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 oringally 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.

The term "Unix time" is defined by [POSIX.1] section 4.16.¶

This document contains non-normative examples of partial and complete HTTP messages. To improve readability, header fields may be split into multiple lines, using the obs-fold syntax. This syntax is deprecated in [MESSAGING], and senders MUST NOT generate messages that include it.¶

Additionally, some examples use '\' line wrapping for long values that contain no whitespace, as per [RFC8792].¶

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 would mandate that the Authorization header be covered to protect the authorization credentials, as well as a *created field to allow replay detection.
A means of retrieving the key material used to verify the signature. An application will usually use the keyid field of the Signature-Input header value and define rules for resolving a key from there.
A means of determining the signature algorithm used to verify the signature content is appropriate for the key material.
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. Identifying and Canonicalizing 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.¶

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. 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.¶

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 [RFC8941] section 4. 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:¶

HTTP/1.1 200 OK
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] 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. List Prefixes

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] on a List containing only the first N members as specified in the list prefix, in the order they appear in the original List.¶

2.3.1. Canonicalization Examples

This section contains non-normative examples of canonicalized values for list prefixes given the following example header fields, whose values are assumed to be Dictionaries:¶

X-List-A: (a b c d e f)
X-List-B: ()

The following table shows example canonicalized values for different content identifiers, given those fields:¶

Non-normative examples of list prefix canonicalization.
Content Identifier	Canonicalized Value
"x-list-a";prefix=0	()
"x-list-a";prefix=1	(a)
"x-list-a";prefix=3	(a, b, c)
"x-list-a";prefix=6	(a, b, c, d, e, f)
"x-list-b";prefix=0	()

2.4. 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 speciality 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 2.4.1
@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 ¶

2.4.1. 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.

2.4.1.1. 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 *

2.4.2. Signature Parameters

The signature parameters special 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. Section 3.1 ¶

Note that an HTTP message could contain multiple signatures, but only the signature parameters used for the current signature are included.¶

2.4.2.1. Canonicalization Examples

Given the following signature parameters:¶

Property	Value
Algorithm	hs2019
Covered Content	@request-target, host, date, cache-control, x-emptyheader, x-example, x-dictionary;key=b, x-dictionary;key=a, x-list;prefix=3
Creation Time	1402174295
Expiration Time	1402174595
Verification Key Material	The public key provided in Appendix B.1.1 and identified by the keyid value "test-key-a".

The signature parameter value is defined as:¶

"@signature-params": ("@request-target" "host" "date" "cache-control" "x-empty-header" "x-example" "x-dictionary";key=b "x-dictionary";key=a "x-list";prefix=3); keyid="test-key-a"; alg="hs2019"; created=1402170695; expires=1402170995

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. Signature Metadata

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

Algorithm:: An HTTP Signature Algorithm defined in the HTTP Signature Algorithms Registry defined in this document, represented as a string. It describes the signing and verification algorithms for the signature.
Creation Time:: A timestamp representing the point in time that the signature was generated, represented as an integer. Sub-second precision is not supported. A signature's Creation Time MAY be undefined, indicating that it is unknown.
Expiration Time:: A timestamp representing the point in time at which the signature expires, represented as an integer. An expired signature always fails verification. A signature's Expiration Time MAY be undefined, indicating that the signature does not expire.
Verification Key Material:: The key material required to verify the signature.
Covered Content:: An ordered list of content identifiers (Section 2) that indicates the metadata and message content that is covered by the signature. This list MUST NOT include the @signature-params content identifier.

The signature metadata is serialized using the rules in [RFC8941] section 4 as follows:¶

Let the output be an empty string.
Serialize the content identifiers as an ordered inner-list according to [RFC8941] section 4.1.1.1 and append this to the output.
Append the signature metadata as parameters according to [RFC8941] section 4.1.1.2 in the any order, skipping fields that are not available:
- alg: Algorithm as an sf-string value.
- keyid: Verification Key Material as an sf-string value.
- created: Creation Time as an sf-integer timestamp value.
- expires: Expiration Time as an sf-integer timestamp value.

