Signing HTTP Messages

This document describes a mechanism for creating, encoding, and verifying digital signatures or message authentication codes over content within 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.¶

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 content within an HTTP message. The mechanism allows applications to create digital signatures or message authentication codes (MACs) over only that content within the message that is 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 mechanism described in this document consists of three parts:¶

A common nomenclature and canonicalization rule set for the different protocol elements and other content within HTTP messages.
Algorithms for generating and verifying signatures over HTTP message content using this nomenclature and rule set.
A mechanism for attaching a signature and related metadata to an HTTP message.

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 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.¶

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).
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).

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 content 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 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.¶

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, "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.¶

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

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 2.3) 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.¶

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 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 2.3.2) 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 2.3.2) 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.¶

2. HTTP Message Signature Covered Content

In order to allow signers and verifiers to establish which content is covered by a signature, this document defines content identifiers for data items covered by an HTTP Message Signature as well as the means for combining these canonicalized values into a signature input string.¶

Some content within HTTP messages can undergo transformations that change the bitwise value without altering meaning of the content (for example, the merging together of header fields with the same name). Message content 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 content identifier that transform the identifier's associated content into such a canonical form.¶

Content identifiers are defined using production grammar defined by RFC8941, Section 4 [RFC8941]. The content identifier is an sf-string value. The content identifier type MAY define parameters which are included using the parameters rule.¶

content-identifier = sf-string parameters

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

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

2.1. HTTP Headers

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.¶

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

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.
Strip leading and trailing whitespace from each item in the list.
Concatenate the list items together, with a comma "," and space " " between each item.

The resulting string is the canonicalized value.¶

2.1.1. Canonicalized Structured HTTP Headers

If value of the the HTTP header in question is a structured field ([RFC8941]), the content identifier MAY include the sf parameter. If this parameter is included, the HTTP header value MUST be canonicalized using the rules specified in Section 4 of RFC8941 [RFC8941]. Note that this process will replace any optional whitespace with a single space.¶

The resulting string is used as the field value input in Section 2.1.¶

2.1.2. Canonicalization Examples

This section contains non-normative examples of canonicalized values for header fields, given the following example HTTP message:¶

Server: 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

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

Non-normative examples of header field canonicalization.
Header Field	Canonicalized Value
"cache-control"	max-age=60, must-revalidate
"date"	Tue, 07 Jun 2014 20:51:35 GMT
"server"	www.example.com
"x-empty-header"
"x-obs-fold-header"	Obsolete line folding.
"x-ows-header"	Leading and trailing whitespace.

2.2. Dictionary Structured Field Members

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.¶

2.2.1. Canonicalization Examples

This section contains non-normative examples of canonicalized values for Dictionary Structured Field Members given the following example header field, whose value is assumed to be a Dictionary:¶

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

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

Non-normative examples of Dictionary member canonicalization.
Content Identifier	Canonicalized Value
"x-dictionary";key=a	1
"x-dictionary";key=b	2;x=1;y=2
"x-dictionary";key=c	(a, b, c)

2.3. Specialty Content Fields

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.¶

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

@request-target: The target request endpoint. (Section 2.3.1)
@signature-params: The signature metadata parameters for this signature. (Section 2.3.2)

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

2.3.1. Request Target

The request target endpoint, consisting of the request method and the path and query of the effective request URI, is identified by the @request-target identifier.¶

Its value is canonicalized as follows:¶

Take the lowercased HTTP method of the message.
Append a space " ".
Append the path and query of the request target of the message, formatted according to the rules defined for the :path pseudo-header in [HTTP2], Section 8.1.2.3. The resulting string is the canonicalized value.

2.3.1.1. Canonicalization Examples

The following table contains non-normative example HTTP messages and their canonicalized @request-target values.¶

Table 1: Non-normative examples of @request-target canonicalization.
HTTP Message	@request-target
POST /?param=value HTTP/1.1 Host: www.example.com	post /?param=value
POST /a/b HTTP/1.1 Host: www.example.com	post /a/b
GET http://www.example.com/a/ HTTP/1.1	get /a/
GET http://www.example.com HTTP/1.1	get /
CONNECT server.example.com:80 HTTP/1.1 Host: server.example.com	connect /
OPTIONS * HTTP/1.1 Host: server.example.com	options *

2.3.2. Signature Parameters

HTTP Message Signatures have metadata properties that provide information regarding the signature's generation and/or verification.¶

The signature parameters specialty content is identified by the @signature-params identifier.¶

Its canonicalized value is the serialization of the signature parameters for this signature, including the covered content list with all associated parameters.¶

alg: The HTTP message signature algorithm from the HTTP Message Signature Algorithm Registry, as an sf-string value.
keyid: The identifier for the key material as an sf-string value.
created: Creation time as an sf-integer UNIX timestamp value. Sub-second precision is not supported.
expires: Expiration time as an sf-integer UNIX timestamp value. Sub-second precision is not supported.
nonce: A random unique value generated for this signature.

