Add SHA256 support for variable message_size to be hashed

[Savio-Sou]

Problem

The SHA256 hash function in the stdlib doesn't currently support hashing of variable actual lengths of inputs.

This hinders use cases that rely on:

• One circuit (and by extension one e.g. verification key / verifier smart contract) for hashing different input sizes
• Hash matching with established services (i.e. customized input-padding that results in different hashes isn't useful)

Happy Case

Extend the SHA256 library to support an additional message_size argument, similar to Keccak256: noir-lang/acvm#313

Project Impact

Blocker

Impact Context

For projects verifying passport ownerships, sizes of data to hash varies from passport to passport.

Workaround

None

Workaround Description

No response

Additional Context

Credit to @madztheo @michaelelliot for flagging.

Would you like to submit a PR for this Issue?

None

Support Needs

No response

[Savio-Sou]

@guipublic mentioned this should be approachable to implement in pure Noir now with the sha256compression opcode AztecProtocol/aztec-packages#4511.

@michaelelliot you mentioned attempting to implement this in pure Noir but was met with poor performance some while ago. Was that before or already using sha256compression?

[guipublic]

Since I said it would be easy, I figured that I might just implement it. I confirm that the performance is good, the cost is the same as if you would hash the full message.