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

The Table 2 values would be serialized as follows:¶

("@request-target" "host" "date" "cache-control" "x-empty-header" "x-example"); keyid="test-key-a"; alg="hs2019"; created=1402170695; expires=1402170995

3.2. Creating a Signature

In order to create a signature, a signer completes the following process:¶

Choose key material and algorithm, and set metadata properties Section 3.2.1
Create the Signature Input Section 3.2.2
Sign the Signature Input Section 3.2.3

The following sections describe each of these steps in detail.¶

3.2.1. Choose and Set Signature Metadata Properties

The signer chooses an HTTP Signature Algorithm from those registered in the HTTP Signature Algorithms Registry defined by this document, and sets the signature's Algorithm property to that value. The signer MUST NOT choose an algorithm marked "Deprecated". The mechanism by which the signer chooses an algorithm is out of scope for this document.
The signer chooses key material to use for signing and verification, and sets the signature's Verification Key Material property to the key material required for verification. 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 key material is out of scope for this document.
The signer sets the signature's Creation Time property to the current time.
The signer sets the signature's Expiration Time property to the time at which the signature is to expire, or to undefined if the signature will not 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.
- Each identifier MUST be one of those defined in Section 2.
- This list MUST NOT be empty, as this would result in creating a signature over the empty string.
- Signers SHOULD include @request-target in the list.
- Signers SHOULD include a date stamp, such as 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, the list order is significant and once established for a given signature it MUST be preserved for that signature.
- Note that the signature metadata is not included in the explicit list of covered content identifiers since its value is always covered.

For example, given the following HTTP message:¶

GET /foo HTTP/1.1
Host: example.org
Date: Sat, 07 Jun 2014 20:51:35 GMT
X-Example: Example header
        with some whitespace.
X-EmptyHeader:
X-Dictionary: a=1, b=2
X-List: (a b c d)
Cache-Control: max-age=60
Cache-Control: must-revalidate

The following table presents a non-normative example of metadata values that a signer may choose:¶

Table 2: Non-normative example metadata values
Property	Value
Algorithm	hs2019
Covered Content	@request-target, host, date, cache-control, x-emptyheader, x-example, x-dictionary;key=b, x-dictionary;key=a, x-list;prefix=3
Creation Time	1402174295
Expiration Time	1402174595
Verification Key Material	The public key provided in Appendix B.1.1 and identified by the keyid value "test-key-a".

3.2.2. Create the Signature Input

The Signature Input is a US-ASCII string containing the content that will be signed. To create it, the signer or verifier concatenates together entries for each identifier in the signature's Covered Content in the order it occurs in the list, with each entry separated by a newline "\n". An identifier's entry is a sf-string followed with a colon ":", a space " ", and the identifier's canonicalized value.¶

The signer or verifier then includes the signature metadata specialty field @signature-params as the last entry in the covered content, separated by a newline "\n". Section 2.4.2 ¶

If Covered Content contains an identifier for a header field that is malformed or is not present in the message, the implementation MUST produce an error.¶

If Covered Content contains an identifier for a Dictionary member that references a header field using the key parameter that is not present, is malformed in the message, or is not a Dictionary Structured Field, the implementation MUST produce an error. If the header field value does not contain the specified member, the implementation MUST produce an error.¶

If Covered Content contains an identifier for a List Prefix that references a header field using the prefix parameter that is not present, is malformed in the message, or is not a List Structured Field, the implementation MUST produce an error. If the header field value contains fewer than the specified number of members, the implementation MUST produce an error.¶

For the non-normative example Signature metadata in Table 2, the corresponding Signature Input is:¶

"@request-target": get /foo
"host": example.org
"date": Tue, 07 Jun 2014 20:51:35 GMT
"cache-control": max-age=60, must-revalidate
"x-emptyheader":
"x-example": Example header with some whitespace.
"x-dictionary";key=b: 2
"x-dictionary";key=a: 1
"x-list";prefix=3: (a, b, c)
"@signature-params": ("@request-target" "host" "date" "cache-control" "x-empty-header" "x-example" "x-dictionary";key=b "x-dictionary";key=b "x-list";prefix=3); keyid="test-key-a"; alg="hs2019"; created=1402170695; expires=1402170995

