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"