Constant-Time Analysis

When to Use

Reviewing code that compares secrets (tokens, passwords, MACs, signatures)
Auditing cryptographic implementations for timing leaks
Checking authentication or authorization code for side-channel risks
Verifying that branching doesn't depend on secret values
Analyzing code paths that process sensitive data with variable timing

When NOT to Use

Non-security-sensitive comparisons (public data)
Performance optimization (different goal)
General code review without cryptographic context

Why Timing Matters

Non-constant-time string comparison reveals information bit by bit:

python

# VULNERABLE: Early exit leaks prefix length
def check_token(provided, expected):
    if len(provided) != len(expected):
        return False
    for a, b in zip(provided, expected):
        if a != b:
            return False  # Exits early - timing reveals match position
    return True

An attacker can measure response times to determine how many characters match, then brute-force one character at a time.

Safe Patterns by Language

Python

python

import hmac
# SAFE: constant-time comparison
hmac.compare_digest(provided_token, expected_token)

# Also safe for bytes
hmac.compare_digest(provided_hash, expected_hash)

Node.js

javascript

const crypto = require('crypto');
// SAFE: constant-time comparison
crypto.timingSafeEqual(
  Buffer.from(provided),
  Buffer.from(expected)
);

Go

import "crypto/subtle"
// SAFE: constant-time comparison
subtle.ConstantTimeCompare([]byte(provided), []byte(expected))

Rust

rust

use subtle::ConstantTimeEq;
// SAFE: constant-time comparison
provided.ct_eq(&expected).into()

Detection Patterns

bash

# Find potentially unsafe secret comparisons
grep -rn "==.*token\|==.*secret\|==.*password\|==.*api_key" --include="*.py" --include="*.js" --include="*.ts" .

# Find safe comparison usage
grep -rn "compare_digest\|timingSafeEqual\|ConstantTimeCompare\|ct_eq" .

Vulnerable Code Patterns

Pattern	Language	Risk
`if token == expected:`	Python	Timing leak
`if (token === expected)`	JavaScript	Timing leak
`strings.Compare(a, b)`	Go	Timing leak
`token.equals(expected)`	Java	Timing leak
`bcrypt.compare(a, b)`	Any	Safe (bcrypt is constant-time)

Beyond String Comparison

Timing leaks can also occur in:

Conditional branches on secret values (if/else based on key bits)
Array indexing with secret indices (cache timing)
Early returns in validation functions
Database lookups that vary by existence (user enumeration)
Regular expressions with backtracking on secret data

Mitigation Strategies

Use language-provided constant-time comparison functions
Avoid branching on secret values
Use constant-time select operations instead of if/else
Add artificial delays to normalize response times (last resort)
Use hash-then-compare for variable-length secrets