# Abstract

 

This document describes a way to add origin authentication, message integrity,

and replay resistance to HTTP REST requests.  It is intended to be used over

the HTTPS protocol.

 

# Copyright Notice

 

Copyright (c) 2011 Joyent, Inc. and the persons identified as document authors.

All rights reserved.

 

Code Components extracted from this document must include MIT License text.

 

# Introduction

 

This protocol is intended to provide a standard way for clients to sign HTTP

requests.  RFC2617 (HTTP Authentication) defines Basic and Digest authentication

mechanisms, and RFC5246 (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.

 

# Signature Authentication Scheme

 

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.

 

## Authorization Header

 

The client is expected to send an Authorization header (as defined in RFC 2617)

with the following parameterization:

 

    credentials := "Signature" params

    params := 1#(keyId | algorithm | [headers] | [ext] | signature)

    digitalSignature := plain-string

 

    keyId := "keyId" "=" <"> plain-string <">

    algorithm := "algorithm" "=" <"> plain-string <">

    headers := "headers" "=" <"> 1#headers-value <">

    ext := "ext" "=" <"> plain-string <">

    signature := "signature" "=" <"> plain-string <">

 

    headers-value := plain-string

    plain-string   = 1*( %x20-21 / %x23-5B / %x5D-7E )

 

### Signature Parameters

 

#### keyId

 

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.

 

#### algorithm

 

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.

 

#### headers

 

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 simply use

`request-line` than to add a separate parameter for it.

 

#### extensions

 

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.

 

#### signature

 

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.

 

### Signing String Composition

 

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.

 

1. If the header name is not `request-line` then append the lowercased header

   name followed with an ASCII colon `:` and an ASCII space ` `.

2. If the header name is `request-line` then appened the HTTP request line,

   otherwise append the header value.

3. If value is not the last value then append an ASCII newline `\n`. The string

   MUST NOT include a trailing ASCII newline.

 

# Example Requests

 

All requests refer to the following request (body ommitted):

 

    POST /foo HTTP/1.1

    Host: example.org

    Date: Tue, 07 Jun 2011 20:51:35 GMT

    Content-Type: application/json

    Content-MD5: h0auK8hnYJKmHTLhKtMTkQ==

    Content-Length: 123

 

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.

 

## Default parameterization

 

The authorization header and signature would be generated as:

 

    Authorization: Signature keyId="rsa-key-1",algorithm="rsa-sha256",signature="Base64(RSA-SHA256(signing string))"

 

The client would compose the signing string as:

 

    date: Tue, 07 Jun 2011 20:51:35 GMT

 

## Header List

 

The authorization header and signature would be generated as:

 

    Authorization: Signature keyId="rsa-key-1",algorithm="rsa-sha256",headers="request-line date content-type content-md5",signature="Base64(RSA-SHA256(signing string))"

 

The client would compose the signing string as (`+ "\n"` inserted for

readability):

 

    POST /foo HTTP/1.1 + "\n"

    date: Tue, 07 Jun 2011 20:51:35 GMT + "\n"

    content-type: application/json + "\n"

    content-md5: h0auK8hnYJKmHTLhKtMTkQ==

 

## Algorithm

 

The authorization header and signature would be generated as:

 

    Authorization: Signature keyId="hmac-key-1",algorithm="hmac-sha1",signature="Base64(HMAC-SHA1(signing string))"

 

The client would compose the signing string as:

 

    date: Tue, 07 Jun 2011 20:51:35 GMT

 

# Signing Algorithms

 

Currently supported algorithm names are:

 

* rsa-sha1

* rsa-sha256

* rsa-sha512

* dsa-sha1

* hmac-sha1

* hmac-sha256

* hmac-sha512

 

# Security Considerations

 

## Default Parameters

 

Note the default parameterization of the `Signature` scheme is only safe if all

requests are carried over a secure transport (i.e., TLS).  Sending the default

scheme over a non-secure transport will leave the request vulnerable to

spoofing, tampering, replay/repudiaton, and integrity violations (if using the

STRIDE threat-modeling methodology).

 

## Insecure Transports

 

If sending the request over plain HTTP, service providers SHOULD require clients

to sign ALL HTTP headers, and the `request-line`.  Additionally, service

providers SHOULD require `Content-MD5` calculations to be performed to ensure

against any tampering from clients.

 

## Nonces

 

Nonces are out of scope for this document simply because many service providers

fail to implement them correctly, or do not adopt security specfiications

because of the infrastructure complexity.  Given the `header` parameterization,

a service provider is fully enabled to add nonce semantics into this scheme by

using something like an `x-request-nonce` header, and ensuring it is signed

with the `Date` header.

 

## Clock Skew

 

As the default scheme is to sign the `Date` header, service providers SHOULD

protect against logged replay attacks by enforcing a clock skew.  The server

SHOULD be synchronized with NTP, and the recommendation in this specification

is to allow 300s of clock skew (in either direction).

 

## Required Headers to Sign

 

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

`Date` header.

