dart_dash_otp

Pure-Dart TOTP and HOTP one-time passwords for two-factor authentication (2FA) and multi-factor authentication (MFA). Implements RFC 6238 (TOTP) and RFC 4226 (HOTP), generates and parses Google Authenticator otpauth:// URIs, and verifies codes with drift tolerance and constant-time comparison.

Features

• TOTP and HOTP — generate and verify time-based (RFC 6238) and counter-based (RFC 4226) one-time passwords.
• RFC test-vector validated — checked against the RFC 4226 Appendix D and RFC 6238 reference vectors.
• Configurable — 6 to 8 digit codes and SHA-1 / SHA-256 / SHA-384 / SHA-512 HMAC algorithms.
• Drift tolerance — a window parameter accepts neighbouring time steps (TOTP) or look-ahead counters (HOTP).
• Constant-time verification — code comparison runs in constant time so response timing does not leak how close a guess was.
• otpauth:// URIs — emit (generateUrl) and parse (TOTP.fromUri / HOTP.fromUri) Google Authenticator Key URI Format links, ready for QR codes.
• Secure random secrets — OTP.randomSecret() generates Base32 keys using Random.secure.
• TOTP countdown — remainingSeconds() reports how long the current code stays valid, for countdown UIs.
• Pure Dart, zero Flutter dependency — runs in Flutter apps, server-side Dart, and CLIs. (On the web, integers are exact only up to 2^53, so the astronomically large HOTP counters beyond that need a native-int target; ordinary counters and TOTP are unaffected.)

Installation

Add the dependency to your pubspec.yaml:

dependencies:
  dart_dash_otp: ^2.0.0

Then fetch it:

dart pub get

Quick start

import 'package:dart_dash_otp/dart_dash_otp.dart';

void main() {
  final totp = TOTP(secret: OTP.randomSecret());

  final code = totp.now();          // current 6-digit code
  final ok = totp.verify(otp: code); // true

  print('Code $code is valid: $ok');
}

Usage

TOTP — time-based (RFC 6238)

import 'package:dart_dash_otp/dart_dash_otp.dart';

void main() {
  final totp = TOTP(secret: 'J22U6B3WIWRRBTAV');

  totp.now();                                 // current 6-digit code
  totp.value(date: DateTime.utc(2024, 1, 1)); // code for a specific instant

  // Strict verification against the current time step.
  totp.verify(otp: '123456');

  // Tolerate +/-1 time step (e.g. 30 s of clock drift) on the server.
  totp.verify(otp: '123456', window: 1);

  // Seconds the current code is still valid, for a countdown indicator.
  final ttl = totp.remainingSeconds(); // 1..interval
  print('Code expires in $ttl s');
}

value() returns null when date is omitted; pass a DateTime to compute a code for a specific instant. Customise the digits, time step and algorithm:

final totp = TOTP(
  secret: 'J22U6B3WIWRRBTAV',
  digits: 8,
  interval: 60,
  algorithm: OTPAlgorithm.SHA256,
);

HOTP — counter-based (RFC 4226)

import 'package:dart_dash_otp/dart_dash_otp.dart';

void main() {
  // The RFC 4226 Appendix D seed, so the codes below match the spec.
  final hotp = HOTP(secret: 'GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ');

  hotp.at(counter: 0);    // '755224' (RFC 4226 Appendix D vector)
  hotp.at(counter: 2019); // code for counter 2019

  // Strict verification.
  hotp.verify(otp: '755224', counter: 0);

  // Look-ahead window to tolerate a few missed client clicks.
  hotp.verify(otp: '755224', counter: 0, window: 3);
}

Persist the counter. HOTP relies on both sides keeping the counter in sync. After a successful verify, store the next counter value (matchedCounter + 1) on the server and never accept it again — this is what prevents replay.

otpauth:// URIs

Export a token a standard authenticator app can scan:

final totp = TOTP(secret: 'J22U6B3WIWRRBTAV');
totp.generateUrl(issuer: 'Acme', account: 'alice@example.com');
// otpauth://totp/Acme:alice%40example.com?secret=J22U6B3WIWRRBTAV
//   &issuer=Acme&digits=6&algorithm=SHA1&period=30

When an issuer is provided the label takes the recommended Issuer:Account form and every query parameter is URL-encoded.

Parse a scanned URI back into a token:

final totp = TOTP.fromUri(
  'otpauth://totp/Acme:alice@example.com'
  '?secret=J22U6B3WIWRRBTAV&digits=6&period=30&algorithm=SHA1',
);
totp.now();

// HOTP URIs require a numeric `counter` parameter.
final hotp = HOTP.fromUri(
  'otpauth://hotp/Acme:alice@example.com'
  '?secret=J22U6B3WIWRRBTAV&counter=0',
);
hotp.at();

fromUri reads only the secret, digits, algorithm and period/counter parameters (with defaults of 6 digits, 30 s period and SHA-1). The label and issuer are display metadata and are not stored on the returned token.

Random secrets

Provision a new token with a cryptographically secure Base32 secret:

final secret = OTP.randomSecret();          // 32 chars = 160-bit key
final shortSecret = OTP.randomSecret(length: 16); // 80-bit RFC 4226 minimum

final totp = TOTP(secret: secret);

Errors

Invalid construction arguments throw ArgumentError at runtime (not only in debug mode):

Source	Argument	Rule
TOTP / HOTP / OTP	digits	between 6 and 8
TOTP / HOTP / OTP	secret	non-empty, valid uppercase Base32 (A–Z, 2–7) that decodes to at least one byte (e.g. "ABC" is rejected)
TOTP	interval	positive integer
HOTP	counter	non-negative integer
verify	window	non-negative integer
OTP.randomSecret	length	at least 16 characters

TOTP.fromUri and HOTP.fromUri throw FormatException for a wrong scheme or host, a missing secret, a missing/non-numeric counter (HOTP), or an unknown algorithm; out-of-range values such as digits=10 still throw ArgumentError.

Documentation

Detailed guides live in doc/:

Page	Description
Overview	Documentation index and where to start.
Getting started	Install, first TOTP/HOTP token, and verification.
TOTP guide	Time-based codes, intervals, drift windows, countdown.
HOTP guide	Counter-based codes, look-ahead, counter persistence.
otpauth:// URIs	Generating and parsing Key URI Format links for QR codes.
Security considerations	Secret storage, replay protection, window sizing.
Flutter integration	Using the package in a Flutter app.
Migration from 1.x	Breaking changes and upgrade steps for 2.0.0.
Publishing	Release and tagging workflow for maintainers.
FAQ	Common questions and gotchas.

Security considerations

• Verify on the server. Treat client-side generation as convenience only; the authoritative verify belongs on a trusted backend.
• Store secrets carefully. Keep shared secrets encrypted at rest and never expose them through logs, URLs or analytics.
• Protect HOTP against replay. After a match, advance and persist the counter so the same code can never be accepted twice.
• Keep the window small. A window of 0 or 1 is enough for normal clock drift; larger windows widen the brute-force surface.

See doc/security-considerations.md for the full discussion.

Migrating from 1.x

Version 2.0.0 is a major rewrite with validated inputs, fromUri parsing, secure secret generation and verification windows. See doc/migration-from-1x.md for the complete list of breaking changes and an upgrade checklist.

Contributing

Issues and pull requests are welcome. CI runs dart analyze and the full test suite (including the RFC test vectors) on every push and pull request via the CI workflow.

Release notes

See CHANGELOG.md.

License

MIT.