Using the same HMAC state without re-init

[Tachi107]

Hi, first of all thank you for this awesome library!

I'm currently looking into using the HMAC-SHA-2 multi-part API, and one limitation I noticed is that it doesn't seem to be possible to reuse the same HMAC state without calling crypto_auth_hmacsha256_init() again. I'm looking for a function similar to MbedTLS' mbedtls_md_hmac_reset(), which can be used after calling _final() to perform another HMAC operation with the same key.

Would it make sense to have such an API in libsodium? Should I just call _init() again with the same key? Would reusing the same state provide any significant performance/efficiency advantage?

Thanks again :)

PS: I tried to subscribe to the mailing list mentioned in the README, but nothing happened. Is it still in use?

[jedisct1]

Hi!

The crypto_auth_*_state values can be copied. Libsodium doesn't do dynamic allocations, so there are no pointers inside and you can just copy them as a blob of bytes.

So, before calling _final(), copy the state, and reuse it afterwards as needed.

[jedisct1]

Oh, I misread your question and thought you wanted to finalize the tag before the entire message was absorbed.

But the answer is the same :) The state can be copied after _init() if you want to reuse it.

That being said _init() is a very fast operation, so there is no significant advantage in doing so.

[Tachi107]

Il giorno mar 27 dic 2022 alle 04:20:53 -08:00:00, Frank Denis ***@***.***> ha scritto:

The crypto_auth_*_state values can be copied. Libsodium doesn't do dynamic allocations, so there are no pointers inside and you can just copy them as a blog of bytes. So, before calling _final(), copy the state, and reuse it afterwards as needed.

Hi Frank, thanks for the fast reply! I'll do as you suggested :)