"}

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

   NOTE: '\' line wrapping per RFC 8792

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

   {"hello": "world"}

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

   NOTE: '\' line wrapping per RFC 8792

   "@path": /foo
   "@query": Param=value&pet=Dog
   "@method": POST
   "@authority": service.internal.example
   "client-cert": :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQ\
     KDBJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBD\
     QTAeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDM\
     FkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXm\
     ckC8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQY\
     DVR0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8B\
     Af8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQ\
     GV4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0\
     Q6bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
   "@signature-params": ("@path" "@query" "@method" "@authority" \
     "client-cert");created=1618884475;keyid="test-key-ecc-p256"

   This results in the following signature:

   NOTE: '\' line wrapping per RFC 8792

   5gudRjXaHrAYbEaQUOoY9TuvqWOdPcspkp7YyKCB0XhyAG9cB715hucPPanEK0OVyiN\
   LJqcoq2Yn1DPWQcnbog==

   Which results in the following signed request sent from the proxy to
   the internal service:

   NOTE: '\' line wrapping per RFC 8792

   POST /foo?Param=value&pet=Dog HTTP/1.1
   Host: service.internal.example
   Date: Tue, 20 Apr 2021 02:07:55 GMT
   Content-Type: application/json
   Content-Length: 18
   Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKD\
     BJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQT\
     AeFw0yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFk\
     wEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmck\
     C8vdgJ1p5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDV\
     R0TBAIwADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf\
     8EBAMCBsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV\
     4YW1wbGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6\
     bMjeSkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
   Signature-Input: ttrp=("@path" "@query" "@method" "@authority" \
     "client-cert");created=1618884475;keyid="test-key-ecc-p256"
   Signature: ttrp=:5gudRjXaHrAYbEaQUOoY9TuvqWOdPcspkp7YyKCB0XhyAG9cB7\
     15hucPPanEK0OVyiNLJqcoq2Yn1DPWQcnbog==:

   {"hello": "world"}

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

Acknowledgements

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

   The editors would also like to thank the following individuals for
   feedback, insight, and implementation of this draft and its
   predecessors (in alphabetical order): Mark Adamcin, Mark Allen, Paul
   Annesley, Karl Boehlmark, Stephane Böhlmark, Stéphane Bortzmeyer, Sarven Capadisli, Liam
   Dennehy, ductm54, Stephen Farrell, Phillip Hallam-Baker, Eric Holmes, Andrey
   Kislyuk, Adam Knight, Dave Lehn, Dave Longley, Ilari Liusvaara, James
   H.  Manger, Kathleen Moriarty, Mark Nottingham, Yoav Nir, Adrian
   Palmer, Lucas Pardue, Roberto Polli, Julian Reschke, Michael
   Richardson, Wojciech Rygielski, Adam Scarr, Cory J.  Slep, Dirk
   Stein, Henry Story, Lukasz Szewc, Chris Webber, and Jeffrey Yasskin.

Document History

   _RFC EDITOR: please remove this section before publication_

   *  draft-ietf-httpbis-message-signatures

      -  -07

         o  Added security and privacy considerations.

         o  Added pointers to algorithm values from definition sections.

         o  Expanded IANA registry sections.

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

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

      -  -06

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

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

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

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

         o  Added request-response binding.

      -  -05

         o  Remove list prefixes.

         o  Clarify signature algorithm parameters.

         o  Update and fix examples.

         o  Add examples for ECC and HMAC.

      -  -04

         o  Moved signature component definitions up to intro.

         o  Created formal function definitions for algorithms to
            fulfill.

         o  Updated all examples.

         o  Added nonce parameter field.

      -  -03

         o  Clarified signing and verification processes.

         o  Updated algorithm and key selection method.

         o  Clearly defined core algorithm set.

         o  Defined JOSE signature mapping process.

         o  Removed legacy signature methods.

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

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

      -  -02

         o  Removed editorial comments on document sources.

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

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

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

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

         o  Fixed up examples.

         o  Updated introduction now that it's adopted.

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

         o  Required signature parameters to be included in signature.

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

      -  -01

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

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

         o  Minor editorial corrections and readability improvements.

      -  -00

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

   *  draft-richanna-http-message-signatures

      -  -00

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

         o  Removed Signature auth-scheme definition and related
            content.

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

         o  Removed Extensions appendix.

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

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

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

         o  Added examples for canonicalization and signature generation
            steps.

         o  Rewrote Signature header definition, retaining normative
            requirements.

         o  Added default values for algorithm and expires parameters.

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

         o  Added IANA HTTP Signature Parameter registry.

         o  Added additional normative and informative references.

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

Authors' Addresses

   Annabelle Backman (editor)
   Amazon
   P.O. Box 81226
   Seattle, WA 98108-1226
   United States of America

   Email: richanna@amazon.com
   URI:   https://www.amazon.com/

   Justin Richer
   Bespoke Engineering

   Email: ietf@justin.richer.org
   URI:   https://bspk.io/

   Manu Sporny
   Digital Bazaar
   203 Roanoke Street W.
   Blacksburg, VA 24060
   United States of America

   Email: msporny@digitalbazaar.com
   URI:   https://manu.sporny.org/