Cryptography Common Mistakes

Also Known As

TL;DR

Explanation

Common mistakes: (1) Rolling your own crypto — use battle-tested libraries only. (2) Reusing IV/nonce in GCM — catastrophically breaks confidentiality and authentication. (3) ECB mode — identical plaintext blocks produce identical ciphertext, leaking patterns. (4) Timing-vulnerable comparison — use hash_equals() not ==. (5) SHA-256 for passwords — fast algorithm, GPU-crackable; use Argon2id. (6) No authenticated encryption — AES-CBC without MAC allows ciphertext tampering. (7) Trusting user-provided algorithm — JWT alg:none, algorithm confusion attacks.

Common Misconception

Why It Matters

Common Mistakes

• AES-CBC without MAC — ciphertext is malleable
• IV/nonce reuse in GCM — recovering the auth key
• SHA-256 for passwords — bcrypt/Argon2id is required
• == for comparing MACs/tokens — timing attack enables brute-force

Code Examples

// Multiple critical mistakes:
$key = 'secret';                         // Short, non-derived key
$iv  = str_repeat('0', 16);             // Static IV — reused every time!
$enc = openssl_encrypt($data, 'AES-128-ECB', $key); // ECB mode!
// No authentication — ciphertext tamperable
if ($provided === $expected) { }        // Timing vulnerable

// Correct authenticated encryption:
$key = random_bytes(32);         // 256-bit key from CSPRNG
$iv  = random_bytes(12);         // Random 96-bit nonce per message
$tag = '';
$enc = openssl_encrypt(
    $data, 'AES-256-GCM', $key, OPENSSL_RAW_DATA, $iv, $tag
);

// Constant-time comparison:
if (!hash_equals($expected, $provided)) {
    throw new SecurityException('Authentication failed');
}

References

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

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