04 Reviewing Cryptography and Secrets

Cryptography is hard to get right. Reviewers must look for hardcoded secrets, weak algorithms, incorrect modes of operation, and insufficient randomness.

1. Hardcoded Secrets

Context: Passwords, API keys, and cryptographic keys should never be embedded in source code.

Vulnerable Pattern (Python)

import hmac

def verify_webhook(payload, signature):
    # BAD: Hardcoded secret key
    secret = b"super_secret_webhook_key_123"
    expected = hmac.new(secret, payload, digestmod="sha256").hexdigest()
    return hmac.compare_digest(expected, signature)

Secure Pattern (Python)

import hmac
import os

def verify_webhook(payload, signature):
    # GOOD: Load secret from environment variables or secret manager
    secret = os.environ.get("WEBHOOK_SECRET").encode('utf-8')
    expected = hmac.new(secret, payload, digestmod="sha256").hexdigest()
    return hmac.compare_digest(expected, signature)

2. Weak Cryptographic Modes (AES-ECB)

Context: Electronic Codebook (ECB) mode encrypts identical plaintext blocks into identical ciphertext blocks, revealing patterns.

Vulnerable Pattern (Node.js)

const crypto = require('crypto');

function encrypt(text, key) {
    // BAD: ECB mode does not use an Initialization Vector (IV) and leaks patterns
    const cipher = crypto.createCipheriv('aes-256-ecb', key, null);
    let encrypted = cipher.update(text, 'utf8', 'hex');
    encrypted += cipher.final('hex');
    return encrypted;
}

Secure Pattern (Node.js)

const crypto = require('crypto');

function encrypt(text, key) {
    // GOOD: Use GCM (Galois/Counter Mode) for authenticated encryption
    const iv = crypto.randomBytes(12); // Generate a random IV
    const cipher = crypto.createCipheriv('aes-256-gcm', key, iv);
    
    let encrypted = cipher.update(text, 'utf8', 'hex');
    encrypted += cipher.final('hex');
    const authTag = cipher.getAuthTag().toString('hex');
    
    return { iv: iv.toString('hex'), ciphertext: encrypted, authTag: authTag };
}

3. Weak Random Number Generation

Context: Standard pseudo-random number generators (PRNGs) like Python's random module are predictable and must not be used for security purposes (like tokens or passwords).

Vulnerable Pattern (Python)

import random
import string

def generate_reset_token():
    # BAD: Predictable random generator
    chars = string.ascii_letters + string.digits
    return ''.join(random.choice(chars) for _ in range(32))

Secure Pattern (Python)

import secrets
import string

def generate_reset_token():
    # GOOD: Cryptographically secure random generator
    chars = string.ascii_letters + string.digits
    return ''.join(secrets.choice(chars) for _ in range(32))