var gcm = new AesGcm(Encoding.UTF8.GetBytes(key));

            byte[] cipherTextBytes = Convert.FromBase64String(_ciphertext);

            byte[] nonce = Encoding.UTF8.GetBytes(_nonce);

            byte[] ciphertext = cipherTextBytes.Take(cipherTextBytes.Length - 16).ToArray();

            byte[] plaintext = new byte[ciphertext.Length];

            byte[] tag = cipherTextBytes.Skip(ciphertext.Length).Take(16).ToArray();

            byte[] associatedData = Encoding.UTF8.GetBytes(_associated_data);

            gcm.Decrypt(nonce, ciphertext, tag, plaintext, associatedData);

            string text = Encoding.UTF8.GetString(plaintext);

提示错误

System.Security.Cryptography.CryptographicException:“The computed authentication tag did not match the input authentication tag.”