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It looks like, given your adversary model, things should be secure. HMAC as a randomness extractor has been shown to be good, especially when we can assume the hash function is collision resistant. That paper also has some results which tell how you could guard against the collision resistance being broken (basically use a hash function with larger output ...


2

The once part inside of the nonce in CTR mode means effectively "once for this particular key". If you use a fresh key for each message (e.g. by encrypting it using public-key crypto or similar), you can use the same nonce for all the messages (or a size-zero nonce). The important part is that the combination of nonce and ctr-value (i.e. what is input into ...


2

It seems that you are trying to implement your own KBKDF (Key Based Key Derivation Function) using HMAC. Maybe it is better to use a pre-defined one. It would be more sensible maybe to use an HSM that is FIPS certified for NIST SP 800-108. These use one of the KBKDFs defined in NIST SP 800-108. You can still use the idea of the random by putting it in the ...



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