Uses the SubtleCrypto interface of the Web Cryptography API to hash a password using PBKDF2, and validate a stored password hash against a subsequently supplied password. Note that both bcrypt and scrypt offer better defence against ASIC/GPU attacks, but are not available within WebCrypto.

crypto-pbkdf2.js

/**
 * Returns PBKDF2 derived key from supplied password.
 *
 * Stored key can subsequently be used to verify that a password matches the original password used
 * to derive the key, using pbkdf2Verify().
 *
 * @param   {String} password - Password to be hashed using key derivation function.
 * @param   {Number} [iterations=1e6] - Number of iterations of HMAC function to apply.
 * @returns {String} Derived key as base64 string.
 *
 * @example
 *   const key = await pbkdf2('pāşšŵōřđ'); // eg 'djAxBRKXWNWPyXgpKWHld8SWJA9CQFmLyMbNet7Rle5RLKJAkBCllLfM6tPFa7bAis0lSTiB'
 */
async function pbkdf2(password, iterations=1e6) {
    const pwUtf8 = new TextEncoder().encode(password);                                           // encode pw as UTF-8
    const pwKey = await crypto.subtle.importKey('raw', pwUtf8, 'PBKDF2', false, ['deriveBits']); // create pw key

    const saltUint8 = crypto.getRandomValues(new Uint8Array(16));                                // get random salt

    const params = { name: 'PBKDF2', hash: 'SHA-256', salt: saltUint8, iterations: iterations }; // pbkdf2 params
    const keyBuffer = await crypto.subtle.deriveBits(params, pwKey, 256);                        // derive key
    
    const keyArray = Array.from(new Uint8Array(keyBuffer));                                      // key as byte array

    const saltArray = Array.from(new Uint8Array(saltUint8));                                     // salt as byte array
   
    const iterHex = ('000000'+iterations.toString(16)).slice(-6);                                // iter’n count as hex
    const iterArray = iterHex.match(/.{2}/g).map(byte => parseInt(byte, 16));                    // iter’ns as byte array

    const compositeArray = [].concat(saltArray, iterArray, keyArray);                            // combined array
    const compositeStr = compositeArray.map(byte => String.fromCharCode(byte)).join('');         // combined as string
    const compositeBase64 = btoa('v01'+compositeStr);                                            // encode as base64

    return compositeBase64;                                                                      // return composite key
}


/**
 * Verifies whether the supplied password matches the password previously used to generate the key.
 *
 * @param   {String}  key - Key previously generated with pbkdf2().
 * @param   {String}  password - Password to be matched against previously derived key.
 * @returns {boolean} Whether password matches key.
 *
 * @example
 *   const match = await pbkdf2Verify(key, 'pāşšŵōřđ'); // true
 */
async function pbkdf2Verify(key, password) {
    let compositeStr = null; // composite key is salt, iteration count, and derived key
    try { compositeStr = atob(key); } catch (e) { throw new Error ('Invalid key'); }             // decode from base64
    
    const version = compositeStr.slice(0, 3);   //  3 bytes
    const saltStr = compositeStr.slice(3, 19);  // 16 bytes (128 bits)
    const iterStr = compositeStr.slice(19, 22); //  3 bytes
    const keyStr  = compositeStr.slice(22, 54); // 32 bytes (256 bits)

    if (version != 'v01') throw new Error('Invalid key');
    
    // -- recover salt & iterations from stored (composite) key
    
    const saltUint8 = new Uint8Array(saltStr.match(/./g).map(ch => ch.charCodeAt(0)));           // salt as Uint8Array
    // note: cannot use TextEncoder().encode(saltStr) as it generates UTF-8
   
    const iterHex = iterStr.match(/./g).map(ch => ch.charCodeAt(0).toString(16)).join('');       // iter’n count as hex
    const iterations = parseInt(iterHex, 16);                                                    // iter’ns
    
    // -- generate new key from stored salt & iterations and supplied password
    
    const pwUtf8 = new TextEncoder().encode(password);                                           // encode pw as UTF-8
    const pwKey = await crypto.subtle.importKey('raw', pwUtf8, 'PBKDF2', false, ['deriveBits']); // create pw key

    const params = { name: 'PBKDF2', hash: 'SHA-256', salt: saltUint8, iterations: iterations }; // pbkdf params
    const keyBuffer = await crypto.subtle.deriveBits(params, pwKey, 256);                        // derive key
    const keyArray = Array.from(new Uint8Array(keyBuffer));                                      // key as byte array
    const keyStrNew = keyArray.map(byte => String.fromCharCode(byte)).join('');                  // key as string
    
    return keyStrNew == keyStr; // test if newly generated key matches stored key
}

@tracker1 this script uses regular equality to compare a 'good' input. regular equality is shortcut, meaning the more correct it is, the more time it will take to verify (as the comparison will bail as soon as a byte fails to compare). The security impact of this is subtle and depends a lot on where this is deployed, but in cases where an attacker can cause this to be called many times and measure response time, they can probably guess the correct PBKDF2 digest.

This took me way longer than it should have so, anybody else who might benefit from it, here you go. I think it's secure...

<script type="text/javascript">
const deriveKeyAndIv = async (password, salt) => {
  const passwordKey = await crypto.subtle.importKey(
    'raw',
    password,
    'PBKDF2',
    false,
    ['deriveBits']
  )
  const keyLength = 32
  const ivLength = 16
  const numBits = (keyLength + ivLength) * 8
  const derviedBytes = await crypto.subtle.deriveBits({
    name: 'PBKDF2',
    hash: 'SHA-512',
    salt,
    iterations: 10000
  }, passwordKey, numBits)
  const key = await crypto.subtle.importKey(
    'raw',
    derviedBytes.slice(0, keyLength),
    'AES-GCM',
    false,
    ['encrypt', 'decrypt']
  )
  const iv = derviedBytes.slice(keyLength, keyLength + ivLength)
  return {
    key,
    iv
  }
}

const encrypt = async (password, salt, plainText) => {
  const { key, iv } = await deriveKeyAndIv(password, salt)
  return crypto.subtle.encrypt({
    name: 'AES-GCM',
    iv
  }, key, plainText)
}

const decrypt = async (password, salt, cipher) => {
  const { key, iv } = await deriveKeyAndIv(password, salt)
  return crypto.subtle.decrypt({
    name: 'AES-GCM',
    iv
  }, key, cipher)
}

const utf8ToUint8Array = (input) => new TextEncoder().encode(input)

const arrayBufferToUtf8 = (input) => new TextDecoder().decode(new Uint8Array(input))

const arrayBufferToHex = (input) => {
  input = new Uint8Array(input)
  const output = []
  for (let i = 0; i < input.length; ++i) {
    output.push(input[i].toString(16).padStart(2, '0'))
  }
  return output.join('')
}

const run = async () => {
  const password = utf8ToUint8Array('fdcf72d4-7c59-4240-a527-6630fc92fcbb')
  const salt = utf8ToUint8Array('233f9fad-7681-4ebd-ad5e-164480bbc3f5')
  const data = utf8ToUint8Array('Hello, world!')
  const cipher = await encrypt(password, salt, data)
  const plainText = await decrypt(password, salt, cipher)
  console.log(arrayBufferToHex(cipher))
  console.log(arrayBufferToUtf8(plainText))
}

run()
</script>