Contents
When to use same nonce in AES-GCM?
It is critical when using AES-GCM that the nonce is never repeated for any given key. The best way to ensure this is to use a cryptographically strong PRNG to generate a new 96-bit nonce for each message, and to re-key at reasonably regular intervals, where “reasonably regular” is defined by how much data and how many messages you’re encrypting.
Is the authentication key derived from the AES key?
The authentication key, which is derived from the AES key, is used in the computation of the authentication tag. Although its compromise will not leak the cipher key, it allows an adversary to conduct ciphertext forgery attacks.
Can a nonce be used to generate unpredictable IVs?
There are two recommended methods for generating unpredictable IVs. The first method is to apply the forward cipher function, under the same key that is used for the encryption of the plaintext, to a nonce. The nonce must be a data block that is unique to each execution of the encryption operation.
Can a stream cipher use the same nonce twice?
AES-GCM is an authenticated stream cipher. The nonce will be used for the key of the stream cipher, and the first rule of stream ciphers is you don’t talk about stream cipher never use the same key/IV twice. Ignoring this rule leads to catastrophic failures in encryption.
How does AES use the same nonce security risk?
The first method is to apply the forward cipher function, under the same key that is used for the encryption of the plaintext, to a nonce. The nonce must be a data block that is unique to each execution of the encryption operation. For example, the nonce may be a counter, as described in Appendix B, or a message number.
When to use different IVs in GCM mode?
For GCM mode, it doesn’t matter in the least whether the IV is predictable to the adversary. What’s critical is that you use a different one each time. If you can manage a counter (e.g. if you restart the program, you’ll pick a fresh key), then using IV=0 for your first message, IV=1 for your second, etc, is the obvious approach.
Is the nonce of a GCM a counter?
The nonce itself does not have to be random, it can be a counter. But it absolutely must be unique for each message encrypted with the same key. Using GCM on two different messages with the same key and nonce basically allows an attacker to decrypt both messages and forge further messages.