signed_json

Encodes and decodes data to a JSON string signed with OpenSSL HMAC. Great for signed cookies.

gem install signed_json

require 'signed_json'
s = SignedJson::Signer.new('your secret')

### encode ###
s.encode 'a string'
s.encode ['an', 'array']
s.encode :a => 'hash'

### decode ###
s.decode '["da7555389d05e04a3367b84aed401cafbbecfe3d","example"]'
# => "example"
s.decode '["da7555389d05e04a3367b84aed401cafbbecfe3d","tampered"]'
# SignedJson::SignatureError

SignedJson::Signer takes any JSON encodable data, and returns the data in a JSON string along with an HMAC signature. The output string can then be decoded back into the original data, with certainty that it was generated using the same secret.

The signature uses OpenSSL::HMAC, with the configurable digest defaulting to SHA1.

Note that the data is not encrypted - it is clearly readable, but altering the data will cause the signature verification to fail. The encoded output looks like this:

["f47bd6c4108cf503b98b82b2e36ce3e7bae712b5",["an","array","of","strings"]]

This is ideal for signed cookies, and allows client cookies to be used as a light-weight session store.

Rails already has a nice signed cookie implementation, but because ActiveSupport::MessageVerifier uses Base64 encoded Marshal.dump instead of JSON, it is barely portable between Ruby versions, let alone different platforms.

SignedJson::Signer, on the other hand, can easily be implemented in other languages, allowing for signed cookies shared between same-domain web applications, for example.

Note that the JSON-encoding must be consistent across implementations. For example in Python, separators=(',',':') must be specified to eliminate whitespace which would invalidate the HMAC digest.

Ported from my PHP implementation, which is running in high-traffic production environments.

RSpec speaks for the Ruby implementation:

$ rake spec

SignedJson
  round trip encoding/decoding
    round-trips a string
    round-trips an array of strings and ints
    round-trips a hash with string keys, string and int values
    round-trips a nested array
    round-trips a hash/array/string/int structure
  Signer#encode
    returns a string
    returns a valid JSON-encoded array
  Signer#decode error handling
    raises SignatureError for incorrect key/signature
    raises InputError for invalid input

Finished in 0.0186 seconds
9 examples, 0 failures

Tested against:

ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-linux]
rubinius 1.1.0 (1.8.7 release 2010-09-23 JI) [x86_64-unknown-linux-gnu]
ruby 1.8.7 (2010-08-16 patchlevel 302) [x86_64-linux]
jruby 1.5.3 (ruby 1.8.7 patchlevel 249) (2010-09-28 7ca06d7) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_22) [amd64-java]

(c) 2010 Paul Annesley, MIT license.