// Plaintext to encrypt.
var plaintext = "I'm cooking MC's like a pound of bacon";

// Create a 32-byte key.
var key = new byte[32];
RandomNumberGenerator.Fill(key);

// Create a 12-byte nonce.
var nonce = new byte[12];
RandomNumberGenerator.Fill(nonce);

// Create a new AesGcmSiv instance. It implements the IDisposable
// interface, so it's best to create it inside using statement.
using (var siv = new AesGcmSiv(key))
{
  // If the message is string, convert it to byte array first.
  var bytes = Encoding.UTF8.GetBytes(plaintext);

  // Encrypt the message.
  var ciphertext = new byte[bytes.Length];
  var tag = new byte[16];
  siv.Encrypt(nonce, bytes, ciphertext, tag);

  // To decrypt the message, call the Decrypt method with the
  // ciphertext and the same nonce that you generated previously.
  siv.Decrypt(nonce, ciphertext, tag, bytes);

  // If the message was originally string,
  // convert if from byte array to string.
  plaintext = Encoding.UTF8.GetString(bytes);

  // Print the decrypted message to the standard output.
  Console.WriteLine(plaintext);
}