Additional parameters can be defined in the HTTP Signature Parameters Registry (Section 5.2.2).¶

The signature parameters are serialized using the rules in Section 4 of RFC8941 [RFC8941] as follows:¶

Let the output be an empty string.
Determine an order for the content identifiers of the covered content. Once this order is chosen, it cannot be changed.
Serialize the content identifiers of the covered content, including all parameters, as an ordered inner-list according to Section 4.1.1.1 of RFC8941 [RFC8941] and append this to the output.
Determine an order for any signature parameters. Once this order is chosen, it cannot be changed.
Append the parameters to the 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 signature.
The output contains the signature parameters value.

Note that the inner-list serialization is used for the covered content value instead of the sf-list serialization in order to facilitate this value's additional inclusion in the Signature-Input header's dictionary, as discussed in Section 4.1.¶

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

("@request-target" "host" "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, but only the signature parameters used for the current signature are included in this field.¶

2.4. Creating the Signature Input String

The signature input is a US-ASCII string containing the content that is covered by the signature. To create the signature input string, the signer or verifier concatenates together entries for each identifier in the signature's covered content and parameters using the following algorithm:¶

Let the output be an empty string.
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 2.3)
5. Append a single newline "\\n"
Append the signature parameters (Section 2.3.2) 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 2.3.2
Return the output string.

If covered content references an identifier that cannot be resolved to a 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 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 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

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 content consists of the @request-target specialty content followed by the Host, Date, Cache-Control, X-Empty-Header, X-Example HTTP headers, in order. The signature creation timestamp is 1618884475 and the key identifier is test-key-rsa-pss. The signature input string for this message with these parameters is:¶

"@request-target": get /foo
"host": example.org
"date": Tue, 20 Apr 2021 02:07:55 GMT
"cache-control": max-age=60, must-revalidate
"x-empty-header": 
"x-example": Example header with some whitespace.
"@signature-params": ("@request-target" "host" "date" "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 content in an HTTP message and metadata about the signature itself. When successfully verified against an HTTP message, it provides cryptographic proof that with respect to the subset of content that was signed, the message is semantically equivalent to the message for which the signature was generated.¶

3.1. Creating a Signature

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

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.
The signer sets the signature's creation time to the current time.
If applicable, the signer sets the signature's expiration time property to the time at which the signature is to expire.
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 2.3 or its associated registry.
- Signers SHOULD include @request-target in the covered content 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.
The signer creates the signature input string. (Section 2.4)
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.
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.

For example, given the HTTP message and signature parameters in the example in Section 2.4, the example signature input string when signed with the test-key-rsa-pss key in Appendix B.1.2 gives the following message signature output value, encoded in Base64:¶

lPxkxqDEPhgrx1yPaKLO7eJ+oPjSwsQ5NjWNRfYP7Jw0FwnK1k8/GH7g5s2q0VTTKVm\
xyfpUDp/HsDphh5Z7Fa/lvtujHyFe/0EP9z7bnVb7YBZrxV52LGvP8p4APhOYuG4yaH\
z478GsJav9BQYK0B2IOHdLFJe8qwWPJs07J47gPewpNwCt0To/zZ2KPpylGX5UHVgJP\
Uom64KjX43u2OwIvSoPEYk4nuBvLR9yxYAHURaTfLoEDUCtY1FsU1hOfG3jAlcT6ill\
fnyS72PEdSSzw1KsxroMj9IYpFhva77YxmJRk4pCIW0F0Kj0ukl7J4y2aZJHMCYI3g8\
yfqh/wQ==

Figure 2: Non-normative example signature value

3.2. Verifying a Signature

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

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

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.
Parse the values of the chosen Signature-Input header field to get the parameters for the signature to be verified.
Parse the value of the corresponding Signature header field to get the byte array value of the signature to be verified.
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)
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.
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.
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 header field for this signature serialized according to the rules described in Section 2.3.2, not including the signature's label from the Signature-Input header.
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.
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.¶

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).
Enforcing a maximum signature age.
Prohibiting the use of certain algorithms, or mandating the use of an algorithm.
Requiring keys to be of a certain size (e.g., 2048 bits vs. 1024 bits).
Enforcing uniqueness of a 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.¶