Figure 1: Non-normative example Signature Input

3.2.3. Sign the Signature Input

The signer signs the Signature Input using the signing algorithm described by the signature's Algorithm property, and the key material chosen by the signer. The signer then encodes the result of that operation as a base 64-encoded string [RFC4648]. This string is the signature value.¶

For the non-normative example Signature metadata in Section 3.2.1 and Signature Input in Figure 1, the corresponding signature value is:¶

K2qGT5srn2OGbOIDzQ6kYT+ruaycnDAAUpKv+ePFfD0RAxn/1BUeZx/Kdrq32DrfakQ6b
PsvB9aqZqognNT6be4olHROIkeV879RrsrObury8L9SCEibeoHyqU/yCjphSmEdd7WD+z
rchK57quskKwRefy2iEC5S2uAH0EPyOZKWlvbKmKu5q4CaB8X/I5/+HLZLGvDiezqi6/7
p2Gngf5hwZ0lSdy39vyNMaaAT0tKo6nuVw0S1MVg1Q7MpWYZs0soHjttq0uLIA3DIbQfL
iIvK6/l0BdWTU7+2uQj7lBkQAsFZHoA96ZZgFquQrXRlmYOh+Hx5D9fJkXcXe5tmAg==

Figure 2: Non-normative example signature value

3.3. Verifying a Signature

In order to verify a signature, a verifier MUST:¶

Examine the signature's metadata 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 header fields or other content are required to be covered by the signature.
Use the received HTTP message and the signature's metadata to recreate the Signature Input, using the process described in Section 3.2.2. The value of the @signature-params input is the value of the signature input header field for this signature, not including the signature's label.
Use the signature's Algorithm and Verification Key Material with the recreated Signing Input to verify the signature value.

A signature with a Creation Time that is in the future or an Expiration Time that is in the past MUST NOT be processed.¶

The verifier MUST ensure that a signature's Algorithm is appropriate for the key material the verifier will use to verify the signature. If the Algorithm is not appropriate for the key material (for example, if it is the wrong size, or in the wrong format), the signature MUST NOT be processed.¶

3.3.1. 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).

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.¶

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. The Signature HTTP header field contains signature values, while the Signature-Input HTTP header field identifies the Covered Content and metadata that describe how each signature was generated.¶

4.1. The 'Signature-Input' HTTP Header

The Signature-Input HTTP header field is a Dictionary Structured Header [RFC8941] containing the metadata for zero 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, described in Section 3.1.¶

Signature-Input: sig1=("@request-target" "host" "date"
    "cache-control" "x-empty-header" "x-example"); keyid="test-key-a";
    alg="hs2019"; created=1402170695; expires=1402170995

To facilitate signature validation, the Signature-Input header MUST contain the same serialization value used in generating the signature input.¶

4.2. The 'Signature' HTTP Header

The Signature HTTP header field is a Dictionary Structured Header [RFC8941] containing zero 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=:K2qGT5srn2OGbOIDzQ6kYT+ruaycnDAAUpKv+ePFfD0RAxn/1BUe\
    Zx/Kdrq32DrfakQ6bPsvB9aqZqognNT6be4olHROIkeV879RrsrObury8L9SCEibe\
    oHyqU/yCjphSmEdd7WD+zrchK57quskKwRefy2iEC5S2uAH0EPyOZKWlvbKmKu5q4\
    CaB8X/I5/+HLZLGvDiezqi6/7p2Gngf5hwZ0lSdy39vyNMaaAT0tKo6nuVw0S1MVg\
    1Q7MpWYZs0soHjttq0uLIA3DIbQfLiIvK6/l0BdWTU7+2uQj7lBkQAsFZHoA96ZZg\
    FquQrXRlmYOh+Hx5D9fJkXcXe5tmAg==:

