using System;

using System.Linq;

using System.Security.Cryptography;

using System.Text;

​

namespace AesGcmNetCoreTestApp

{

    internal class Program

    {

        static void Main(string[] args)

        {

            try

            {

                var cipherText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

                cipherText = cipherText.Replace(" ", "");

                var cipherTextBytes = ConvertHexStringToByteArray(cipherText);

​

                var encryptionKey = "5AD84121D9D20B364B7A11F3C1B5827F";

                var encryptionKeyBytes = ConvertHexStringToByteArray(encryptionKey);

                var authenticationKey = "AFB3F93E3E7204EDB3C27F96DBD51AE0";

                var authenticationKeyBytes = ConvertHexStringToByteArray(authenticationKey);

                var peek = ConvertByteArrayToString(encryptionKeyBytes);

​

                var systemTitle = cipherTextBytes.SelectIndexRange(2, 10);

                var initializationVector = systemTitle.MergeWith(cipherTextBytes.SelectIndexRange(14, 18));

                var additionalAuthenticatedData = cipherTextBytes.SelectIndexRange(13, 14).MergeWith(authenticationKeyBytes);

                var authenticationTag = cipherTextBytes.SelectIndexRange(cipherTextBytes.Length - 12, cipherTextBytes.Length);

​

                var actualCipherTextBytes = cipherTextBytes.SelectIndexRange(18, cipherTextBytes.Length - 12); // pass actual ciphertext

                var res = DecryptGcm(actualCipherTextBytes, encryptionKeyBytes, authenticationTag, initializationVector, additionalAuthenticatedData); // pass AAD

                Console.WriteLine(ConvertByteArrayToString(res));

            }

            catch (Exception e)

            {

                Console.WriteLine(e);

                Console.ReadLine();

            }

        }

​

        static byte[] DecryptGcm(byte[] cipherBytes, byte[] encKey, byte[] authTag, byte[] nonce, byte[] aad) // additionally pass aad

        {

            byte[] plainBytes = new byte[cipherBytes.Length];

​

            using (AesGcm aesGcm = new AesGcm(encKey))

            {

                aesGcm.Decrypt(nonce, cipherBytes, authTag, plainBytes, aad); // pass AAD as 5th parameter

            }

​

            return plainBytes;

        }

​

        public static string ConvertByteArrayToString(byte[] ba)

        {

            StringBuilder hex = new StringBuilder(ba.Length * 2);

            foreach (byte b in ba)

                hex.AppendFormat("{0:x2}", b);

            return hex.ToString();

        }

​

        public static byte[] ConvertHexStringToByteArray(string hex)

        {

            return Enumerable.Range(0, hex.Length)

                .Where(x => x % 2 == 0)

                .Select(x => Convert.ToByte(hex.Substring(x, 2), 16))

                .ToArray();

        }

    }

    public static class ExtensionMethods

    {

        public static byte[] SelectIndexRange(this byte[] bytes, int from, int to)

        {

            if (bytes == null) throw new ArgumentNullException("Bytes array cannot be empty.");

            if (bytes.Length == 0) throw new ArgumentException("Bytes array cannot be empty.");

​

            if (bytes.Length < from) throw new ArgumentException("From index cannot be lower than length!");

            if (bytes.Length < to) throw new ArgumentException("To index cannot be higher than length.");

​

            var res = bytes.Skip(from).Take(to - from).ToArray();

            return res;

        }

​

        public static byte[] MergeWith(this byte[] origByte, byte[] mergeWithBytes)

        {

            if (origByte.Length == 0 && mergeWithBytes.Length == 0) return Array.Empty<byte>();

​

            var totalLength = origByte.Length + mergeWithBytes.Length;

​

            var resBytes = new byte[totalLength];

​

            for (int i = 0; i < origByte.Length; i++)

            {

                resBytes[i] = origByte[i];

            }

​

            for (int i = 0; i < mergeWithBytes.Length; i++)

            {

                var index = i + origByte.Length;

                resBytes[index] = mergeWithBytes[i];

            }

​

            return resBytes;

        }

    }

}