X25519 key exchange (RFC 7748) #51

Phase 5: Pure Rust Crypto#

Implement X25519 Diffie-Hellman key exchange in the crypto crate per RFC 7748.

Requirements#

• Field arithmetic in GF(2^255 - 19): add, sub, mul, pow, inverse
• Montgomery ladder scalar multiplication on Curve25519
• x25519(scalar, u_coordinate) -> u_coordinate
• Key generation: clamp private key, compute public key from basepoint
• Shared secret derivation: x25519(my_private, their_public)
• Constant-time implementation to prevent timing side-channels

Acceptance Criteria#

• Field arithmetic for GF(2^255 - 19) with 5x51-bit limb representation
• Montgomery ladder scalar multiplication
• x25519_base(private_key) -> public_key (basepoint multiplication)
• x25519(private_key, public_key) -> shared_secret
• Pass RFC 7748 §6.1 test vectors
• Pass iterated test vector (1,000 iterations)
• Constant-time (no secret-dependent branches or memory accesses)
• cargo test -p we-crypto passes
• cargo clippy -p we-crypto -- -D warnings clean