4.3. Examples

The following is a non-normative example of Signature-Input and Signature HTTP header fields representing the signature in Figure 2:¶

# NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig1=("@request-target" "host" "date"
    "cache-control" "x-empty-header" "x-example"); keyid="test-key-a";
    alg="hs2019"; created=1402170695; expires=1402170995
Signature: sig1=:K2qGT5srn2OGbOIDzQ6kYT+ruaycnDAAUpKv+ePFfD0RAxn/1BUe\
    Zx/Kdrq32DrfakQ6bPsvB9aqZqognNT6be4olHROIkeV879RrsrObury8L9SCEibe\
    oHyqU/yCjphSmEdd7WD+zrchK57quskKwRefy2iEC5S2uAH0EPyOZKWlvbKmKu5q4\
    CaB8X/I5/+HLZLGvDiezqi6/7p2Gngf5hwZ0lSdy39vyNMaaAT0tKo6nuVw0S1MVg\
    1Q7MpWYZs0soHjttq0uLIA3DIbQfLiIvK6/l0BdWTU7+2uQj7lBkQAsFZHoA96ZZg\
    FquQrXRlmYOh+Hx5D9fJkXcXe5tmAg==:

Since Signature-Input and Signature are both defined as Dictionary Structured Headers, they can be used to easily include multiple signatures within the same HTTP message. For example, a signer may include multiple signatures signing the same content with different keys and/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, and may also include a signature over those fields and the client's signature. The following is a non-normative example of header fields a reverse proxy might add to a forwarded request that contains the signature in the above example:¶

# NOTE: '\' line wrapping per RFC 8792

X-Forwarded-For: 192.0.2.123
Signature-Input: reverse_proxy_sig=("host" "date"
    "signature";key=sig1 "x-forwarded-for"); keyid="test-key-a";
    alg="hs2019"; created=1402170695; expires=1402170695
Signature: reverse_proxy_sig=:ON3HsnvuoTlX41xfcGWaOEVo1M3bJDRBOp0Pc/O\
    jAOWKQn0VMY0SvMMWXS7xG+xYVa152rRVAo6nMV7FS3rv0rR5MzXL8FCQ2A35DCEN\
    LOhEgj/S1IstEAEFsKmE9Bs7McBsCtJwQ3hMqdtFenkDffSoHOZOInkTYGafkoy78\
    l1VZvmb3Y4yf7McJwAvk2R3gwKRWiiRCw448Nt7JTWzhvEwbh7bN2swc/v3NJbg/w\
    JYyYVbelZx4IywuZnYFxgPl/qvqbAjeEVvaLKLgSMr11y+uzxCHoMnDUnTYhMrmOT\
    4O8lBLfRFOcoJPKBdoKg9U0a96U2mUug1bFOozEVYFg==:

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.¶

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 description of the algorithm used to sign the signing string when generating an HTTP Message Signature, or instructions on how to determine that algorithm. When the description specifies an algorithm, it MUST include a reference to the document or documents that define the algorithm.

5.1.2. Initial Contents

(( MS: The references in this section are problematic as many of the specifications that they refer to are too implementation specific, rather than just pointing to the proper signature and hashing specifications. A better approach might be just specifying the signature and hashing function specifications, leaving implementers to connect the dots (which are not that hard to connect). ))¶

5.1.2.1. hs2019

Algorithm Name:

hs2019

Status:

active

Description:

Derived from metadata associated with keyid. Recommend support for:

RSASSA-PSS [RFC8017] using SHA-512 [RFC6234]

HMAC [RFC2104] using SHA-512 [RFC6234]

ECDSA using curve P-256 DSS [FIPS186-4] and SHA-512 [RFC6234]

Ed25519ph, Ed25519ctx, and Ed25519 [RFC8032]

5.1.2.2. rsa-sha1

Algorithm Name:: rsa-sha1
Status:: Deprecated; SHA-1 not secure.
Description:: RSASSA-PKCS1-v1_5 [RFC8017] using SHA-1 [RFC6234]

