decrypt_signed_cookies.rb

# This is decryption of signed cookies (note that doesn't apply for encrypted cookies)
# Only for rails 4.2
SIGNED_COOKIE_SALT = 'signed cookie' # it's just constant in rails code.
secret_key_base = ''
key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000)
secret = key_generator.generate_key(SIGNED_COOKIE_SALT)
verifier = ActiveSupport::MessageVerifier.new(
                      secret, 
                      digest: 'SHA1', 
                      serializer: ActiveSupport::MessageEncryptor::NullSerializer
                    )
verifier.verify(URI.decode(cookie))