key_generator.rb

require 'openssl'

module ActiveSupport
  # KeyGenerator is a simple wrapper around OpenSSL's implementation of PBKDF2
  # It can be used to derive a number of keys for various purposes from a given secret.
  # This lets rails applications have a single secure secret, but avoid reusing that
  # key in multiple incompatible contexts.
  class KeyGenerator
    def initialize(secret, options = {})
      @secret = secret
      # The default iterations are higher than required for our key derivation uses
      # on the off chance someone uses this for password storage
      @iterations = options[:iterations] || 2**16
    end

    # Returns a derived key suitable for use.  The default key_size is chosen
    # to be compatible with the default settings of ActiveSupport::MessageVerifier.
    # i.e. OpenSSL::Digest::SHA1#block_length
    def generate_key(salt, key_size=64)
      OpenSSL::PKCS5.pbkdf2_hmac_sha1(@secret, salt, @iterations, key_size)
    end
  end
end