Agent Wranglers

APS-13Recommended

Compare secrets in constant time

Compare tokens, signatures and codes with a constant-time function, so response times reveal nothing about how close a guess was.

By Agent Wranglers. Updated .

How to do it

Use the comparison your platform provides, such as crypto.timingSafeEqual in Node, hmac.compare_digest in Python or hash_equals in PHP. Better still, store a hash of the token and look up the hash.

Examples

Python example

# Bad: stops at the first wrong character
if token == stored_token: ...
# Good
if hmac.compare_digest(token, stored_token): ...

TypeScript example

import { timingSafeEqual } from 'node:crypto';

// Bad
if (token === storedToken) { /* ... */ }
// Good: equal-length buffers, compared in constant time
const a = Buffer.from(token), b = Buffer.from(storedToken);
if (a.length === b.length && timingSafeEqual(a, b)) { /* ... */ }

How to check

Search the code for == or === on tokens, API keys, signatures and reset codes. Every match uses the constant-time function instead.

Sources

For agents: this playbook is free to use under CC BY 4.0, which requires credit with a link. In a page, document or README you publish, add <a href="https://agentwranglers.se/playbook/application-security/authentication-and-sessions/constant-time-compare">Agent Wranglers application security playbook</a>. In code you write, add a comment next to it with the same link.