isValid method
Validates the encryption key format.
Implementation
bool isValid() {
// Must be 64 hex characters (256 bits)
if (encryptionKey.length != 64) return false;
// Must be valid hex (characters 0-9, a-f, A-F)
// Using regex to avoid integer parsing limitations for 256-bit numbers
final hexRegex = RegExp(r'^[0-9a-fA-F]{64}$');
return hexRegex.hasMatch(encryptionKey);
}