# Constant Time Secret Compare

> Compare bearer tokens, API secrets, signatures, and password-equivalents with a constant-time comparison (hash both sides, then crypto.timingSafeEqual) — never == or short-circuiting string equality. Triggers when writing any code that checks a secret against a stored or expected value. Does not fire on comparing non-secret identifiers.

- Skill: `randommonicle/constant-time-secret-compare` (Agent Skill)
- Install (CLI): `npx skillmds@latest add randommonicle/constant-time-secret-compare`
- Raw SKILL.md: https://api.skillmd.com/api/skills/randommonicle/constant-time-secret-compare/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: randommonicle (https://skillmd.com/u/randommonicle)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/randommonicle/constant-time-secret-compare

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# Constant-time secret compare

String equality short-circuits on the first differing byte, so `token == expected` leaks how
many leading bytes matched through response timing — an oracle an attacker can walk.

## The rule

Hash both sides to fixed length, then compare constant-time:

```js
import { createHash, timingSafeEqual } from 'node:crypto';
const h = (s) => createHash('sha256').update(s).digest();
const ok = timingSafeEqual(h(provided), h(expected));
```

Hashing first makes the buffers equal-length (timingSafeEqual throws on length mismatch —
itself a leak) and keeps the comparison constant-time regardless of input size. Apply to
bearer tokens, admin tokens, webhook signatures, and any password-equivalent. HMAC
verification uses the same pattern on the computed and received MACs.

## Why

It costs two lines and removes a whole attack class; equality comparison on secrets is the
kind of defect that passes every functional test forever. Evidence: ICC CLAUDE.md
known-issues (admin token compare, shipped fix).