HTTP Signatures

This protocol is intended to provide a standard way for clients to sign HTTP requests. RFC 2617 (HTTP Authentication) defines Basic and Digest authentication mechanisms, and RFC 5246 (TLS 1.2) defines client-auth, both of which are widely employed on the Internet today. However, it is common place that the burdens of PKI prevent web service operators from deploying that methodoloy, and so many fall back to Basic authentication, which has poor security characteristics. Additionally, OAuth provides a fully-specified alternative for authorization of web service requests, but is not (always) ideal for machine to machine communication, as the key acquisition steps (generally) imply a fixed infrastructure that may not make sense to a service provider (e.g., symmetric keys). Several web service providers have invented their own schemes for signing HTTP requests, but to date, none have been placed in the public domain as a standard. This document serves that purpose. There are no techniques in this proposal that are novel beyond previous art, however, this aims to be a simple mechanism for signing these requests.

The "signature" authentication scheme is based on the model that the client must authenticate itself with a digital signature produced by either a private asymmetric key (e.g., RSA) or a shared symmetric key (e.g., HMAC). The scheme is parameterized enough such that it is not bound to any particular key type or signing algorithm. However, it does explicitly assume that clients can send an HTTP `Date` header.

The client is expected to send an Authorization header (as defined in RFC 2617) with the following parameterization:

credentials := "Signature" SP params params := keyId "," algorithm [", " headers] [", " ext] ", " signature keyId := "keyId" "=" plain-string algorithm := "algorithm" "=" DQUOTE ( rsa-sha1 / rsa-sha256 / rsa-sha512 / dsa-sha1 / hmac-sha1 / hmac-sha256 / hmac-sha512 ) DQUOTE headers := "headers" "=" plain-string ext := "ext" "=" plain-string signature := "signature" "=" plain-string plain-string = DQUOTE *( %x20-21 / %x23-5B / %x5D-7E ) DQUOTE

The following section details the signature parameters of the Authorization Header.

REQUIRED. The `keyId` field is an opaque string that the server can use to look up the component they need to validate the signature. It could be an SSH key fingerprint, an LDAP DN, etc. Management of keys and assignment of `keyId` is out of scope for this document.

REQUIRED. The `algorithm` parameter is used if the client and server agree on a non-standard digital signature algorithm. The full list of supported signature mechanisms is listed below.

OPTIONAL. The `headers` parameter is used to specify the list of HTTP headers used to sign the request. If specified, it should be a quoted list of HTTP header names, separated by a single space character. By default, only one HTTP header is signed, which is the `Date` header. Note that the list MUST be specified in the order the values are concatenated together during signing. To include the HTTP request line in the signature calculation, use the special `request-line` value. While this is overloading the definition of `headers` in HTTP linguism, the request-line is defined in RFC 2616, and as the outlier from headers in useful signature calculation, it is deemed simpler to use `request-line` than to add a separate parameter for it.

OPTIONAL. The `extensions` parameter is used to include additional information which is covered by the request. The content and format of the string is out of scope for this document, and expected to be specified by implementors.

REQUIRED. The `signature` parameter is a `Base64` encoded digital signature generated by the client. The client uses the `algorithm` and `headers` request parameters to form a canonicalized `signing string`. This `signing string` is then signed with the key associated with `keyId` and the algorithm corresponding to `algorithm`. The `signature` parameter is then set to the `Base64` encoding of the signature.

In order to generate the string that is signed with a key, the client MUST take the values of each HTTP header specified by `headers` in the order they appear. It is out of scope for this document to dictate what headers a service provider will want to enforce, but service providers SHOULD at minimum include the request line, Host, and Date headers. If the header name is not `request-line` then append the lowercased header name followed with an ASCII colon `:` and an ASCII space ` `. If the header name is `request-line` then appened the HTTP request line, otherwise append the header value. If value is not the last value then append an ASCII newline `\n`. The string MUST NOT include a trailing ASCII newline. The rest of this section uses the following HTTP request as an example.

POST /foo HTTP/1.1 Host: example.org Date: Tue, 07 Jun 2014 20:51:35 GMT Content-Type: application/json Digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= Content-Length: 18 {"hello": "world"} The following sections also assume that the "rsa-key-1" keyId refers to a private key known to the client and a public key known to the server. The "hmac-key-1" keyId refers to key known to the client and server.

