class ActiveSupport::MessageEncryptor

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"

Constants

OpenSSLCipherError

Attributes

verifier[R]

Public Class Methods

key_len(cipher = default_cipher) click to toggle source

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

# File lib/active_support/message_encryptor.rb, line 164
def self.key_len(cipher = default_cipher)
  OpenSSL::Cipher.new(cipher).key_len
end
new(secret, sign_secret = nil, cipher: nil, digest: nil, serializer: nil) click to toggle source

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 lib/active_support/message_encryptor.rb, line 142
def initialize(secret, sign_secret = nil, cipher: nil, digest: nil, serializer: nil)
  @secret = secret
  @sign_secret = sign_secret
  @cipher = cipher || self.class.default_cipher
  @digest = digest || "SHA1" unless aead_mode?
  @verifier = resolve_verifier
  @serializer = serializer || Marshal
end

Public Instance Methods

decrypt_and_verify(data, purpose: nil, **) click to toggle source

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 lib/active_support/message_encryptor.rb, line 159
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) click to toggle source

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 lib/active_support/message_encryptor.rb, line 153
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

Private Instance Methods

_decrypt(encrypted_message, purpose) click to toggle source
# File lib/active_support/message_encryptor.rb, line 186
def _decrypt(encrypted_message, purpose)
  cipher = new_cipher
  encrypted_data, iv, auth_tag = encrypted_message.split("--").map { |v| ::Base64.strict_decode64(v) }

  # Currently the OpenSSL bindings do not raise an error if auth_tag is
  # truncated, which would allow an attacker to easily forge it. See
  # https://github.com/ruby/openssl/issues/63
  raise InvalidMessage if aead_mode? && (auth_tag.nil? || auth_tag.bytes.length != 16)

  cipher.decrypt
  cipher.key = @secret
  cipher.iv  = iv
  if aead_mode?
    cipher.auth_tag = auth_tag
    cipher.auth_data = ""
  end

  decrypted_data = cipher.update(encrypted_data)
  decrypted_data << cipher.final

  message = Messages::Metadata.verify(decrypted_data, purpose)
  @serializer.load(message) if message
rescue OpenSSLCipherError, TypeError, ArgumentError
  raise InvalidMessage
end
_encrypt(value, **metadata_options) click to toggle source
# File lib/active_support/message_encryptor.rb, line 169
def _encrypt(value, **metadata_options)
  cipher = new_cipher
  cipher.encrypt
  cipher.key = @secret

  # Rely on OpenSSL for the initialization vector
  iv = cipher.random_iv
  cipher.auth_data = "" if aead_mode?

  encrypted_data = cipher.update(Messages::Metadata.wrap(@serializer.dump(value), **metadata_options))
  encrypted_data << cipher.final

  blob = "#{::Base64.strict_encode64 encrypted_data}--#{::Base64.strict_encode64 iv}"
  blob = "#{blob}--#{::Base64.strict_encode64 cipher.auth_tag}" if aead_mode?
  blob
end
aead_mode?() click to toggle source
# File lib/active_support/message_encryptor.rb, line 218
def aead_mode?
  @aead_mode ||= new_cipher.authenticated?
end
new_cipher() click to toggle source
# File lib/active_support/message_encryptor.rb, line 212
def new_cipher
  OpenSSL::Cipher.new(@cipher)
end
resolve_verifier() click to toggle source
# File lib/active_support/message_encryptor.rb, line 222
def resolve_verifier
  if aead_mode?
    NullVerifier
  else
    MessageVerifier.new(@sign_secret || @secret, digest: @digest, serializer: NullSerializer)
  end
end