generateHMACKey method
Implementation
Future<String> generateHMACKey(List<String> masterKeys, String email) async {
final mk = masterKeys.last;
if (mk.isEmpty) throw Exception('No master keys available');
final emailBytes = utf8.encode(email.toLowerCase());
final mkBytes = utf8.encode(mk);
final derived = pbkdf2(mkBytes, emailBytes, 1, 32);
return HEX.encode(derived).toLowerCase();
}