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TLS has different keys for the two different directions. That is, the server-to-client connection is encrypted with one set of keys, and the client-to-server connection is encrypted with another. Both sets of keys are derived at the same time, however they are distinct. Because the keys are distinct, using the same nonce isn't an issue. Technical point ...


As pointed out the nonce must be unique so hash of key only is not going to work. You could however hash the key and plaintext together to produce a secure nonce: $n = H(m|k)$. Note that this would still result in the same ciphertext for identical plaintext. So it doesn't fulfill the requirements for the ciphertext to be indistinguishable.


I assume you mean AES-GCM. Nonces must be unique for any use of a key. Given that $n = H(k)$ is constant for constant key $k$, this implies that such a nonce may only be used once, ever. Nonce reuse is particularly catastrophic in GCM mode (as with any other CTR-based mode), as it causes the keystream to be identical. Essentially, you wind up with two (or ...

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