# 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 methodology, 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 append the HTTP request line,
     otherwise append the header value.
  3. If value is not the last value then append an ASCII newline `
  `. The string
     MUST NOT include a trailing ASCII newline.
  
  # Example Requests
  
  All requests refer to the following request (body omitted):
  
      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
  
  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 2014 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-target) date content-type digest",signature="Base64(RSA-SHA256(signing string))"
  
  The client would compose the signing string as (`+ "
  "` inserted for
  readability):
  
      (request-target) post /foo + "
  "
      date: Tue, 07 Jun 2011 20:51:35 GMT + "
  "
      content-type: application/json + "
  "
      digest: SHA-256=Base64(SHA256(Body))
  
  ## 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/repudiation, 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 specifications
  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: mark.cavage@joyent.com
      URI: http://www.joyent.com
  
  # Appendix A - Test Values
  
  The following test data uses the RSA (1024b) 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:
  
  <!-- httpreq -->
  
      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"}
  
  <!-- /httpreq -->
  
  ### Default
  
  The string to sign would be:
  
  <!-- sign {"name": "Default", "options": {"keyId":"Test", "algorithm": "rsa-sha256"}} -->
  <!-- signstring -->
  
      date: Thu, 05 Jan 2014 21:31:40 GMT
  
  <!-- /signstring -->
  
  The Authorization header would be:
  
  <!-- authz -->
  
      Authorization: Signature keyId="Test",algorithm="rsa-sha256",headers="date",signature="jKyvPcxB4JbmYY4mByyBY7cZfNl4OW9HpFQlG7N4YcJPteKTu4MWCLyk+gIr0wDgqtLWf9NLpMAMimdfsH7FSWGfbMFSrsVTHNTk0rK3usrfFnti1dxsM4jl0kYJCKTGI/UWkqiaxwNiKqGcdlEDrTcUhhsFsOIo8VhddmZTZ8w="
  
  <!-- /authz -->
  
  ### All Headers
  
  Parameterized to include all headers, the string to sign would be (`+ "
  "`
  inserted for readability):
  
  <!-- sign {"name": "All Headers", "options": {"keyId":"Test", "algorithm": "rsa-sha256", "headers": ["(request-target)", "host", "date", "content-type", "digest", "content-length"]}} -->
  <!-- signstring -->
  
      (request-target): post /foo?param=value&pet=dog
      host: example.com
      date: Thu, 05 Jan 2014 21:31:40 GMT
      content-type: application/json
      digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
      content-length: 18
  
  <!-- /signstring -->
  
  The Authorization header would be:
  
  <!-- authz -->
  
      Authorization: Signature keyId="Test",algorithm="rsa-sha256",headers="(request-target) host date content-type digest content-length",signature="Ef7MlxLXoBovhil3AlyjtBwAL9g4TN3tibLj7uuNB3CROat/9KaeQ4hW2NiJ+pZ6HQEOx9vYZAyi+7cmIkmJszJCut5kQLAwuX+Ms/mUFvpKlSo9StS2bMXDBNjOh4Auj774GFj4gwjS+3NhFeoqyr/MuN6HsEnkvn6zdgfE2i0="
  
  <!-- /authz -->
  
  ## Generating and verifying signatures using `openssl`
  
  The `openssl` commandline tool can be used to generate or verify the signatures listed above.
  
  Compose the signing string as usual, and pipe it into the the `openssl dgst` command, then into `openssl enc -base64`, as follows:
  
      $ printf 'date: Thu, 05 Jan 2014 21:31:40 GMT' | \
        openssl dgst -binary -sign /path/to/private.pem -sha256 | \
        openssl enc -base64
      jKyvPcxB4JbmYY4mByyBY7cZfNl4OW9Hp...
      $
  
  The `-sha256` option is necessary to produce an `rsa-sha256` signature. You can select other hash algorithms such as `sha1` by changing this argument.
  
  To verify a signature, first save the signature data, Base64-decoded, into a file, then use `openssl dgst` again with the `-verify` option:
  
      $ echo 'jKyvPcxB4JbmYY4mByy...' | openssl enc -A -d -base64 > signature
      $ printf 'date: Thu, 05 Jan 2014 21:31:40 GMT' | \
        openssl dgst -sha256 -verify /path/to/public.pem -signature ./signature
      Verified OK
      $
  
  ## Generating and verifying signatures using `sshpk-sign`
  
  You can also generate and check signatures using the `sshpk-sign` tool which is
  included with the `sshpk` package in `npm`.
  
  Compose the signing string as above, and pipe it into `sshpk-sign` as follows:
  
      $ printf 'date: Thu, 05 Jan 2014 21:31:40 GMT' | \
        sshpk-sign -i /path/to/private.pem
      jKyvPcxB4JbmYY4mByyBY7cZfNl4OW9Hp...
      $
  
  This will produce an `rsa-sha256` signature by default, as you can see using
  the `-v` option:
  
      sshpk-sign: using rsa-sha256 with a 1024 bit key
  
  You can also use `sshpk-verify` in a similar manner:
  
      $ printf 'date: Thu, 05 Jan 2014 21:31:40 GMT' | \
        sshpk-verify -i ./public.pem -s 'jKyvPcxB4JbmYY...'
      OK
      $