The authorization header and signature would be generated as:

Authorization: Signature keyId="rsa-key-1",algorithm="rsa-sha256", headers="request-line host date digest content-length", signature="Base64(RSA-SHA256(signing string))" The client would compose the signing string as:

POST /foo HTTP/1.1\n host: example.org\n date: Tue, 07 Jun 2014 20:51:35 GMT\n digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=\n content-length: 18 Note that the '\n' symbols above are included to demonstrate where the new line character should be inserted. There is no new line on the final line of the signing string. For an RSA-based signature, the authorization header and signature would then be generated as:

Authorization: Signature keyId="rsa-key-1",algorithm="rsa-sha256", headers="request-line host date digest content-length", signature="Base64(RSA-SHA256(signing string))"

For an HMAC-based signature without a list of headers specified, the authorization header and signature would be generated as:

Authorization: Signature keyId="hmac-key-1",algorithm="hmac-sha1", headers="request-line host date digest content-length", signature="Base64(HMAC-SHA1(signing string))" The only difference between the RSA Example and the HMAC Example is the signature algorithm that is used. The client would compose the signing string in the same way as the RSA Example above:

POST /foo HTTP/1.1\n host: example.org\n date: Tue, 07 Jun 2014 20:51:35 GMT\n digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=\n content-length: 18

There are a number of security considerations to take into account when implementing or utilizing this specification. A thorough security analysis of this protocol, including its strengths and weaknesses, can be found in Security Considerations for HTTP Signatures.

This specification was designed to be simple, modular, and extensible. There are a number of other specifications that build on this one. For example, the HTTP Signature Nonces specification details how to use HTTP Signatures over a non-secured channel like HTTP and the HTTP Signature Trailers specification explains how to apply HTTP Signatures to streaming content. Developers that desire more functionality than this specification provides are urged to ensure that an extension specification doesn't already exist before implementing a proprietary extension.

The following test data uses the following RSA 2048-bit keys, which we will refer to as `keyId=Test` in the following samples:

POST /foo?param=value&pet=dog HTTP/1.1 Host: example.com Date: Thu, 05 Jan 2014 21:31:40 GMT Content-Type: application/json Digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= Content-Length: 18 {"hello": "world"}

If a list of headers is not included, the date is the only header that is signed by default. The string to sign would be:

date: Thu, 05 Jan 2014 21:31:40 GMT The Authorization header would be:

Authorization: Signature keyId="Test",algorithm="rsa-sha256", signature="ATp0r26dbMIxOopqw0OfABDT7CKMIoENumuruOtarj8n/97Q3htH FYpH8yOSQk3Z5zh8UxUym6FYTb5+A0Nz3NRsXJibnYi7brE/4tx5But9kkFGzG+ xpUmimN4c3TMN7OFH//+r8hBf7BT9/GmHDUVZT2JzWGLZES2xDOUuMtA="

The minimum recommended data to sign is the request-line, host, and date. In this case, the string to sign would be:

POST /foo?param=value&pet=dog HTTP/1.1 host: example.com date: Thu, 05 Jan 2014 21:31:40 GMT The Authorization header would be:

Authorization: Signature keyId="Test",algorithm="rsa-sha256", headers="request-line host date", signature="KcLSABBj/m3v2Dhxi CKJmzYJvnx74tDO1SaURD8Dr8XpugN5wpy8iBVJtpkHUIp4qBYpzx2QvD16t8X 0BUMiKc53Age+baQFWwb2iYYJzvuUL+krrl/Q7H6fPBADBsHqEZ7IE8rR0Ys3l b7J5A6VB9J/4yVTRiBcxTypW/mpr5w="

A strong signature including all of the headers and a digest of the body of the HTTP request would result in the following signing string:

POST /foo?param=value&pet=dog HTTP/1.1 host: example.com date: Thu, 05 Jan 2014 21:31:40 GMT content-type: application/json digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE= content-length: 18 The Authorization header would be:

Authorization: Signature keyId="Test",algorithm="rsa-sha256", headers="request-line host date content-type digest content-length", signature="jgSqYK0yKclIHfF9zdApVEbDp5eqj8C4i4X76pE+XHoxugXv7q nVrGR+30bmBgtpR39I4utq17s9ghz/2QFVxlnToYAvbSVZJ9ulLd1HQBugO0j Oyn9sXOtcN7uNHBjqNCqUsnt0sw/cJA6B6nJZpyNqNyAXKdxZZItOuhIs78w="