stanford-crypto-patterns

/stanford-crypto-patterns

When to Use

• Environments where timing-attack resistance is critical (secret comparison)
• AES-CCM authenticated encryption (alternative to GCM with simpler nonce handling)
• SJCL PRNG for cryptographic random number generation
• Academic/compliance contexts requiring vetted, peer-reviewed crypto library

Do NOT use for

• New Node.js code — prefer built-in crypto module (WebCrypto API) for standard cases
• Performance-critical bulk encryption (pure JS SJCL is slower than native C bindings)

---

Constant-time comparison (timing-attack safe)

import sjcl from 'sjcl'

// WRONG: regular string comparison leaks timing info
// if (secretA === secretB) ...  ← fails early on first mismatch

// RIGHT: constant-time comparison via XOR of bitArrays
function constantTimeEqual(a: string, b: string): boolean {
  if (a.length !== b.length) return false
  const aWords = sjcl.codec.utf8String.toBits(a)
  const bWords = sjcl.codec.utf8String.toBits(b)

  let diff = 0
  for (let i = 0; i < aWords.length; i++) {
    diff |= aWords[i] ^ bWords[i]
  }
  return diff === 0
}

---

AES-CCM authenticated encryption

import sjcl from 'sjcl'

function encryptSJCL(plaintext: string, passphrase: string): string {
  // sjcl.encrypt handles: PBKDF2 key derivation + AES-CCM + random IV + MAC
  const encrypted = sjcl.encrypt(passphrase, plaintext, {
    ks:  256,      // AES key size: 128 | 192 | 256
    ts:  128,      // authentication tag size: 64 | 96 | 128 bits
    iter: 100_000, // PBKDF2 iterations
    mode: 'ccm',   // ccm | gcm | ocb2
  })
  return encrypted  // JSON string with iv, salt, ct, adata, mac
}

function decryptSJCL(encrypted: string, passphrase: string): string {
  try {
    return sjcl.decrypt(passphrase, encrypted)
  } catch (e) {
    throw new Error('[sjcl] decryption failed — wrong passphrase or tampered data')
  }
}

---

Cryptographic random bytes

// SJCL PRNG (seeded from system entropy via window.crypto or a custom entropy function)
function secureRandom(bytes: number): string {
  const wordCount = Math.ceil(bytes / 4)
  const words     = sjcl.random.randomWords(wordCount, 10)  // paranoia level 10
  return sjcl.codec.hex.fromBits(words).slice(0, bytes * 2)
}

// Session nonce
const nonce = secureRandom(16)   // 32-char hex = 128-bit nonce

---

PBKDF2 key derivation

function deriveKeySJCL(passphrase: string, saltHex: string, iterations = 100_000): string {
  const salt  = sjcl.codec.hex.toBits(saltHex)
  const key   = sjcl.misc.pbkdf2(passphrase, salt, iterations, 256)
  return sjcl.codec.hex.fromBits(key)
}

---

SHA-256 / HMAC

const hash = sjcl.codec.hex.fromBits(sjcl.hash.sha256.hash('data'))

const key  = sjcl.codec.utf8String.toBits(process.env.HMAC_KEY!)
const mac  = sjcl.codec.hex.fromBits(new sjcl.misc.hmac(key).encrypt('message'))

---

Anti-Fake-Pass Checklist

❌ sjcl.random.randomWords without seeding → throws on missing entropy in Node.js (use node-crypto seed)
❌ sjcl.encrypt paranoia < 6 → low entropy tolerance warning; use 8-10 for production
❌ Comparing MAC tags with === → timing attack; use constantTimeEqual above
❌ AES-CCM nonce reused → catastrophic, breaks authentication (SJCL uses random IV by default)
❌ sjcl in browser vs Node.js → window.crypto auto-seed in browser, must manually add entropy in Node
❌ sjcl output is JSON object — stringify before storing, parse before decrypting