Each signature algorithm method takes as its input the signature input string as a set of byte values (I), the signing key material (Ks), and outputs the signed content as a set of byte values (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), the verification key material (Kv), and the presented signature to be verified as a set of byte values (S) and outputs the verification result (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 2.3.2, 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 (K, M) function [RFC8017] with the signer's private signing key (K) and the signature input string (M) (Section 2.4). The mask generation function is MGF1 as specified in [RFC8017] with a hash function of SHA-512 [RFC6234]. The salt length (sLen) is 64 bytes. The hash function (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) is the HTTP message signature output used in Section 3.1.¶

To verify using this algorithm, the verifier applies the RSASSA-PSS-VERIFY ((n, e), M, S) function [RFC8017] using the public key portion of the verification key material ((n, e)) and the signature input string (M) re-created as described in Section 3.2. The mask generation function is MGF1 as specified in [RFC8017] with a hash function of SHA-512 [RFC6234]. The salt length (sLen) is 64 bytes. The hash function (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) 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 this algorithm, the signer applies the RSASSA-PKCS1-V1_5-SIGN (K, M) function [RFC8017] with the signer's private signing key (K) and the signature input string (M) (Section 2.4). 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) is the HTTP message signature output used in Section 3.1.¶

To verify using this algorithm, the verifier applies the RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S) function [RFC8017] using the public key portion of the verification key material ((n, e)) and the signature input string (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) 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.¶

3.3.3. HMAC using SHA-256

To sign and verify using this algorithm, the signer applies the HMAC function [RFC2104] with the shared signing key (K) and the signature input string (text) (Section 2.4). 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) 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.¶

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

To sign using this algorithm, the signer applies the ECDSA algorithm [FIPS186-4] using curve P-256 with the signer's private signing key and the signature input string (Section 2.4). 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 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) 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.¶

3.3.5. 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) 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 and MUST NOT be any algorithm with a JOSE Implementation Requirement of Prohibited.¶

4. Including a Message Signature in a Message

Message signatures can be included within an HTTP message via the Signature-Input and Signature HTTP header fields, both defined within this specification.¶

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

4.1. The 'Signature-Input' HTTP Header

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 2.3.2.¶

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

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.¶

4.2. The 'Signature' HTTP Header

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.¶

Signature: sig1=:lPxkxqDEPhgrx1yPaKLO7eJ+oPjSwsQ5NjWNRfYP7Jw0FwnK1k\
  8/GH7g5s2q0VTTKVmxyfpUDp/HsDphh5Z7Fa/lvtujHyFe/0EP9z7bnVb7YBZrxV5\
  2LGvP8p4APhOYuG4yaHz478GsJav9BQYK0B2IOHdLFJe8qwWPJs07J47gPewpNwCt\
  0To/zZ2KPpylGX5UHVgJPUom64KjX43u2OwIvSoPEYk4nuBvLR9yxYAHURaTfLoED\
  UCtY1FsU1hOfG3jAlcT6illfnyS72PEdSSzw1KsxroMj9IYpFhva77YxmJRk4pCIW\
  0F0Kj0ukl7J4y2aZJHMCYI3g8yfqh/wQ==:

4.3. Multiple Signatures

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.¶

The following is a non-normative example of header fields a reverse proxy sets in addition to the examples in the previous sections. The original signature is included under the identifier sig1, and the reverse proxy's signature is included under proxy_sig. The proxy uses the key rsa-test-key to create its signature using the rsa-v1_5-sha256 signature value. This results in a signature input string of:¶

"signature";key="sig1": \
  :lPxkxqDEPhgrx1yPaKLO7eJ+oPjSwsQ5NjWNRfYP7Jw0FwnK1k8/GH7g5s2q0VTT\
  KVmxyfpUDp/HsDphh5Z7Fa/lvtujHyFe/0EP9z7bnVb7YBZrxV52LGvP8p4APhOYu\
  G4yaHz478GsJav9BQYK0B2IOHdLFJe8qwWPJs07J47gPewpNwCt0To/zZ2KPpylGX\
  5UHVgJPUom64KjX43u2OwIvSoPEYk4nuBvLR9yxYAHURaTfLoEDUCtY1FsU1hOfG3\
  jAlcT6illfnyS72PEdSSzw1KsxroMj9IYpFhva77YxmJRk4pCIW0F0Kj0ukl7J4y2\
  aZJHMCYI3g8yfqh/wQ==:
"x-forwarded-for": 192.0.2.123
"@signature-params": ("signature";key="sig1" "x-forwarded-for")\
  ;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"

