2fa-kit

2fa-kit

Add Google-Authenticator-style 2FA to your app. Zero dependencies, runs on Node 20+, Bun, Deno, edge workers, and browsers.

What you need

• Node.js 20 or newer (nothing else to install, no native modules)
• One environment variable: MASTER_KEY, any long random string (used to encrypt stored secrets, same idea as a JWT secret)
• Three columns on your user record: totp_secret, totp_salt, and totp_last_step (an integer, for replay protection)

Install

npm install 2fa-kit

The whole flow in two steps

 enrol:   server makes a secret --> user scans QR --> server stores it encrypted
 login:   user types 6-digit code --> server decrypts secret --> verify --> allow/deny

Step 1: enrol a user

import { createVault, generateSecret, buildUri } from "2fa-kit";

const vault = await createVault(process.env.MASTER_KEY!);

const secret = await generateSecret();
const uri = buildUri({ label: user.email, secret, issuer: "Acme" });
// show `uri` as a QR code (see below) - the user scans it once

const { encrypted, salt } = await vault.encrypt(secret);
// save `encrypted` and `salt` on the user record

Step 2: verify at login

import { verifyTotpWithDelta } from "2fa-kit";

const secret = await vault.decrypt(user.totpSecret, user.totpSalt);
const { valid, step } = await verifyTotpWithDelta(secret, codeFromLoginForm);

if (!valid || step! <= user.totpLastStep) deny();
user.totpLastStep = step!; // persist, then allow

The step check is replay protection, and it is not optional: codes stay valid for up to 90 seconds of clock drift, so a code that is only checked with a boolean can be intercepted and used again. Storing the last accepted step and requiring each login to beat it closes that door (RFC 6238 requires it). verifyTotp still exists and returns a plain boolean for cases where replay is handled elsewhere.

That is the entire integration. Everything below is optional extras.

Showing the QR code

The library gives you the URI; any QR library renders it:

import QRCode from "qrcode";
const dataUrl = await QRCode.toDataURL(uri); // <img src={dataUrl} />

Extras

Backup codes - one-time recovery codes when the user loses their phone. Store only the hashes. Pass your master key so a leaked database row cannot be brute-forced offline:

const { codes, hashed } = await generateBackupCodes({ key: masterKey });
// show `codes` once, store `hashed`

const { valid, remaining } = await verifyBackupCode(input, user.backupCodes, { key: masterKey });
if (!valid) deny();
user.backupCodes = remaining; // codes are single-use: persist, then allow

verifyBackupCode accepts any case, with or without dashes, and compares in constant time.

Import from Google Authenticator - decode a "Transfer accounts" export QR:

const accounts = await parseMigrationUri(migrationUri); // -> ParsedOtpauth[]

API

Full signatures and options are in the TypeScript types (dist/index.d.ts).

Notes

• Defaults match the authenticator ecosystem: SHA-1, 6 digits, 30 seconds.
• Secrets are accepted lowercase, unpadded, or with spaces, just like the apps display them.
• The vault never stores plaintext: wrong master key or tampered data throws instead of decrypting.
• Losing MASTER_KEY makes stored secrets unrecoverable. Back it up.

License

MIT - see LICENSE.