Optimize Argon2 on the Web/JS platform

[dipu-bd]

Summary

Optimizes the Web/JS (Node/browser) implementation of Argon2 by removing BigInt from the compression function's hot path. This resolves the long-standing - Support for Argon2 in Node platform (TODO: requires optimization) note in CHANGELOG.md.

Problem

Argon2's hottest operation is the BLaMka G-function's 64-bit fused multiply-add, z = x + y + 2 * (lo32(x) * lo32(y)). On the VM this runs in native Uint64List arithmetic, but JS has no 64-bit integers, so the web twin _fBlaMka in lib/src/algorithms/argon2/argon2_32bit.dart implemented it with heap-allocating BigInt. _fBlaMka is called 4× per _mix, 8 _mix per mixer, 16 mixers per block, over thousands of blocks — so BigInt allocation and arbitrary-precision math sat on the single hottest line of the whole KDF.

Change

Replace the BigInt-based _fBlaMka with pure 32-bit integer math:

• a full 32×32→64 low-word product built from 16-bit limbs, and
• carry-based addition of the (low, high) pairs,

with every intermediate kept below 2^53 so results stay exact on IEEE-754 doubles. This mirrors the BigInt-free pattern the sibling blake2b_32bit.dart already uses (_add2/_add3 carry idiom). No public API, signature, or exported symbol changes — it is an internal swap behind the existing conditional import.

Correctness

Output is byte-identical to the previous implementation. The Argon2 known-answer vectors (test/argon2_test.dart, sourced from argon2.online, argon2i/d/id at m=16 and m=256) are not vm-only, so they exercise the JS path directly — all pass, which is the proof a carry bug would break.

• dart format --output=none --set-exit-if-changed . — clean
• dart analyze --fatal-infos — zero infos
• dart test -p vm — 736 passed
• dart test -p node — 601 passed

Performance

Measured on the compiled JS build under Node, argon2id at Argon2Security.moderate, averaged over 10 rounds:

	ms/op
Before (BigInt)	13,738.90
After (32-bit math)	882.80

≈ 15.6× faster on the web platform. The VM path (argon2_64bit.dart) is unchanged.

Notes

• Bumps version to 2.4.3 and adds the matching CHANGELOG.md entry.
• BENCHMARK.md is intentionally untouched (regenerated only on the maintainer's reference hardware).

🤖 Generated by Claude Code