SHA-2 hash functions: SHA-256, SHA-384, SHA-512 #46

Phase 5: Pure Rust Crypto#

Implement the SHA-2 family of cryptographic hash functions in the crypto crate per FIPS 180-4.

Requirements#

• SHA-256: 256-bit digest, 64-byte block size
• SHA-384: 384-bit digest, 128-byte block size (truncated SHA-512)
• SHA-512: 512-bit digest, 128-byte block size
• Streaming API: new(), update(&[u8]), finalize() -> [u8; N]
• One-shot convenience: sha256(data) -> [u8; 32], etc.
• Message padding per FIPS 180-4 §5.1
• Correct round constants and initial hash values

Acceptance Criteria#

• Sha256, Sha384, Sha512 structs with streaming and one-shot APIs
• Pass NIST test vectors (empty string, "abc", 448-bit message, 896-bit message)
• Pass long message test vectors (1 million "a" characters)
• cargo test -p we-crypto passes
• cargo clippy -p we-crypto -- -D warnings clean
• unsafe only for constant-time operations or assembly optimizations (if needed)