BIP-39 Passphrase (25th Word)

Try it Live

Run BIP39 examples in the interactive playground

Overview

BIP-39 supports an optional passphrase (sometimes called “25th word”) that modifies seed derivation. This enables plausible deniability, two-factor security, and enhanced entropy.

How Passphrases Work

PBKDF2 Salt Modification

Passphrase is appended to the PBKDF2 salt:

Without passphrase:
salt = "mnemonic"

With passphrase:
salt = "mnemonic" + passphrase

seed = PBKDF2(mnemonic, salt, 2048, SHA512, 64 bytes)

Different Passphrases = Different Wallets

import * as Bip39 from '@tevm/voltaire/Bip39';

const mnemonic = 'abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about';

// No passphrase
const seed1 = await Bip39.mnemonicToSeed(mnemonic);

// Empty string (equivalent to no passphrase)
const seed2 = await Bip39.mnemonicToSeed(mnemonic, '');

// With passphrase
const seed3 = await Bip39.mnemonicToSeed(mnemonic, 'secret');

console.log(seed1.every((b, i) => b === seed2[i])); // true (same)
console.log(seed1.some((b, i) => b !== seed3[i])); // true (different)

Use Cases

1. Plausible Deniability

Create decoy wallets with different passphrases:

const mnemonic = Bip39.generateMnemonic(256);

// Decoy wallet (small amount, no passphrase)
const decoySeed = await Bip39.mnemonicToSeed(mnemonic);
const decoyWallet = HDWallet.fromSeed(decoySeed);

// Real wallet (main funds, with passphrase)
const realSeed = await Bip39.mnemonicToSeed(mnemonic, 'my real secret');
const realWallet = HDWallet.fromSeed(realSeed);

/**
 * Under duress:
 * - Provide mnemonic without passphrase
 * - Reveals decoy wallet only
 * - Real funds remain hidden
 */

2. Two-Factor Security

Separate storage of mnemonic and passphrase:

/**
 * Factor 1: Mnemonic (physical backup)
 * - Written on paper
 * - Stored in safe
 * - Can be duplicated
 *
 * Factor 2: Passphrase (memorized)
 * - Never written down
 * - Only in your head
 * - Cannot be stolen physically
 */

const mnemonic = Bip39.generateMnemonic(256);
const memorizedPassphrase = 'correct horse battery staple ancient wisdom';

// Attacker needs both to access funds
const seed = await Bip39.mnemonicToSeed(mnemonic, memorizedPassphrase);

3. Enhanced Entropy

Add entropy even if mnemonic is compromised:

// Even if mnemonic generation was weak
const potentiallyWeakMnemonic = Bip39.generateMnemonic(128); // 12 words

// Strong passphrase adds significant entropy
const strongPassphrase = 'my very long and complex passphrase with 128+ bits of entropy';

const seed = await Bip39.mnemonicToSeed(potentiallyWeakMnemonic, strongPassphrase);
// Combined security is much stronger

4. Inheritance/Time-Lock

/**
 * Split security:
 * - Mnemonic: in will/safe deposit box
 * - Passphrase: given to heirs separately
 *
 * Heirs need both to access funds
 */

const mnemonic = Bip39.generateMnemonic(256);
const inheritancePassphrase = 'revealed-in-will';

// Document in will: "Use passphrase: [inheritancePassphrase]"

Passphrase Strength

Weak Passphrases

// ❌ Too weak - easily guessed
const weak = [
  '',                    // No passphrase
  '1234',               // Trivial
  'password',           // Dictionary word
  'myname',             // Personal info
  'qwerty',             // Keyboard pattern
];

// Attacker can brute force these
for (const p of weak) {
  const seed = await Bip39.mnemonicToSeed(mnemonic, p);
  // Try to find funds at derived addresses
}

