[Cfrg] SHA256 based AEAD construction

[Fihlman Emil <[e-mail]>]

Hi

Let me first apologise if this is not the correct forum (if so, any pointers towards a place of discussion are appreciated).

I recently implemented a SHA256 based AEAD crypto (base file https://emil.fi/d/sha256.c and a test program / simple utility https://emil.fi/d/sha256crypt.c).

In a nutshell:
We make an intermediate key kv by taking sha256(key|vector), which is then used to
a) generate a random keystream by taking sha256(kv|counter), with which input is xorred by and
b) take a hmac of the ciphertext for authentication.

This concept is nothing new nor special. Some positives provided by this scheme are a) the base key is in the system only for a brief moment, b) it's very, very simple and easy to audit and use.

My current idea is to release this as a foss library after refinements, mainly targeted for resource limited systems.
I greatly appreciate any criticism, guidance, ideas and suggestions (especially improvements on reducing ram and cpu requirements and hardening against any timing/side channel attacks are welcomed, I suspect there are quite a few low hanging fruits at the moment).

Emil Fihlman