Active Support Message Encryptor

MessageEncryptor is a simple way to encrypt values which get stored somewhere you don’t trust.

The cipher text and initialization vector are base64 encoded and returned to you.

This can be used in situations similar to the MessageVerifier, but where you don’t want users to be able to determine the value of the payload.

len   = ActiveSupport::MessageEncryptor.key_len
salt  = SecureRandom.random_bytes(len)
key   = ActiveSupport::KeyGenerator.new('password').generate_key(salt, len) # => "\x89\xE0\x156\xAC..."
crypt = ActiveSupport::MessageEncryptor.new(key)                            # => #<ActiveSupport::MessageEncryptor ...>
encrypted_data = crypt.encrypt_and_sign('my secret data')                   # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
crypt.decrypt_and_verify(encrypted_data)                                    # => "my secret data"

The decrypt_and_verify method will raise an ActiveSupport::MessageEncryptor::InvalidMessage exception if the data provided cannot be decrypted or verified.

crypt.decrypt_and_verify('not encrypted data') # => ActiveSupport::MessageEncryptor::InvalidMessage

Confining messages to a specific purpose

By default any message can be used throughout your app. But they can also be confined to a specific :purpose.

token = crypt.encrypt_and_sign("this is the chair", purpose: :login)

Then that same purpose must be passed when verifying to get the data back out:

crypt.decrypt_and_verify(token, purpose: :login)    # => "this is the chair"
crypt.decrypt_and_verify(token, purpose: :shipping) # => nil
crypt.decrypt_and_verify(token)                     # => nil

Likewise, if a message has no purpose it won’t be returned when verifying with a specific purpose.

token = crypt.encrypt_and_sign("the conversation is lively")
crypt.decrypt_and_verify(token, purpose: :scare_tactics) # => nil
crypt.decrypt_and_verify(token)                          # => "the conversation is lively"

Making messages expire

By default messages last forever and verifying one year from now will still return the original value. But messages can be set to expire at a given time with :expires_in or :expires_at.

crypt.encrypt_and_sign(parcel, expires_in: 1.month)
crypt.encrypt_and_sign(doowad, expires_at: Time.now.end_of_year)

Then the messages can be verified and returned up to the expire time. Thereafter, verifying returns nil.

Rotating keys

MessageEncryptor also supports rotating out old configurations by falling back to a stack of encryptors. Call rotate to build and add an encryptor so decrypt_and_verify will also try the fallback.

By default any rotated encryptors use the values of the primary encryptor unless specified otherwise.

You’d give your encryptor the new defaults:

crypt = ActiveSupport::MessageEncryptor.new(@secret, cipher: "aes-256-gcm")

Then gradually rotate the old values out by adding them as fallbacks. Any message generated with the old values will then work until the rotation is removed.

crypt.rotate old_secret            # Fallback to an old secret instead of @secret.
crypt.rotate cipher: "aes-256-cbc" # Fallback to an old cipher instead of aes-256-gcm.

Though if both the secret and the cipher was changed at the same time, the above should be combined into:

crypt.rotate old_secret, cipher: "aes-256-cbc"

Namespace

Class

Methods

Constants

OpenSSLCipherError = OpenSSL::Cipher::CipherError

Class Public methods

key_len(cipher = default_cipher)

Given a cipher, returns the key length of the cipher to help generate the key of desired size

📝 Source code 
# File activesupport/lib/active_support/message_encryptor.rb, line 252
    def self.key_len(cipher = default_cipher)
      OpenSSL::Cipher.new(cipher).key_len
    end
🔎 See on GitHub

new(secret, sign_secret = nil, **options)

Initialize a new MessageEncryptor. secret must be at least as long as the cipher key size. For the default ‘aes-256-gcm’ cipher, this is 256 bits. If you are using a user-entered secret, you can generate a suitable key by using ActiveSupport::KeyGenerator or a similar key derivation function.

The first additional parameter is used as the signature key for MessageVerifier. This allows you to specify keys to encrypt and sign data. Ignored when using an AEAD cipher like ‘aes-256-gcm’.

ActiveSupport::MessageEncryptor.new('secret', 'signature_secret')

Options

:cipher

Cipher to use. Can be any cipher returned by OpenSSL::Cipher.ciphers. Default is ‘aes-256-gcm’.

:digest

Digest used for signing. Ignored when using an AEAD cipher like ‘aes-256-gcm’.

