HTTP Message Signatures

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 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). 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 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.
• 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 components 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.¶

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) that indicates the set of message components covered by the signature, not including the @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 algorithm to produce 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 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 component 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 2.3.1) 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 parameter of the signature parameters (Section 2.3.1) 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. 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.¶

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) when using them as component identifiers.¶

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 comma "," and space " " between each item.

The resulting string is the canonicalized component value.¶

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 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 item.¶

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.¶

@signature-params

The signature metadata parameters for this signature. (Section 2.3.1)

@method

The method used for a request. (Section 2.3.2)

@target-uri

The full target URI for a request. (Section 2.3.3)

@authority

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

@scheme

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

@request-target

The request target. (Section 2.3.6)

@path

The absolute path portion of the target URI for a request. (Section 2.3.7)

@query

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

@query-params

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

@status

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

@request-response

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

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

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 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)
   5. Append a single newline "\\n"
3. Append the signature parameters component (Section 2.3.1) as follows:
   1. Append the component identifier for the signature parameters serialized according to the component-identifier rule, i.e. "@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
4. Return the output string.

• 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 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 member identifier that references a member that is not present in the 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

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 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 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 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, or some combination thereof.
   • Signers SHOULD include the created signature metadata parameter to indicate when the signature was created.
   • The @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)
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 field 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 fields 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. If an applicable 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 field to get the parameters for the signature to be verified.
3. Parse the value of the corresponding 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. The value of the @signature-params input is the value of the SignatureInput field for this signature serialized according to the rules described in Section 2.3.1, not including the signature's label from the Signature-Input field.
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 or produce an error, 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 2.4.¶

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, by reference to the key material, or through mutual agreement between the signer and verifier.¶

The 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.¶

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

The signer MAY include the 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 HTTP field be included only as a header to avoid signatures being inadvertently stripped from a message.¶

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

The 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 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 HTTP field that does not have a corresponding member in the HTTP message's Signature-Input HTTP field MUST be ignored.¶

The signer MAY include the 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 HTTP field be included only as a header to avoid signatures being inadvertently stripped from a message.¶

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

Multiple distinct signatures MAY be included in a single message. Since Signature-Input and 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. 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.¶

These values are added to the HTTP request message by the proxy. The original signature is included under the identifier sig1, and the reverse proxy's signature is included under the label proxy_sig. The proxy uses the key test-key-rsa to create its signature using the rsa-v1_5-sha256 signature algorithm, while the client's original signature was made using the key id of test-key-rsa-pss and an RSA PSS signature algorithm.¶

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.¶

The 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.¶

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 and nonce parameters.¶

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 and Signature header values and associate them with the label
3. Optionally create any additional Signature-Input and Signature values, with unique labels not found in the Accept-Signature field
4. Combine all labeled Signature-Input and 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 field.¶

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 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, and sha512 portions of the identifier to determine parameters of the signing algorithm from the string.¶

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.¶

This document defines a method for canonicalizing HTTP message components, including components that can be generated 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 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.¶

8.1. 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., Nottingham, M., and J. , “HTTP/1.1”, Internet-Draft draft-ietf-httpbis-messaging-17 (work in progress), July 2021.

[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>.

[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/info/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/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., Nottingham, M., and J. , “HTTP Semantics”, Internet-Draft draft-ietf-httpbis-semantics-17 (work in progress), July 2021.

8.2. Informative References

[I-D.ietf-httpbis-client-cert-field]

Campbell, B. and M. , “Client-Cert HTTP Header Field: Conveying Client Certificate Information from TLS Terminating Reverse Proxies to Origin Server Applications”, Internet-Draft draft-ietf-httpbis-client-cert-field-00 (work in progress), June 2021.

[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.¶

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