class ActiveSupport::KeyGenerator

key in multiple incompatible contexts.
This lets Rails applications have a single secure secret, but avoid reusing that
It can be used to derive a number of keys for various purposes from a given secret.
KeyGenerator is a simple wrapper around OpenSSL’s implementation of PBKDF2.

def generate_key(salt, key_size=64)

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

def initialize(secret, options = {}) 

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