Strong Passphrases

// ✅ Strong - high entropy, memorable
const strong = [
  'correct horse battery staple ancient wisdom mountain river',  // Diceware (7+ words)
  'My cat Mittens was born on July 4th 2015 at 3:47 PM',       // Personal sentence
  'L3t$_M@k3-A_R@nd0m!P@$$phr@$3#2024',                        // Complex alphanumeric
  'ILikeToEat🍕OnSundaysWhileWatching📺',                       // Unicode + emoji
];

// High entropy passphrases are effectively unbreakable

Passphrase Entropy Calculator

function estimateEntropyBits(passphrase: string): number {
  const hasLower = /[a-z]/.test(passphrase);
  const hasUpper = /[A-Z]/.test(passphrase);
  const hasDigit = /\d/.test(passphrase);
  const hasSpecial = /[^a-zA-Z0-9]/.test(passphrase);

  let charsetSize = 0;
  if (hasLower) charsetSize += 26;
  if (hasUpper) charsetSize += 26;
  if (hasDigit) charsetSize += 10;
  if (hasSpecial) charsetSize += 32; // Estimate

  const entropyPerChar = Math.log2(charsetSize);
  const totalEntropy = entropyPerChar * passphrase.length;

  return totalEntropy;
}

console.log(estimateEntropyBits('password')); // ~38 bits (weak)
console.log(estimateEntropyBits('correct horse battery staple')); // ~132 bits (strong)
console.log(estimateEntropyBits('L3t$_M@k3-A_R@nd0m!#2024')); // ~156 bits (very strong)

Passphrase Management

Memorization

/**
 * Diceware method (recommended):
 * - Roll dice to select words
 * - 7+ words = 90+ bits entropy
 * - Memorable phrase
 */

const dicewarePassphrase = 'correct horse battery staple ancient wisdom mountain';

// Practice recovery before funding:
async function testRecovery() {
  const recoveredSeed = await Bip39.mnemonicToSeed(mnemonic, dicewarePassphrase);
  const recoveredWallet = HDWallet.fromSeed(recoveredSeed);
  const address = getFirstAddress(recoveredWallet);

  console.log('Recovered:', address);
  // Verify matches original
}

Storage (If Must Write)

/**
 * If passphrase must be written:
 * - NEVER store with mnemonic
 * - Encrypt differently
 * - Use different physical location
 * - Consider split storage
 */

// ❌ NEVER
const backup = {
  mnemonic: '...',
  passphrase: '...'
}; // Single file = single point of failure

// ✅ BETTER
// Mnemonic: Safe deposit box A
// Passphrase: Safe deposit box B (different bank)

Hint System

interface PassphraseHints {
  // NEVER include actual passphrase
  hints: string[];

  // Can include structure
  structure: {
    words: number;
    type: 'diceware' | 'sentence' | 'random';
  };

  // Recovery test
  firstAddressChecksum?: string; // First 8 chars to verify
}

const hints: PassphraseHints = {
  hints: [
    'Favorite book title',
    'Wedding anniversary year',
    'Pet name',
  ],
  structure: {
    words: 7,
    type: 'diceware'
  },
  firstAddressChecksum: '0x1234abcd'
};

// User can reconstruct from hints
// Hints alone are useless to attacker

Testing Passphrases

Verification

async function verifyPassphrase(
  mnemonic: string,
  passphrase: string,
  expectedAddress: string
): Promise<boolean> {
  // Derive seed
  const seed = await Bip39.mnemonicToSeed(mnemonic, passphrase);

  // Derive first address
  const root = HDWallet.fromSeed(seed);
  const eth0 = HDWallet.deriveEthereum(root, 0, 0);
  const actualAddress = deriveAddress(eth0);

  // Compare
  return actualAddress.toLowerCase() === expectedAddress.toLowerCase();
}