5.1.2.3. rsa-sha256

Algorithm Name:: rsa-sha256
Status:: Deprecated; specifying signature algorithm enables attack vector.
Description:: RSASSA-PKCS1-v1_5 [RFC8017] using SHA-256 [RFC6234]

5.1.2.4. hmac-sha256

Algorithm Name:: hmac-sha256
Status:: Deprecated; specifying signature algorithm enables attack vector.
Description:: HMAC [RFC2104] using SHA-256 [RFC6234]

5.1.2.5. ecdsa-sha256

Algorithm Name:: ecdsa-sha256
Status:: Deprecated; specifying signature algorithm enables attack vector.
Description:: ECDSA using curve P-256 DSS [FIPS186-4] and SHA-256 [RFC6234]

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 3.1 of this document
created	Active	Section 3.1 of this document
expires	Active	Section 3.1 of this document
keyid	Active	Section 3.1 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 2.4.1 of this document
@signature-params	Active	Section 2.4.2 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.¶

B.1.1. Example Key RSA test

The following key is a 2048-bit RSA public and private key pair:¶

B.2. Example keyid Values

The table below maps example keyid values to associated algorithms and/or keys. These are example mappings that are valid only within the context of examples in examples within this and future documents that reference this section. Unless otherwise specified, within the context of examples it should be assumed that the signer and verifier understand these keyid mappings. These keyid values are not reserved, and deployments are free to use them, with these associations or others.¶

keyid	Algorithm	Verification Key
test-key-a	hs2019, using RSASSA-PSS [RFC8017] and SHA-512 [RFC6234]	The public key specified in Appendix B.1.1
test-key-b	rsa-sha256	The public key specified in Appendix B.1.1

B.3. 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 message:¶

POST /foo?param=value&pet=dog HTTP/1.1
Host: example.com
Date: Tue, 07 Jun 2014 20:51:35 GMT
Content-Type: application/json
Digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
Content-Length: 18

{"hello": "world"}

B.3.1. Signature Generation

B.3.1.1. hs2019 signature over minimal recommended content

This presents metadata for a Signature using hs2019, over minimum recommended data to sign:¶

Property	Value
Algorithm	hs2019, using RSASSA-PSS [RFC8017] using SHA-512 [RFC6234]
Covered Content	@request-target
Creation Time	8:51:35 PM GMT, June 7th, 2014
Expiration Time	Undefined
Verification Key Material	The public key specified in Appendix B.1.1.

The Signature Input is:¶

"@request-target": post /foo?param=value&pet=dog
"@signature-params": ("@request-target"); keyid="test-key-a"; created=1402170695

The signature value is:¶

QaVaWYfF2da6tG66Xtd0GrVFChJ0fOWUe/C6kaYESPiYYwnMH9egOgyKqgLLY9NQJFk7b
QY834sHEUwjS5ByEBaO3QNwIvqEY1qAAU/2MX14tc9Yn7ELBnaaNHaHkV3xVO9KIuLT7V
6e4OUuGb1axfbXpMgPEql6CEFrn6K95CLuuKP5/gOEcBtmJp5L58gN4VvZrk2OVA6U971
YiEDNuDa4CwMcQMvcGssbc/L3OULTUffD/1VcPtdGImP2uvVQntpT8b2lBeBpfh8MuaV2
vtzidyBYFtAUoYhRWO8+ntqA1q2OK4LMjM2XgDScSVWvGdVd459A0wI9lRlnPap3zg==

A possible Signature-Input and Signature header containing this signature is:¶

# NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig1=("@request-target");
    keyid="test-key-a"; created=1402170695
Signature: sig1=:QaVaWYfF2da6tG66Xtd0GrVFChJ0fOWUe/C6kaYESPiYYwnMH9eg\
    OgyKqgLLY9NQJFk7bQY834sHEUwjS5ByEBaO3QNwIvqEY1qAAU/2MX14tc9Yn7ELB\
    naaNHaHkV3xVO9KIuLT7V6e4OUuGb1axfbXpMgPEql6CEFrn6K95CLuuKP5/gOEcB\
    tmJp5L58gN4VvZrk2OVA6U971YiEDNuDa4CwMcQMvcGssbc/L3OULTUffD/1VcPtd\
    GImP2uvVQntpT8b2lBeBpfh8MuaV2vtzidyBYFtAUoYhRWO8+ntqA1q2OK4LMjM2X\
    gDScSVWvGdVd459A0wI9lRlnPap3zg==:

