generateNonce function
Generate random nonce for one-time use.
Used for selective encryption where each field needs unique nonce.
Implementation
String generateNonce() {
final timestamp = DateTime.now().millisecondsSinceEpoch;
final micro = DateTime.now().microsecond;
final random = '$timestamp$micro';
return sha256.convert(utf8.encode(random)).toString().substring(0, 16);
}