nonce method
Generates a cryptographically secure random nonce
Implementation
String nonce([int length = 32]) {
const charset =
'0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._';
return List.generate(length, (_) => charset[nextInt(charset.length)])
.join();
}