// Test before using in production
const correct = await verifyPassphrase(
  mnemonic,
  'my passphrase',
  '0x1234...'
);

if (!correct) {
  console.error('Wrong passphrase!');
}

Typo Detection

/**
 * Passphrases are case-sensitive and exact:
 */

const original = 'correct horse battery staple';

const typos = [
  'correct horse battery staples',  // Added 's'
  'correct horse battery',          // Missing word
  'Correct horse battery staple',   // Capital 'C'
  'correct  horse battery staple',  // Extra space
];

// Each produces completely different wallet
for (const typo of typos) {
  const seed1 = await Bip39.mnemonicToSeed(mnemonic, original);
  const seed2 = await Bip39.mnemonicToSeed(mnemonic, typo);

  console.log('Different:', seed1.some((b, i) => b !== seed2[i])); // true
}

// NO ERROR OR WARNING - all valid passphrases!

Edge Cases

Empty String

// Empty string and no passphrase are equivalent
const seed1 = await Bip39.mnemonicToSeed(mnemonic);
const seed2 = await Bip39.mnemonicToSeed(mnemonic, '');

console.log(seed1.every((b, i) => b === seed2[i])); // true

Whitespace

// Leading/trailing whitespace matters
const pass1 = 'password';
const pass2 = ' password';
const pass3 = 'password ';

const seed1 = await Bip39.mnemonicToSeed(mnemonic, pass1);
const seed2 = await Bip39.mnemonicToSeed(mnemonic, pass2);
const seed3 = await Bip39.mnemonicToSeed(mnemonic, pass3);

// All different!
console.log(seed1.some((b, i) => b !== seed2[i])); // true
console.log(seed1.some((b, i) => b !== seed3[i])); // true

Unicode Characters

// Full Unicode support via NFKD normalization
const unicodePassphrases = [
  'café',                    // Accented characters
  'パスワード',              // Japanese
  'пароль',                  // Cyrillic
  '🔑🔐🗝️',                  // Emoji
];

// All valid, normalized to NFKD before hashing
for (const p of unicodePassphrases) {
  const seed = await Bip39.mnemonicToSeed(mnemonic, p);
  console.log('Seed length:', seed.length); // 64
}

Unicode Normalization

// Different Unicode representations normalized
const form1 = 'café'; // U+00E9 (composed é)
const form2 = 'café'; // U+0065 + U+0301 (e + combining accent)

const seed1 = await Bip39.mnemonicToSeed(mnemonic, form1);
const seed2 = await Bip39.mnemonicToSeed(mnemonic, form2);

// NFKD normalization ensures same seed
console.log(seed1.every((b, i) => b === seed2[i])); // true

Security Considerations

Forgetting Passphrase

/**
 * CRITICAL WARNING:
 * - Forgotten passphrase = permanent loss
 * - No recovery mechanism exists
 * - No customer support can help
 * - Funds are unrecoverable
 */

// Before using passphrase in production:
async function passphraseRecoveryTest() {
  const mnemonic = Bip39.generateMnemonic(256);
  const passphrase = 'my secret passphrase';

  // 1. Generate wallet
  const seed = await Bip39.mnemonicToSeed(mnemonic, passphrase);
  const originalAddress = deriveFirstAddress(seed);

  // 2. User writes mnemonic only (not passphrase)
  console.log('Mnemonic backup:', mnemonic);

  // 3. Later, user tries to recover without passphrase
  const seedWithoutPass = await Bip39.mnemonicToSeed(mnemonic);
  const recoveredAddress = deriveFirstAddress(seedWithoutPass);

  // 4. DIFFERENT ADDRESS - funds lost!
  console.log('Original:', originalAddress);
  console.log('Recovered:', recoveredAddress);
  console.log('Match:', originalAddress === recoveredAddress); // false
}

