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.
Deterministic AEAD can also be used to tie ciphertext to specific associated
data. Suppose you have a database with the fields
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.
- 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.