class ActiveSupport::MessageVerifier

MessageVerifier makes it easy to generate and verify messages which are signed to prevent tampering.

This is useful for cases like remember-me tokens and auto-unsubscribe links where the session store isn't suitable or available.

Remember Me:

cookies[:remember_me] = @verifier.generate([@user.id, 2.weeks.from_now])

In the authentication filter:

id, time = @verifier.verify(cookies[:remember_me])
if time < Time.now
  self.current_user = User.find(id)
end

By default it uses Marshal to serialize the message. If you want to use another serialization method, you can set the serializer attribute to something that responds to dump and load, e.g.:

@verifier.serializer = YAML

Public Class Methods

new(secret, options = {}) click to toggle source
# File lib/active_support/message_verifier.rb, line 30
def initialize(secret, options = {})
  unless options.is_a?(Hash)
    ActiveSupport::Deprecation.warn "The second parameter should be an options hash. Use :digest => 'algorithm' to specify the digest algorithm."
    options = { :digest => options }
  end

  @secret = secret
  @digest = options[:digest] || 'SHA1'
  @serializer = options[:serializer] || Marshal
end

Public Instance Methods

generate(value) click to toggle source
# File lib/active_support/message_verifier.rb, line 52
def generate(value)
  data = ::Base64.strict_encode64(@serializer.dump(value))
  "#{data}--#{generate_digest(data)}"
end
verify(signed_message) click to toggle source
# File lib/active_support/message_verifier.rb, line 41
def verify(signed_message)
  raise InvalidSignature if signed_message.blank?

  data, digest = signed_message.split("--")
  if data.present? && digest.present? && secure_compare(digest, generate_digest(data))
    @serializer.load(::Base64.decode64(data))
  else
    raise InvalidSignature
  end
end

Private Instance Methods

generate_digest(data) click to toggle source
# File lib/active_support/message_verifier.rb, line 69
def generate_digest(data)
  require 'openssl' unless defined?(OpenSSL)
  OpenSSL::HMAC.hexdigest(OpenSSL::Digest.const_get(@digest).new, @secret, data)
end
secure_compare(a, b) click to toggle source

constant-time comparison algorithm to prevent timing attacks

# File lib/active_support/message_verifier.rb, line 59
def secure_compare(a, b)
  return false unless a.bytesize == b.bytesize

  l = a.unpack "C#{a.bytesize}"

  res = 0
  b.each_byte { |byte| res |= byte ^ l.shift }
  res == 0
end