Deterministic Authenticated Encryption with Associated Data (Deterministic AEAD)

Deterministic AEAD provides encryption with a deterministic property: encrypting the same data always yields the same ciphertext. This type of encryption is useful for key wrapping or for some schemes for searching on encrypted data (see RFC 5297, Section 1.3 for more info). Because of its deterministic property, implementations of this primitive can lead to loss of secrecy because an attacker only needs to find out the ciphertext for a given message to identify other instances of that message.

Deterministic AEAD has the following properties:

• Secrecy: Nothing about the plaintext is known, except its length and the equality of repeated plaintexts.
• Authenticity: It is impossible to change the encrypted plaintext underlying the ciphertext without being detected.
• Symmetric: Encrypting the plaintext and decrypting the ciphertext is done with the same key.
• Deterministic: As long as the primary key is not changed, encrypting a plaintext twice with the same parameters results in the same ciphertext.

Note: A deterministic AEAD protects data almost as well as a normal AEAD. However, if you send the same message twice, an attacker can notice that the two messages are equal. If this is not the intended behaviour, see AEAD.

Associated data

Caution: Associated data is authenticated but NOT encrypted.

Deterministic AEAD can also be used to tie ciphertext to specific associated data. Suppose you have a database with the fields user-id and encrypted-medical-history. In this scenario, user-id can be used as associated data when encrypting encrypted-medical-history. This prevents an attacker from moving medical history from one user to another.

Choose a key type

We recommend the AES256_SIV key type for all use cases.

Note: Tink doesn't offer AES128_SIV because it doesn't provide 128-bit security in multi-user scenarios.

Security guarantees

• At least 80-bit authentication strength.
• The plaintext and associated data can have arbitrary lengths (within the range 0..2³² bytes).
• 128-bit security level against key recovery attacks, and also in multi-user attacks with up to 2³² keys — that means if an adversary obtains 2³² ciphertexts of the same message encrypted under 2³² keys, they need to do 2¹²⁸ computations to obtain a single key.
• The ability to safely encrypt 2³⁸ messages, provided each is less than 1MB in length.

Caution: Deterministic AEAD offers no secrecy guarantees for associated data.

Example use case

See I want to encrypt data deterministically and I want to bind ciphertext to its context.