Signing HTTP Messages

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 signed content. Since the raw bytes of the message cannot be relied upon as signed content, the signer and verifier must derive the signed content 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 header field) is relevant. Thus a general purpose solution must provide signers with some degree of control over which message content is 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). These applications need to be able to generate and verify signatures despite incomplete knowledge of the HTTP message.¶

• Re-ordering of header fields with different header field names ([MESSAGING], Section 3.2.2).
• Combination of header fields with the same field name ([MESSAGING], Section 3.2.2).
• Removal of header fields listed in the Connection header field ([MESSAGING], Section 6.1).
• Addition of header fields that indicate control options ([MESSAGING], Section 6.1).
• Addition or removal of a transfer coding ([MESSAGING], Section 5.7.2).
• Addition of header fields such as Via ([MESSAGING], Section 5.7.1) and Forwarded ([RFC7239], Section 4).

• 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 content 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.
• Changes to the request-target and 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 content not covered by the signature are considered safe.¶

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, "effective request URI", "gateway", "header field", "intermediary", 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 and keyed MACs. 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.¶

Signer:

The entity that is generating or has generated an HTTP Message Signature.

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.

Covered Content:

An ordered list of content identifiers for headers (Section 2.1) and specialty content (Section 2.4) that indicates the metadata and message content that is covered by the signature, not including the @signature-params specialty field itself.

HTTP Signature Algorithm:

A cryptographic algorithm that describes the signing and verification process for the signature. When expressed explicitly, the value maps to a string defined in the HTTP Signature Algorithms Registry defined in this document.

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 which the signature expires, 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.¶

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.¶

• The set of content identifiers (Section 2) that are expected and required. For example, an authorization protocol could mandate that the Authorization header be covered to protect the authorization credentials and mandate the signature parameters contain a created parameter, while an API expecting HTTP message bodies could require the 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 parameter of the signature parameters (Section 2.4.2) 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 content is appropriate for the key material. For example, the process could use the alg parameter of the signature parameters (Section 2.4.2) 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.

The details of this kind of profiling are the purview of the application and outside the scope of this specification.¶

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

1. Create an ordered list of the field values of each instance of the header 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 comma "," and space " " between each item.

The resulting string is the canonicalized value.¶

An individual member in the value of a Dictionary Structured Field is identified by using the parameter key on the content identifier for the header. The value of this parameter is a the key being identified, without any parameters present on that key in the original dictionary.¶

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 member.¶

A prefix of a List Structured Field consisting of the first N members in the field's value (where N is an integer greater than 0 and less than or equal to the number of members in the List) is identified by the parameter prefix with the value of N as an integer.¶

A list prefix value is canonicalized by applying the serialization algorithm described in Section 4.1.1 of RFC8941 [RFC8941] on a List containing only the first N members as specified in the list prefix, in the order they appear in the original List.¶

Content not found in an HTTP header can be included in the signature base string by defining a content identifier and the canonicalization method for its content.¶

@request-target

The target request endpoint. (Section 2.4.1)

@signature-params

The signature metadata parameters for this signature. (Section 2.4.2)

Additional specialty content identifiers MAY be defined and registered in the HTTP Signatures Specialty Content Identifier Registry. (Section 5.3)¶

1. Let the output be an empty string.
2. For each covered content item in the covered content list (in order):
   1. Append the identifier for the covered content serialized according to the content-identifier rule.
   2. Append a single colon ":"
   3. Append a single space " "
   4. Append the covered content's canonicalized value, as defined by the covered content type. (Section 2.1 and Section 2.4)
   5. Append a single newline "\\n"
3. Append the signature parameters (Section 2.4.2) as follows:
   1. Append the identifier for the signature parameters serialized according to the content-identifier rule, "@signature-params"
   2. Append a single colon ":"
   3. Append a single space " "
   4. Append the signature parameters' canonicalized value as defined in Section 2.4.2
4. Return the output string.

• The signer or verifier does not understand the content identifier.
• The identifier identifies a header field that is not present in the message or whose value is malformed.
• The identifier is a Dictionary member identifier that references a header field that is not present in the message, is not a Dictionary Structured Field, or whose value is malformed.
• The identifier is a List Prefix member identifier that references a header field that is not present in the message, is not a List Structured Field, or whose value is malformed.
• The identifier is a Dictionary member identifier that references a member that is not present in the header field value, or whose value is malformed. E.g., the identifier is "x-dictionary";key=c and the value of the x-dictionary header field is a=1, b=2
• The identifier is a List Prefix member identifier that specifies more List members than are present the header field. E.g., the identifier is "x-list";prefix=3 and the value of the x-list header field is (1, 2).

