sha256asm

SHA-256 in Protected Mode Assembly

This implementation of the SHA-256 hash was specially designed for boot loaders. As such it's written for protected mode, has very small footprint (970 bytes), only uses general purpose registers, and it's very easy on the stack. It was written in fasm.

I haven't unrolled the sha loop for compact code size, but I've used a lookup table to avoid cubic root calculation. If you're looking for a really fast SHA implementation (also for long mode, using SSE and AVX registers), check out project Nayuki and also read about the hardware features of Intel SHA Extensions (only available if CPUID(EAX=7,ECX=0) returns EBX bit 29 set, and has strict alignment requirements).

Application Interface

Uses the same API as OpenSSL, except for simplicity this implementation uses one implicit context in the bss segment.

sha256_init

Initializes the context. Clobbers EAX.

sha256_update buffer, length

Adds a buffer to the hash. The underlying function receives it's arguments in ESI and ECX. Clobbers EAX, EDI.

sha256_final checksumbuffer

Generates the final hash into the given 32 bytes long buffer. The underlying function receives the address in EDI. Clobbers EAX, EBX, ECX.

That's all, hope it will be useful!

Contributors

I'd like to say thanks to @CandyMan and @WinLego for testing this code and helping with finding bugs!