C# Encrypt/Decrypt with BouncyCastle

AesGcmAead.cs

using System;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;

namespace TnnCrypto
{
    public class AesGcmAead
    {
        private const int MacSize = 16 * 8;
        
        private const int NonceSizeBytes = 12;
        
        public byte[] Encrypt(byte[] keyBytes, byte[] plainBytes, byte[] associatedBytes)
        {
            var rdNonce = new byte[NonceSizeBytes];
            var srng = new SecureRandom();
            srng.NextBytes(rdNonce);

            // encrypt
            GcmBlockCipher c = CreateGcmCipher(true, keyBytes, associatedBytes, rdNonce);
            var cipherBytes = new byte[c.GetOutputSize(plainBytes.Length)];
            int len = c.ProcessBytes(plainBytes, 0, plainBytes.Length, cipherBytes, 0);
            c.DoFinal(cipherBytes, len);
            
            // append nonce to the beginning
            return ConcatBytes(rdNonce, cipherBytes);
        }

        public byte[] Decrypt(byte[] keyBytes, byte[] cipherBytes, byte[] associatedBytes)
        {
            // extract the nonce
            byte[] nonce = new byte[NonceSizeBytes];
            Buffer.BlockCopy(cipherBytes, 0, nonce, 0, NonceSizeBytes);
            
            // extract
            GcmBlockCipher c = CreateGcmCipher(false, keyBytes, associatedBytes, nonce);
            int rawCipherLen = cipherBytes.Length - NonceSizeBytes;
            var plainBytes = new byte[c.GetOutputSize(rawCipherLen)];
            int len = c.ProcessBytes(cipherBytes, NonceSizeBytes, rawCipherLen, plainBytes, 0);
            c.DoFinal(plainBytes, len);
            
            // return
            return plainBytes;
        }
        
        private static GcmBlockCipher CreateGcmCipher(bool forEncryption, byte[] keyBytes, byte[] associatedBytes, byte[] rdNonce)
        {
            var parameters = new AeadParameters(new KeyParameter(keyBytes), MacSize, rdNonce, associatedBytes);
            var c = new GcmBlockCipher(new AesEngine());
            c.Init(forEncryption, parameters);
            return c;
        }

        private static byte[] ConcatBytes(byte[] bt1, byte[] bt2)
        {
            var combinedBytes = new byte[bt1.Length + bt2.Length];
            Buffer.BlockCopy(bt1, 0, combinedBytes, 0, bt1.Length);
            Buffer.BlockCopy(bt2, 0, combinedBytes, bt1.Length, bt2.Length);
            return combinedBytes;
        }
    }
}