Brute Force Resistance

// Passphrase strength vs attack time
const passphraseStrengths = [
  { passphrase: '1234', bits: 13 },
  { passphrase: 'password', bits: 38 },
  { passphrase: 'correct horse battery staple', bits: 132 },
  { passphrase: 'L3t$_M@k3-A_R@nd0m!P@$$phr@$3#2024', bits: 156 },
];

for (const { passphrase, bits } of passphraseStrengths) {
  const combinations = Math.pow(2, bits);
  const secondsAt1B = combinations / 1e9; // 1 billion attempts/second
  const years = secondsAt1B / (365.25 * 24 * 3600);

  console.log(`Passphrase: "${passphrase}"`);
  console.log(`  Bits: ${bits}`);
  console.log(`  Crack time: ${years.toExponential(2)} years`);
}

// 1234: 8.2e-05 years (instantly)
// password: 8.7 years (weak)
// diceware: 1.7e23 years (strong)
// complex: 1.4e30 years (overkill but good)

Best Practices

1. Choose Strong Passphrases

// ✅ Use diceware (7+ words)
const diceware = 'correct horse battery staple ancient wisdom mountain';

// ✅ Use memorable sentence
const sentence = 'My first cat was named Mittens and born in 2015';

// ✅ Use password manager generated
const manager = 'X7$mK9#pL2@nQ5!wR8^vT3&';

2. Test Recovery Before Funding

async function fullRecoveryTest() {
  // 1. Generate
  const mnemonic = Bip39.generateMnemonic(256);
  const passphrase = 'test passphrase';

  // 2. Derive address
  const seed = await Bip39.mnemonicToSeed(mnemonic, passphrase);
  const original = deriveFirstAddress(seed);

  // 3. Write mnemonic and passphrase separately
  console.log('Write mnemonic:', mnemonic);
  console.log('Write passphrase (separately):', passphrase);

  // 4. Simulate recovery
  const writtenMnemonic = prompt('Enter mnemonic:');
  const writtenPassphrase = prompt('Enter passphrase:');

  // 5. Verify
  const recovered = await Bip39.mnemonicToSeed(writtenMnemonic, writtenPassphrase);
  const recoveredAddress = deriveFirstAddress(recovered);

  if (original !== recoveredAddress) {
    throw new Error('Recovery failed! Check backup.');
  }

  console.log('✅ Recovery successful');
}

3. Document Passphrase Usage

interface WalletDocumentation {
  // Safe to document
  hasPassphrase: boolean;

  // Safe to document (helps recovery)
  passphraseType: 'none' | 'memorized' | 'written-separately';

  // NEVER document actual passphrase
  hints?: string[];
}

const docs: WalletDocumentation = {
  hasPassphrase: true,
  passphraseType: 'memorized',
  hints: ['Favorite book + wedding year']
};

4. Never Reuse Passphrases

// ❌ Reusing passphrase across wallets
const passphrase = 'shared secret';
const wallet1 = await Bip39.mnemonicToSeed(mnemonic1, passphrase);
const wallet2 = await Bip39.mnemonicToSeed(mnemonic2, passphrase);

// ✅ Unique passphrase per wallet
const wallet1Pass = 'unique secret for wallet 1';
const wallet2Pass = 'unique secret for wallet 2';

Examples

Passphrase Usage - Plausible deniability with passphrases
Full Workflow - Complete wallet creation with passphrase

Try it Live

​Overview

​How Passphrases Work

​PBKDF2 Salt Modification

​Different Passphrases = Different Wallets

​Use Cases

​1. Plausible Deniability

​2. Two-Factor Security

​3. Enhanced Entropy

​4. Inheritance/Time-Lock

​Passphrase Strength

​Weak Passphrases

​Strong Passphrases

​Passphrase Entropy Calculator

​Passphrase Management

​Memorization

​Storage (If Must Write)

​Hint System

​Testing Passphrases

​Verification

​Typo Detection

​Edge Cases

​Empty String

​Whitespace

​Unicode Characters

​Unicode Normalization

​Security Considerations

​Forgetting Passphrase

​Brute Force Resistance

​Best Practices

​Examples

​References