using System;

using System.Security.Cryptography;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        AESGCMEncryptor ob=new AESGCMEncryptor();

        string enctext=ob.Encrypt("eVIloWh9y5BdNWq0FBXiCrtJq5QteSJ4","Hello World");

        Console.WriteLine("encrypted text-->"+enctext);

        string plain=ob.Decrypt("eVIloWh9y5BdNWq0FBXiCrtJq5QteSJ4","B0McNWvIue6U8XZMzf6IvewpVUim");

        Console.WriteLine("decrypt text-->"+plain);

    }

​

public class AESGCMEncryptor

{

    private const int GCM_IV_LENGTH = 12;  

    private const int GCM_TAG_LENGTH = 16; // GCM authentication tag length (128 bits)

    public string Encrypt(string Key,string Plaintext)

    {

        // Convert the key into a byte array (ensure key length matches AES expectations)

        byte[] key = Encoding.UTF8.GetBytes(Key);

         string ivKey = Key.Substring(0, 12);

        // Generate a random IV (Initialization Vector)

        byte[] iv = Encoding.UTF8.GetBytes(ivKey);

        //using (var rng = new RNGCryptoServiceProvider())

        //{

          //  rng.GetBytes(iv); // Fill the IV with random values

        //}

​

        // Convert the plaintext to bytes

        byte[] plaintext = Encoding.UTF8.GetBytes(Plaintext);

        // Prepare the AES GCM cipher

        using (AesGcm aesGcm = new AesGcm(key,16))

        {

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

            byte[] tag = new byte[GCM_TAG_LENGTH];

​

            // Encrypt the plaintext using AES GCM

            aesGcm.Encrypt(iv, plaintext, ciphertext, tag);

​

            // Combine the IV, ciphertext, and tag to send it all together

            byte[] result = new byte[ ciphertext.Length + GCM_TAG_LENGTH];

           // Array.Copy(iv, 0, result, 0, GCM_IV_LENGTH);

            Array.Copy(ciphertext, 0, result, 0, ciphertext.Length);

            Array.Copy(tag, 0, result, ciphertext.Length, GCM_TAG_LENGTH);

​

            // Return the result as a Base64 encoded string

            return Convert.ToBase64String(result);

        }

    }

     public string Decrypt(string Key,string enctext)

    {

         try

         {

        // Convert the key into a byte array

        byte[] key =Convert.FromBase64String(Key); // Encoding.UTF8.GetBytes(Key);

         Console.WriteLine("Key== " +Key);

             Console.WriteLine("Key length== " +key.Length);

        // Decode the Base64 encoded ciphertext (IV + Ciphertext + Tag)

        byte[] cipherTextWithTag = Convert.FromBase64String(enctext);

        string ivKey = Key.Substring(0, 16);

                 Console.WriteLine("ivKey== " +ivKey);

        // Generate a random IV (Initialization Vector)

       byte[] iv =Convert.FromBase64String(ivKey); //Encoding.UTF8.GetBytes(ivKey);

             Console.WriteLine("ivKey length== " +iv.Length);

        // Extract the IV, Ciphertext, and Tag

        //byte[] iv = new byte[GCM_IV_LENGTH];

            // byte[] iv = new byte[12];

        //byte[] cipherText = new byte[cipherTextWithTag.Length -  GCM_TAG_LENGTH];

             byte[] cipherText = new byte[cipherTextWithTag.Length -  16];

            //byte[] cipherText = Convert.FromBase64String(enctext);

        byte[] tag = new byte[16];

 Console.WriteLine("cipherText length== " +cipherText.Length);

              Console.WriteLine("tag length== " +tag.Length);

       //Array.Copy(cipherTextWithTag, 0, iv, 0, GCM_IV_LENGTH);

        Array.Copy(cipherTextWithTag, 0, cipherText, 0, cipherTextWithTag.Length-16);

        Array.Copy(cipherTextWithTag,  cipherTextWithTag.Length-16, tag, 0, GCM_TAG_LENGTH);

        // Decrypt the data using AES GCM

       using (AesGcm aesGcm = new AesGcm(key,16))

        {

            byte[] plainText = new byte[cipherTextWithTag.Length-16];

           aesGcm.Decrypt(iv, cipherText, tag, plainText);

           //aesGcm.Decrypt(plainText);

            return Encoding.UTF8.GetString(plainText);

        }

         }

         catch (Exception ex)

        {

            Console.WriteLine(ex.Message);

        }

         return null;

    }

}

}