# Tag Info

6

The point of the IV is to prevent the same (key,IV) from ever being used for two different messages in practice. This is an absolute requirement for stream ciphers or block cipher modes such as CTR that are effectively stream ciphers, because re-using the same (key,IV) pair lets an eavesdropper trivially obtain the XOR of two plaintext messages, which means ...

4

SIV is considered determanistic authenticated encryption because: It is deterministic; given a key, a plaintext maps to a specific ciphertext; there is no randomness involved. It is authenticated encryption; it provides privacy (that is, someone with a set of ciphertexts but without the key gets no information about the plaintexts, other than its length, ...

4

The synthesized IV does not need to be random. AES-SIV is a deterministic authenticated encryption mode: it can be used without any nonce when it is not a concern if the attacker can tell that the same message is being transmitted (under the same key) multiple times. Privacy and authentication are still guaranteed. SIV recommends to use a nonce (more ...

3

Correctly implemented, it should be secure deterministic authenticated encryption. In fact, it is SIV, in the wider sense of using the "SIV construction" as defined in Deterministic Authenticated-Encryption by Rogaway and Shrimpton (except for lacking a header input). The proof of the security of the SIV construction is that: We will now show that if $F$ ...

3

ChaCha20-Poly1305-SIV is not well defined, and does not have the advantages of SIV-mode if you do define it. The SIV mode is essentially MAC-then-encrypt, with the MAC reused as nonce. The MAC in ChaCha20-Poly1305 requires a nonce, because it uses ChaCha20 to encrypt the Poly1305 authenticator (you cannot reveal the raw authenticator). So you cannot use it ...

2

The definition of DAE security, as given in Rogaway and Shrimpton's original paper (which both defines the security notion and proves that SIV mode satisfies it), does effectively require that a DAE scheme must protect ciphertext integrity. Specifically, the definition of DAE security (definition 1 in the paper) says that an encryption scheme is DAE secure ...

2

Rogways's paper on the algorithm goes deeper into detail: http://web.cs.ucdavis.edu/~rogaway/papers/keywrap.pdf and discusses more about design rationale and provides proof (more than the RFC). I find a lot of benefits in SIV and S2V. For me, the most important is that it can be used without nonce. The second most important is that the mode builds on widely ...

2

Do I have to do all the "zero" stuff and doubling and XORing? If you are implementing the S2V primitive, using an AES-CMAC function, then yes, you will need to do all that "zero" stuff and doubling and XORing. Yes, if you peek inside the AES-CMAC function, you will find some logic that looks sort of like this; processing an all zero block, doubling the ...

1

Both: Secure deterministic encryption for one-block messages. ECB: No expansion for block-sized messages. Faster. SIV: Authentication. Works for messages of any size.

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