B.3.1.2. hs2019 signature covering all header fields

This presents metadata for a Signature using hs2019 that covers all header fields in the request:¶

Property	Value
Algorithm	hs2019, using RSASSA-PSS [RFC8017] using SHA-512 [RFC6234]
Covered Content	@request-target, host, date, content-type, digest, content-length
Creation Time	8:51:35 PM GMT, June 7th, 2014
Expiration Time	Undefined
Verification Key Material	The public key specified in Appendix B.1.1.

The Signature Input is:¶

"@request-target": post /foo?param=value&pet=dog
"host": example.com
"date": Tue, 07 Jun 2014 20:51:35 GMT
"content-type": application/json
"digest": SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
"content-length": 18
"@signature-params": ("@request-target" "host" "date" "content-type" "digest" "content-length"); keyid="test-key-a"; alg="hs2019"; created=1402170695

The signature value is:¶

B24UG4FaiE2kSXBNKV4DA91J+mElAhS3mncrgyteAye1GKMpmzt8jkHNjoudtqw3GngGY
3n0mmwjdfn1eA6nAjgeHwl0WXced5tONcCPNzLswqPOiobGeA5y4WE8iBveel30OKYVel
0lZ1OnXOmN5TIEIIPo9LrE+LzZis6A0HA1FRMtKgKGhT3N965pkqfhKbq/V48kpJKT8+c
Zs0TOn4HFMG+OIy6c9ofSBrXD68yxP6QYTz6xH0GMWawLyPLYR52j3I05fK1ylAb6K0ox
PxzQ5nwrLD+mUVPZ9rDs1En6fmOX9xfkZTblG/5D+s1fHHs9dDXCOVkT5dLS8DjdIA==

A possible Signature-Input and Signature header containing this signature is:¶

# NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig1=("@request-target" "host" "date"
        "content-type" "digest" "content-length"); keyid="test-key-a";
    alg="hs2019"; created=1402170695
Signature: sig1=:B24UG4FaiE2kSXBNKV4DA91J+mElAhS3mncrgyteAye1GKMpmzt8\
    jkHNjoudtqw3GngGY3n0mmwjdfn1eA6nAjgeHwl0WXced5tONcCPNzLswqPOiobGe\
    A5y4WE8iBveel30OKYVel0lZ1OnXOmN5TIEIIPo9LrE+LzZis6A0HA1FRMtKgKGhT\
    3N965pkqfhKbq/V48kpJKT8+cZs0TOn4HFMG+OIy6c9ofSBrXD68yxP6QYTz6xH0G\
    MWawLyPLYR52j3I05fK1ylAb6K0oxPxzQ5nwrLD+mUVPZ9rDs1En6fmOX9xfkZTbl\
    G/5D+s1fHHs9dDXCOVkT5dLS8DjdIA==:

B.3.2. Signature Verification

B.3.2.1. Minimal Required Signature Header

This presents a Signature-Input and Signature header containing only the minimal required parameters:¶

# NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig1=(); keyid="test-key-a"; created=1402170695
Signature: sig1=:cxieW5ZKV9R9A70+Ua1A/1FCvVayuE6Z77wDGNVFSiluSzR9TYFV\
    vwUjeU6CTYUdbOByGMCee5q1eWWUOM8BIH04Si6VndEHjQVdHqshAtNJk2Quzs6WC\
    2DkV0vysOhBSvFZuLZvtCmXRQfYGTGhZqGwq/AAmFbt5WNLQtDrEe0ErveEKBfaz+\
    IJ35zhaj+dun71YZ82b/CRfO6fSSt8VXeJuvdqUuVPWqjgJD4n9mgZpZFGBaDdPiw\
    pfbVZHzcHrumFJeFHWXH64a+c5GN+TWlP8NPg2zFdEc/joMymBiRelq236WGm5VvV\
    9a22RW2/yLmaU/uwf9v40yGR/I1NRA==:

The corresponding signature metadata derived from this header field is:¶

Property	Value
Algorithm	hs2019, using RSASSA-PSS using SHA-256
Covered Content	``
Creation Time	8:51:35 PM GMT, June 7th, 2014
Expiration Time	Undefined
Verification Key Material	The public key specified in Appendix B.1.1.

