---

name: cryptography description: | Application-level cryptography. Password hashing (bcrypt, argon2), encryption (AES-GCM), digital signatures, key management, and secure random generation.

USE WHEN: user mentions "encryption", "hashing", "bcrypt", "argon2", "AES", "cryptography", "digital signature", "key management", "HMAC"

DO NOT USE FOR: TLS/HTTPS configuration - use infrastructure skills; JWT tokens - use jwt; OAuth flows - use oauth2 allowed-tools: Read, Grep, Glob, Write, Edit

Cryptography

Password Hashing

bcrypt (recommended for most apps)

import bcrypt from 'bcrypt';

const SALT_ROUNDS = 12;

async function hashPassword(password: string): Promise<string> {
  return bcrypt.hash(password, SALT_ROUNDS);
}

async function verifyPassword(password: string, hash: string): Promise<boolean> {
  return bcrypt.compare(password, hash);
}

Argon2 (recommended for high-security)

import argon2 from 'argon2';

async function hashPassword(password: string): Promise<string> {
  return argon2.hash(password, {
    type: argon2.argon2id,
    memoryCost: 65536,  // 64 MB
    timeCost: 3,
    parallelism: 4,
  });
}

async function verifyPassword(password: string, hash: string): Promise<boolean> {
  return argon2.verify(hash, password);
}

Symmetric Encryption (AES-256-GCM)

import { createCipheriv, createDecipheriv, randomBytes } from 'crypto';

const ALGORITHM = 'aes-256-gcm';

function encrypt(plaintext: string, key: Buffer): string {
  const iv = randomBytes(12);
  const cipher = createCipheriv(ALGORITHM, key, iv);
  const encrypted = Buffer.concat([cipher.update(plaintext, 'utf8'), cipher.final()]);
  const authTag = cipher.getAuthTag();
  return Buffer.concat([iv, authTag, encrypted]).toString('base64');
}

function decrypt(ciphertext: string, key: Buffer): string {
  const buf = Buffer.from(ciphertext, 'base64');
  const iv = buf.subarray(0, 12);
  const authTag = buf.subarray(12, 28);
  const encrypted = buf.subarray(28);
  const decipher = createDecipheriv(ALGORITHM, key, iv);
  decipher.setAuthTag(authTag);
  return decipher.update(encrypted) + decipher.final('utf8');
}

// Key from environment (32 bytes for AES-256)
const key = Buffer.from(process.env.ENCRYPTION_KEY!, 'hex');

HMAC (Message Authentication)

import { createHmac, timingSafeEqual } from 'crypto';

function signPayload(payload: string, secret: string): string {
  return createHmac('sha256', secret).update(payload).digest('hex');
}

function verifySignature(payload: string, signature: string, secret: string): boolean {
  const expected = signPayload(payload, secret);
  return timingSafeEqual(Buffer.from(signature), Buffer.from(expected));
}

Secure Random Values

import { randomBytes, randomUUID } from 'crypto';

const token = randomBytes(32).toString('hex');  // 64-char hex token
const uuid = randomUUID();                       // UUID v4

Python

from passlib.hash import argon2
import os
from cryptography.fernet import Fernet

# Password hashing
hashed = argon2.hash("password")
is_valid = argon2.verify("password", hashed)

# Symmetric encryption
key = Fernet.generate_key()  # Store securely
f = Fernet(key)
encrypted = f.encrypt(b"sensitive data")
decrypted = f.decrypt(encrypted)

# Secure random
token = os.urandom(32).hex()

Anti-Patterns

Anti-Pattern	Fix
MD5/SHA for passwords	Use bcrypt or argon2
ECB mode encryption	Use GCM (authenticated encryption)
Hardcoded keys	Use environment variables or KMS
Math.random() for tokens	Use crypto.randomBytes()
String comparison for signatures	Use timingSafeEqual() to prevent timing attacks
Reusing IV/nonce	Generate fresh random IV for each encryption

Production Checklist

• bcrypt (cost 12+) or argon2id for passwords
• AES-256-GCM for symmetric encryption
• Keys in environment variables or KMS
• crypto.randomBytes for all random tokens
• timingSafeEqual for signature verification
• Key rotation plan documented