sha256_6502

Speed-optimized SHA-256 hashing for the venerable 6502.

Just-for-fun implementation of SHA-256. It is much more computationally intensive than MD5, so this time I went for performance, code size be damned. What resulted is probably the fastest SHA-256 for the 6502, hashing at a blistering pace of 410 bytes/s on a C64. All that with a minimal use of ZP.

You will need cc65 (https://github.com/cc65/cc65) to build.

How to use:

In order to calculate SHA-256 hash of a message:

1. Call _sha256_init.
2. Call _sha256_next_block for every 64-byte block of the message, passing a pointer to beginning of the block in A/X (lo/hi) and size of the block in Y. Only the final block's size can be smaller than 64 bytes!
3. Call _sha256_finalize.
4. Find computed SHA-256 hash in 32 bytes starting at _sha256_hash.

See tests.s for examples.

Limitations

Currently messages of up to 2MiB are supported. This should not be a problem in practice, but if the need arises, the limit can be increased easily.

Testing

Sure. Run make test. If it fails, you optimized too much.

Did you say Commodore 64?

Yes. make sha256.prg and have fun benchmarking.

Author

Milek Smyk (@laubzega)