 

# References

 

## Normative References

 

* [RFC2616] Hypertext Transfer Protocol -- HTTP/1.1

* [RFC2617] HTTP Authentication: Basic and Digest Access Authentication

* [RFC5246] The Transport Layer Security (TLS) Protocol Version 1.2

 

## Informative References

 

    Name: Mark Cavage (editor)

    Company: Joyent, Inc.

    Email: [email protected]

    URI: http://www.joyent.com

 

# Appendix A - Test Values

 

The following test data uses the RSA (2048b) keys, which we will refer

to as `keyId=Test` in the following samples:

 

   -----BEGIN PUBLIC KEY-----

   MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDCFENGw33yGihy92pDjZQhl0C3

   6rPJj+CvfSC8+q28hxA161QFNUd13wuCTUcq0Qd2qsBe/2hFyc2DCJJg0h1L78+6

   Z4UMR7EOcpfdUE9Hf3m/hs+FUR45uBJeDK1HSFHD8bHKD6kv8FPGfJTotc+2xjJw

   oYi+1hqp1fIekaxsyQIDAQAB

   -----END PUBLIC KEY-----

 

    -----BEGIN RSA PRIVATE KEY-----

    MIICXgIBAAKBgQDCFENGw33yGihy92pDjZQhl0C36rPJj+CvfSC8+q28hxA161QF

    NUd13wuCTUcq0Qd2qsBe/2hFyc2DCJJg0h1L78+6Z4UMR7EOcpfdUE9Hf3m/hs+F

    UR45uBJeDK1HSFHD8bHKD6kv8FPGfJTotc+2xjJwoYi+1hqp1fIekaxsyQIDAQAB

    AoGBAJR8ZkCUvx5kzv+utdl7T5MnordT1TvoXXJGXK7ZZ+UuvMNUCdN2QPc4sBiA

    QWvLw1cSKt5DsKZ8UETpYPy8pPYnnDEz2dDYiaew9+xEpubyeW2oH4Zx71wqBtOK

    kqwrXa/pzdpiucRRjk6vE6YY7EBBs/g7uanVpGibOVAEsqH1AkEA7DkjVH28WDUg

    f1nqvfn2Kj6CT7nIcE3jGJsZZ7zlZmBmHFDONMLUrXR/Zm3pR5m0tCmBqa5RK95u

    412jt1dPIwJBANJT3v8pnkth48bQo/fKel6uEYyboRtA5/uHuHkZ6FQF7OUkGogc

    mSJluOdc5t6hI1VsLn0QZEjQZMEOWr+wKSMCQQCC4kXJEsHAve77oP6HtG/IiEn7

    kpyUXRNvFsDE0czpJJBvL/aRFUJxuRK91jhjC68sA7NsKMGg5OXb5I5Jj36xAkEA

    gIT7aFOYBFwGgQAQkWNKLvySgKbAZRTeLBacpHMuQdl1DfdntvAyqpAZ0lY0RKmW

    G6aFKaqQfOXKCyWoUiVknQJAXrlgySFci/2ueKlIE1QqIiLSZ8V8OlpFLRnb1pzI

    7U1yQXnTAEFYM560yJlzUpOb1V4cScGd365tiSMvxLOvTA==

    -----END RSA PRIVATE KEY-----

 

And all examples use this request:

 

    POST /foo?param=value&pet=dog HTTP/1.1

    Host: example.com

    Date: Thu, 05 Jan 2012 21:31:40 GMT

    Content-Type: application/json

    Content-MD5: Sd/dVLAcvNLSq16eXua5uQ==

    Content-Length: 18

 

    {"hello": "world"}

 

### Default

 

The string to sign would be:

 

    date: Thu, 05 Jan 2012 21:31:40 GMT

 

The Authorization header would be:

 

    Authorization: Signature keyId="Test",algorithm="rsa-sha256",signature="JldXnt8W9t643M2Sce10gqCh/+E7QIYLiI+bSjnFBGCti7s+mPPvOjVb72sbd1FjeOUwPTDpKbrQQORrm+xBYfAwCxF3LBSSzORvyJ5nRFCFxfJ3nlQD6Kdxhw8wrVZX5nSem4A/W3C8qH5uhFTRwF4ruRjh+ENHWuovPgO/HGQ="

 

### All Headers

 

Parameterized to include all headers, the string to sign would be (`+ "\n"`

inserted for readability):

 

    POST /foo?param=value&pet=dog HTTP/1.1 + "\n"

    host: example.com + "\n"

    date: Thu, 05 Jan 2012 21:31:40 GMT + "\n"

    content-type: application/json + "\n"

    content-md5: Sd/dVLAcvNLSq16eXua5uQ== + "\n"

    content-length: 18

 

The Authorization header would be:

 

    Authorization: Signature keyId="Test",algorithm="rsa-sha256",headers="request-line host date content-type content-md5 content-length",signature="Gm7W/r+e90REDpWytALMrft4MqZxCmslOTOvwJX17ViEBA5E65QqvWI0vIH3l/vSsGiaMVmuUgzYsJLYMLcm5dGrv1+a+0fCoUdVKPZWHyImQEqpLkopVwqEH67LVECFBqFTAKlQgBn676zrfXQbb+b/VebAsNUtvQMe6cTjnDY="