And a signature output value of:¶

XD1O/vEh772WVpY7jYvReXop2+b7xTIIPKH8/OCYzPn78Wd9jodCwAJPF5TYCn9L6n6\
8j4EjGsqFOMkVLVdSQEZqMLjEbvMEdIe8m1a0CLd5kydeaAwoHoglqod6ijkwhhEtxt\
aD8tDZmihQw2mZEH8u4aMSnRntqy7ExCNld0JLharsHV0iCbRO9jIP+d2ApD7gB+eZp\
n3pIvvVJZlxTwPkahFpxKlQtNMPaSqa1lvejURx+ST8CEuz4sS+G/oLJiX3MZenuUoO\
R8HeOHDnjN/VLzrEN4x44iF7WIL+iY2PtK87LUWRAsJAX9GqHL/upsGh1nxIdoVaoLV\
V5w+fRw==

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.¶

X-Forwarded-For: 192.0.2.123
Signature-Input: sig1=("@request-target" "host" "date" \
    "cache-control" "x-empty-header" "x-example")\
    ;created=1618884475;keyid="test-key-rsa-pss", \
  proxy_sig=("signature";key="sig1" "x-forwarded-for")\
    ;created=1618884480;keyid="test-key-rsa";alg="rsa-v1_5-sha256"
Signature: sig1=:lPxkxqDEPhgrx1yPaKLO7eJ+oPjSwsQ5NjWNRfYP7Jw0FwnK1k\
    8/GH7g5s2q0VTTKVmxyfpUDp/HsDphh5Z7Fa/lvtujHyFe/0EP9z7bnVb7YBZrx\
    V52LGvP8p4APhOYuG4yaHz478GsJav9BQYK0B2IOHdLFJe8qwWPJs07J47gPewp\
    NwCt0To/zZ2KPpylGX5UHVgJPUom64KjX43u2OwIvSoPEYk4nuBvLR9yxYAHURa\
    TfLoEDUCtY1FsU1hOfG3jAlcT6illfnyS72PEdSSzw1KsxroMj9IYpFhva77Yxm\
    JRk4pCIW0F0Kj0ukl7J4y2aZJHMCYI3g8yfqh/wQ==:, \
  proxy_sig=:XD1O/vEh772WVpY7jYvReXop2+b7xTIIPKH8/OCYzPn78Wd9jodCwA\
    JPF5TYCn9L6n68j4EjGsqFOMkVLVdSQEZqMLjEbvMEdIe8m1a0CLd5kydeaAwoH\
    oglqod6ijkwhhEtxtaD8tDZmihQw2mZEH8u4aMSnRntqy7ExCNld0JLharsHV0i\
    CbRO9jIP+d2ApD7gB+eZpn3pIvvVJZlxTwPkahFpxKlQtNMPaSqa1lvejURx+ST\
    8CEuz4sS+G/oLJiX3MZenuUoOR8HeOHDnjN/VLzrEN4x44iF7WIL+iY2PtK87LU\
    WRAsJAX9GqHL/upsGh1nxIdoVaoLVV5w+fRw==:

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.¶

5. IANA Considerations

5.1. 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 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.¶

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.¶

5.1.1. 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" - "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.
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.

5.1.2. Initial Contents

5.1.2.1. rsa-pss-sha512

Algorithm Name:: rsa-pss-sha512
Status:: Active
Definition:: RSASSA-PSS using SHA-256
Specification document(s):: [[This document]], Section 3.3.1

5.1.2.2. rsa-v1_5-sha256

Algorithm Name:: rsa-v1_5-sha256
Status:: Active
Description:: RSASSA-PKCS1-v1_5 using SHA-256
Specification document(s):: [[This document]], Section 3.3.2

5.1.2.3. hmac-sha256

Algorithm Name:: hmac-sha256
Status:: Active
Description:: HMAC using SHA-256
Specification document(s):: [[This document]], Section 3.3.3

5.1.2.4. ecdsa-p256-sha256

Algorithm Name:: ecdsa-p256-sha256
Status:: Active
Description:: ECDSA using curve P-256 DSS and SHA-256
Specification document(s):: [[This document]], Section 3.3.4

5.2. HTTP Signature Metadata Parameters Registry

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.¶

5.2.1. Registration Template

5.2.2. 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.¶

Initial contents of the HTTP Signature Metadata Parameters Registry.
Name	Status	Reference(s)
alg	Active	Section 2.3.2 of this document
created	Active	Section 2.3.2 of this document
expires	Active	Section 2.3.2 of this document
keyid	Active	Section 2.3.2 of this document
nonce	Active	Section 2.3.2 of this document

5.3. HTTP Signature Specialty Content Identifiers Registry

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.¶

5.3.1. Registration Template

5.3.2. Initial Contents

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

Initial contents of the HTTP Signature Specialty Content Identifiers Registry.
Name	Status	Reference(s)
@request-target	Active	Section 2.3.1 of this document
@signature-params	Active	Section 2.3.2 of this document

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:¶

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:¶

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.¶

-----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.¶

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 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 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 Header using rsa-pss-sha512

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

The corresponding signature input is:¶

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