Timing Attack on Password Verification

Medium+100 XPA07:2021 Identification and Authentication FailuresCWE-916

Timing AttackConstant-Time ComparisonPassword Verification

Scenario

A login service compares a supplied password hash against the stored hash using the === operator (or equivalent). This short-circuits on the first mismatched character.

An attacker can measure how long each comparison takes. A guess that matches the first character takes slightly longer than one that doesn't — leaking one character at a time until the full hash is reconstructed.

The fix is to compare every character in both strings regardless of early mismatches, ensuring a constant execution time.

Timing side-channels allow remote attackers to reconstruct secrets one bit at a time. Constant-time comparison is mandatory for any security-sensitive equality check — tokens, HMACs, and password hashes.

Your Tasks

Fix verifyPassword to use a constant-time comparison that does not short-circuit on a mismatch.
Return true when both hashes match, false otherwise.
The function must never throw — return false for all non-matching inputs.

Examples

Example 1 — Match — returns true

verifyPassword('hash:abc', 'hash:abc')
// → true

Example 2 — Mismatch — returns false

verifyPassword('hash:xyz', 'hash:abc')
// → false

Constraints

›Do not use === or == for the final comparison.
›Iterate over every character and accumulate a difference flag — only return the result after the full loop.
›Return a boolean; do not throw.

Hint

References

CWE-208: Observable Timing Discrepancy OWASP Testing Guide — Testing for Timing Attacks Node.js crypto.timingSafeEqual documentation

solution.js

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