class ActiveSupport::KeyGenerator

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