The corresponding Signature Input is:¶

"@signature-params": sig1=(); alg="hs2019"; keyid="test-key-a"; created=1402170695

B.3.2.2. Minimal Recommended Signature Header

This presents a Signature-Input and Signature header containing only the minimal required and recommended parameters:¶

# NOTE: '\' line wrapping per RFC 8792

Signature-Input: sig1=(); alg="hs2019"; keyid="test-key-a";
    created=1402170695
Signature: sig1=:cxieW5ZKV9R9A70+Ua1A/1FCvVayuE6Z77wDGNVFSiluSzR9TYFV\
    vwUjeU6CTYUdbOByGMCee5q1eWWUOM8BIH04Si6VndEHjQVdHqshAtNJk2Quzs6WC\
    2DkV0vysOhBSvFZuLZvtCmXRQfYGTGhZqGwq/AAmFbt5WNLQtDrEe0ErveEKBfaz+\
    IJ35zhaj+dun71YZ82b/CRfO6fSSt8VXeJuvdqUuVPWqjgJD4n9mgZpZFGBaDdPiw\
    pfbVZHzcHrumFJeFHWXH64a+c5GN+TWlP8NPg2zFdEc/joMymBiRelq236WGm5VvV\
    9a22RW2/yLmaU/uwf9v40yGR/I1NRA==:

The corresponding signature metadata derived from this header field is:¶

Property	Value
Algorithm	hs2019, using RSASSA-PSS using SHA-512
Covered Content	``
Creation Time	8:51:35 PM GMT, June 7th, 2014
Expiration Time	Undefined
Verification Key Material	The public key specified in Appendix B.1.1.

The corresponding Signature Input is:¶

"@signature-params": sig1=(); alg="rsa-sha256"; keyid="test-key-b"

B.3.2.3. Minimal Signature Header using rsa-sha256

This presents a minimal Signature-Input and Signature header for a signature using the rsa-sha256 algorithm:¶

# NOTE: '\' line wrapping per RFC 8792

Signature: sig1=("date"); alg=rsa-sha256; keyid="test-key-b"
Signature: sig1=:HtXycCl97RBVkZi66ADKnC9c5eSSlb57GnQ4KFqNZplOpNfxqk62\
    JzZ484jXgLvoOTRaKfR4hwyxlcyb+BWkVasApQovBSdit9Ml/YmN2IvJDPncrlhPD\
    VDv36Z9/DiSO+RNHD7iLXugdXo1+MGRimW1RmYdenl/ITeb7rjfLZ4b9VNnLFtVWw\
    rjhAiwIqeLjodVImzVc5srrk19HMZNuUejK6I3/MyN3+3U8tIRW4LWzx6ZgGZUaEE\
    P0aBlBkt7Fj0Tt5/P5HNW/Sa/m8smxbOHnwzAJDa10PyjzdIbywlnWIIWtZKPPsoV\
    oKVopUWEU3TNhpWmaVhFrUL/O6SN3w==:

The corresponding signature metadata derived from this header field is:¶

Property	Value
Algorithm	rsa-sha256
Covered Content	date
Creation Time	Undefined
Expiration Time	Undefined
Verification Key Material	The public key specified in Appendix B.1.1.

The corresponding Signature Input is:¶

"date": Tue, 07 Jun 2014 20:51:35 GMT
"@signature-params": ("date"); alg=rsa-sha256; keyid="test-key-b"

7. References

7.1. Normative References

7.2. Informative References