C# AES/GCM/NoPadding

AESGCMUtility.cs

using System;
using System.Text;
using System.Linq;

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

/**
 * Author:		Satyendra Singh
 * Created:		19-08-2022
 * Description:	Perform AES encryption based on pre-generated secret key and IV/Nonce.
 * 
 * Disclaimer:	This code is intended for demo purpose only, basis application need ensure implementation of error and exception handling.
*/
namespace AES.Class {
    public class AESGCMUtility {
        private const int KEY_BIT_SIZE = 256;
        private const int MAC_BIT_SIZE = 128;

		/// Encrypt method to encrypt message(plain-text) based on secret key and nonce
        public static string encrypt(string message, string secretKey, string nonce) {            
            var plainText = Encoding.UTF8.GetBytes(message);
			var decodedKey = hexStringToByteArray(secretKey);            
			var decodedNounce = hexStringToByteArray(nonce);
            var cipherText = encrypt(plainText, decodedKey, decodedNounce);
            return convertByteArrayToHexString(cipherText);
        }

        private static byte[] encrypt(byte[] text, byte[] key, byte[] nonce) {            
            var cipher = new GcmBlockCipher(new AesEngine());
            var parameters = new AeadParameters(new KeyParameter(key), MAC_BIT_SIZE, nonce);
            cipher.Init(true, parameters);

			var cipherText = new byte[cipher.GetOutputSize(text.Length)];
            var len = cipher.ProcessBytes(text, 0, text.Length, cipherText, 0);
            cipher.DoFinal(cipherText, len);

            return cipherText;
        }

		/// Convert HEX string into byte array.
		private static byte[] hexStringToByteArray(string hex) {
    		return Enumerable.Range(0, hex.Length)
                     .Where(x => x % 2 == 0)
                     .Select(x => Convert.ToByte(hex.Substring(x, 2), 16))
                     .ToArray();
		}

		/// Convert byte array into HEX string format.
		private static string convertByteArrayToHexString(byte[] data) {
			return BitConverter.ToString(data).Replace("-", string.Empty);
		}
 
		/// Demo code to call encrypt method.
		public static void Main(string[] args) {
            string value = encrypt("plain text message or data", "secret key in hex format", "nonce in hex format");
            Console.WriteLine(value);
        }
    }
}