Key Derivation Functions

Also Known As

TL;DR

Explanation

Passwords must not be stored directly — a KDF transforms a password into a hash that cannot be reversed, and is deliberately slow to compute. bcrypt: adaptive cost factor, max 72 chars, PHP's password_hash() default. scrypt: memory-hard (GPU/ASIC resistant) — good for application-level KDFs. Argon2 (Argon2id recommended): winner of Password Hashing Competition, configurable time+memory+parallelism — PHP password_hash() with PASSWORD_ARGON2ID. PBKDF2: NIST-approved, widely supported, less memory-hard than Argon2/scrypt. Never use MD5/SHA (fast — trivially brute-forced).

Common Misconception

Why It Matters

Common Mistakes

• MD5 or SHA-1 for passwords — deprecated and trivially cracked; any data breach is catastrophic.
• bcrypt with passwords > 72 characters — bcrypt silently truncates; use Argon2id for long passwords.
• Low cost/iteration count — makes hashing fast and brute force feasible; use the highest cost your server tolerates.
• Not using PHP's password_verify() — manual comparison with == is vulnerable to timing attacks; password_verify() uses constant-time comparison.

Code Examples

// MD5 password hash — trivially cracked:
$hash = md5($password);          // Cracked in seconds with GPU
$hash = sha256($password);       // Fast — millions/sec on GPU
$hash = md5(md5($password));     // Still fast — do not use

// Manual comparison — timing attack:
if ($storedHash === md5($password)) { /* login */ }

// Argon2id — slow, memory-hard, PHP native:
$hash = password_hash($password, PASSWORD_ARGON2ID, [
    'memory_cost' => 65536,  // 64 MB
    'time_cost'   => 4,      // 4 iterations
    'threads'     => 2,
]);

// Verify with constant-time comparison:
if (password_verify($password, $hash)) {
    // Login — rehash if needs_rehash():
    if (password_needs_rehash($hash, PASSWORD_ARGON2ID)) {
        $newHash = password_hash($password, PASSWORD_ARGON2ID);
        updateStoredHash($userId, $newHash);
    }
}

References

• ↗ https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html

Tags

Related Terms