digestAuthEmailOtpCode function

String digestAuthEmailOtpCode({ required String code, required String secret, })

Produces the keyed digest persisted for a low-entropy email OTP.

A plain SHA-256 digest does not protect a short numeric OTP from offline enumeration after a database leak. This boundary therefore requires an application secret and domain-separates email OTP material from other HMAC uses. The raw code must only be retained long enough for delivery.

Implementation

String digestAuthEmailOtpCode({required String code, required String secret}) {
  final normalized = code.trim();
  final key = utf8.encode(secret);
  if (normalized.isEmpty || normalized.length > authEmailOtpMaximumLength) {
    throw ArgumentError(
      'must be non-empty and at most $authEmailOtpMaximumLength characters',
      'code',
    );
  }
  if (key.length < authEmailOtpDigestKeyMinimumLength) {
    throw ArgumentError(
      'must contain at least $authEmailOtpDigestKeyMinimumLength UTF-8 bytes',
      'secret',
    );
  }
  return Hmac(
    sha256,
    key,
  ).convert(utf8.encode('routed-auth:email-otp:$normalized')).toString();
}