Symmetric Encryption

Also Known As

TL;DR

Explanation

Symmetric algorithms (AES, ChaCha20) use one key for both operations. AES-256-GCM is the current gold standard: 256-bit key, Galois/Counter Mode provides both encryption and authentication (AEAD). A unique random IV (nonce) must be used for every encryption with the same key. ChaCha20-Poly1305 is preferred on systems without AES hardware acceleration. The key distribution problem — securely sharing the key — is solved using asymmetric encryption or key exchange protocols.

Common Misconception

Why It Matters

Common Mistakes

• Using ECB mode — identical plaintext blocks produce identical ciphertext, revealing patterns.
• Reusing the same IV with the same key — completely breaks GCM security.
• Not using authenticated encryption (GCM) — unauthenticated ciphertext can be silently tampered with.
• Storing the IV separately from the ciphertext in an unreliable way — prepend IV to ciphertext for simple management.

Code Examples

// AES-256-ECB — insecure mode, no authentication:
$encrypted = openssl_encrypt($data, 'AES-256-ECB', $key);
// Identical 16-byte blocks encrypt identically — patterns visible
// No IV — deterministic, dictionary-attackable
// No authentication — tampering undetectable

// AES-256-GCM — authenticated, random IV:
function encrypt(string $plaintext, string $key): string {
    $iv = random_bytes(12); // GCM standard IV size
    $tag = '';
    $ciphertext = openssl_encrypt(
        $plaintext, 'AES-256-GCM', $key,
        OPENSSL_RAW_DATA, $iv, $tag
    );
    return base64_encode($iv . $tag . $ciphertext); // IV + auth tag + ciphertext
}

function decrypt(string $encoded, string $key): string {
    $data = base64_decode($encoded);
    $iv = substr($data, 0, 12);
    $tag = substr($data, 12, 16);
    $ciphertext = substr($data, 28);
    return openssl_decrypt($ciphertext, 'AES-256-GCM', $key, OPENSSL_RAW_DATA, $iv, $tag);
}

References

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

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