:serializer

The serializer used to serialize message data. You can specify any object that responds to dump and load, or you can choose from several preconfigured serializers: :marshal, :json_allow_marshal, :json, :message_pack_allow_marshal, :message_pack.

The preconfigured serializers include a fallback mechanism to support multiple deserialization formats. For example, the :marshal serializer will serialize using Marshal, but can deserialize using Marshal, ActiveSupport::JSON, or ActiveSupport::MessagePack. This makes it easy to migrate between serializers.

The :marshal, :json_allow_marshal, and :message_pack_allow_marshal serializers support deserializing using Marshal, but the others do not. Beware that Marshal is a potential vector for deserialization attacks in cases where a message signing secret has been leaked. If possible, choose a serializer that does not support Marshal.

The :message_pack and :message_pack_allow_marshal serializers use ActiveSupport::MessagePack, which can roundtrip some Ruby types that are not supported by JSON, and may provide improved performance. However, these require the msgpack gem.

When using Rails, the default depends on config.active_support.message_serializer. Otherwise, the default is :marshal.

:url_safe

By default, MessageEncryptor generates RFC 4648 compliant strings which are not URL-safe. In other words, they can contain “+” and “/”. If you want to generate URL-safe strings (in compliance with “Base 64 Encoding with URL and Filename Safe Alphabet” in RFC 4648), you can pass true.

:force_legacy_metadata_serializer

Whether to use the legacy metadata serializer, which serializes the message first, then wraps it in an envelope which is also serialized. This was the default in Rails 7.0 and below.

If you don’t pass a truthy value, the default is set using config.active_support.use_message_serializer_for_metadata.

📝 Source code 
# File activesupport/lib/active_support/message_encryptor.rb, line 183
    def initialize(secret, sign_secret = nil, **options)
      super(**options)
      @secret = secret
      @cipher = options[:cipher] || self.class.default_cipher
      @aead_mode = new_cipher.authenticated?
      @verifier = if !@aead_mode
        MessageVerifier.new(sign_secret || secret, **options, serializer: NullSerializer)
      end
    end
🔎 See on GitHub

Instance Public methods

decrypt_and_verify(message, **options)

Decrypt and verify a message. We need to verify the message in order to avoid padding attacks. Reference: www.limited-entropy.com/padding-oracle-attacks/.

Options

:purpose

The purpose that the message was generated with. If the purpose does not match, decrypt_and_verify will return nil.

message = encryptor.encrypt_and_sign("hello", purpose: "greeting")
encryptor.decrypt_and_verify(message, purpose: "greeting") # => "hello"
encryptor.decrypt_and_verify(message)                      # => nil

message = encryptor.encrypt_and_sign("bye")
encryptor.decrypt_and_verify(message)                      # => "bye"
encryptor.decrypt_and_verify(message, purpose: "greeting") # => nil
📝 Source code 
# File activesupport/lib/active_support/message_encryptor.rb, line 241
    def decrypt_and_verify(message, **options)
      catch_and_raise :invalid_message_format, as: InvalidMessage do
        catch_and_raise :invalid_message_serialization, as: InvalidMessage do
          catch_and_ignore :invalid_message_content do
            read_message(message, **options)
          end
        end
      end
    end
🔎 See on GitHub

encrypt_and_sign(value, **options)

Encrypt and sign a message. We need to sign the message in order to avoid padding attacks. Reference: www.limited-entropy.com/padding-oracle-attacks/.

Options

:expires_at

The datetime at which the message expires. After this datetime, verification of the message will fail.

message = encryptor.encrypt_and_sign("hello", expires_at: Time.now.tomorrow)
encryptor.decrypt_and_verify(message) # => "hello"
# 24 hours later...
encryptor.decrypt_and_verify(message) # => nil
:expires_in

The duration for which the message is valid. After this duration has elapsed, verification of the message will fail.

message = encryptor.encrypt_and_sign("hello", expires_in: 24.hours)
encryptor.decrypt_and_verify(message) # => "hello"
# 24 hours later...
encryptor.decrypt_and_verify(message) # => nil
:purpose

The purpose of the message. If specified, the same purpose must be specified when verifying the message; otherwise, verification will fail. (See decrypt_and_verify.)

📝 Source code 
# File activesupport/lib/active_support/message_encryptor.rb, line 220
    def encrypt_and_sign(value, **options)
      create_message(value, **options)
    end
🔎 See on GitHub