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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"

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 upto 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
Methods
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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

# File activesupport/lib/active_support/message_encryptor.rb, line 159
def self.key_len(cipher = default_cipher)
  OpenSSL::Cipher.new(cipher).key_len
end
new(secret, *signature_key_or_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.

First additional parameter is used as the signature key for MessageVerifier. This allows you to specify keys to encrypt and sign data.

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 - String of digest to use for signing. Default is SHA1. Ignored when using an AEAD cipher like 'aes-256-gcm'.

  • :serializer - Object serializer to use. Default is Marshal.

# File activesupport/lib/active_support/message_encryptor.rb, line 135
def initialize(secret, *signature_key_or_options)
  options = signature_key_or_options.extract_options!
  sign_secret = signature_key_or_options.first
  @secret = secret
  @sign_secret = sign_secret
  @cipher = options[:cipher] || self.class.default_cipher
  @digest = options[:digest] || "SHA1" unless aead_mode?
  @verifier = resolve_verifier
  @serializer = options[:serializer] || Marshal
end
Instance Public methods
decrypt_and_verify(data, purpose: nil, **)

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/.

# File activesupport/lib/active_support/message_encryptor.rb, line 154
def decrypt_and_verify(data, purpose: nil, **)
  _decrypt(verifier.verify(data), purpose)
end
encrypt_and_sign(value, expires_at: nil, expires_in: nil, purpose: nil)

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/.

# File activesupport/lib/active_support/message_encryptor.rb, line 148
def encrypt_and_sign(value, expires_at: nil, expires_in: nil, purpose: nil)
  verifier.generate(_encrypt(value, expires_at: expires_at, expires_in: expires_in, purpose: purpose))
end