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.
4. The signer creates an ordered list of content identifiers representing the message content and signature metadata to be covered by the signature, and assigns this list as the signature's Covered Content.
   • Once an order of covered content is chosen, the order MUST NOT change for the life of the signature.
   • Each covered content identifier MUST either reference an HTTP header in the request message Section 2.1 or reference a specialty content field listed in Section 2.4 or its associated registry.
   • Signers SHOULD include @request-target in the covered content list list.
   • Signers SHOULD include a date stamp in some form, such as using the date header. Alternatively, the created signature metadata parameter can fulfil this role.
   • Further guidance on what to include in this list and in what order is out of scope for this document. However, note that the list order is significant and once established for a given signature it MUST be preserved for that signature.
   • Note that the @signature-params specialty identifier is not explicitly listed in the list of covered content 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.
5. The signer creates the signature input string. (Section 2.5)
6. The signer signs the signature input with the chosen signing algorithm using the key material chosen by the signer. Several 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 header as defined in Section 4.2.

A verifier processes a signature and its associated signature input parameters in concert with each other.¶

1. Parse the Signature and Signature-Input headers 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 request. If an appropriate signature is not found, produce an error.
   2. If the chosen Signature value does not have a corresponding Signature-Input value, produce an error.
2. Parse the values of the chosen Signature-Input header field to get the parameters for the signature to be verified.
3. Parse the value of the corresponding Signature header 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 contents 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 2.5. The value of the @signature-params input is the value of the SignatureInput header field for this signature serialized according to the rules described in Section 2.4.2, not including the signature's label from the Signature-Input header.
8. If the key material is appropriate for the algorithm, apply the verification algorithm to the signature, recalculated signature input, signature parameters, key material, and algorithm. Several algorithms are defined in Section 3.3.
9. The results of the verification algorithm function are the final results of the signature verification.

If any of the above steps fail, the signature validation fails.¶

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 2.5.¶

This section contains several common algorithm methods. The method to use can be communicated through the algorithm signature parameter defined in Section 2.4.2, by reference to the key material, or through mutual agreement between the signer and verifier.¶

The Signature-Input HTTP header field is a Dictionary Structured Header [RFC8941] containing the metadata for one or more message signatures generated from content 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 content including all signature metadata parameters, using the serialization process defined in Section 2.4.2.¶

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

The Signature HTTP header field is a Dictionary Structured Header [RFC8941] containing one or more message signatures generated from content within the HTTP message. Each member's name is a signature identifier that is present as a member name in the Signature-Input Structured Header 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 HTTP header field that does not have a corresponding member in the HTTP message's Signature-Input HTTP header field MUST be ignored.¶

Since Signature-Input and Signature are both defined as Dictionary Structured Headers, they can be used to include multiple signatures within the same HTTP message. For example, a signer may include multiple signatures signing the same content with different keys or algorithms to support verifiers with different capabilities, or a reverse proxy may include information about the client in header fields when forwarding the request to a service host, including a signature over those fields and the client's original signature.¶

These values are added to the HTTP request message by the proxy. The different signature values are wrapped onto separate lines to increase human-readability of the result.¶

The proxy's signature and the client's original signature can be verified independently for the same message, depending on the needs of the application.¶

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 5.1.2. 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 5.1.1.¶

This document defines the Signature-Input Structured Header, whose member values 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-Input Structured Header. Initial values for this registry are given in Section 5.2.2. 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 5.2.1.¶

This document defines a method for canonicalizing HTTP message content, including content that can be generated from the context of the HTTP message outside of the HTTP headers. This content is identified by a unique key. IANA is asked to create and maintain a new registry typed "HTTP Signature Specialty Content Identifiers" to record and maintain the set of non-header content identifiers and their canonicalization method. Initial values for this registry are given in Section 5.3.2. 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 5.3.1.¶

7.1. Normative References

[FIPS186-4]

“Digital Signature Standard (DSS)”, 2013, <https://csrc.nist.gov/publications/detail/fips/186/4/final>.

[HTTP2]

Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <https://www.rfc-editor.org/info/rfc7540>.

[MESSAGING]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014, <https://www.rfc-editor.org/info/rfc7230>.

[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/info/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/info/rfc2119>.

[RFC8174]

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

[RFC8792]

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

[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/info/rfc8941>.

[SEMANTICS]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, RFC 7231, DOI 10.17487/RFC7231, June 2014, <https://www.rfc-editor.org/info/rfc7231>.

7.2. Informative References

[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/info/rfc6234>.

[RFC7239]

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

[RFC7517]

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

[RFC7518]

Jones, M., “JSON Web Algorithms (JWA)”, RFC 7518, DOI 10.17487/RFC7518, May 2015, <https://www.rfc-editor.org/info/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/info/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/info/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/info/rfc8446>.

[WP-HTTP-Sig-Audit]

“Security Considerations for HTTP Signatures”, 2013, <https://web-payments.org/specs/source/http-signatures-audit/>.

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.¶