using System;

using System.IO;

using System.Security.Cryptography;

using System.Text;

​

public class Program {

    public static void Main() {

        Console.WriteLine("AES GCM 256 String decryption");

​

        // decryption

        Console.WriteLine("\n* * * Decryption * * *");

        string password = "A random password to encrypt";

        //generate hash of password # # # this is UNSECURE # # #

        SHA256 mySHA256 = SHA256.Create();

        byte[] Key = mySHA256.ComputeHash(Encoding.UTF8.GetBytes(password));

        // ciphertext taken from encryption function in PHP     

        string soCiphertextBase64 = "aV+gDmSBbi9PjOT9FD8LcuISbEQ5F3q0X8qzf3MKiDzxo12WQVirsnltbApLMMG9JScVfTXx7PJw7EVFoKz8JLMYLMu/JsRGcfvihSK+d/yeRTBEuJHL74Hv2Zr7b4CoMJhEUmYF3KT2Onlj4lI5ChOjmgXvpSev/xc=";

        Console.WriteLine("ciphertext (Base64): " + soCiphertextBase64);

        string soDecryptedtext = soAesGcmDecryptFromBase64(Key, soCiphertextBase64);

        Console.WriteLine("plaintext: " + soDecryptedtext);

    }

​

    static string soAesGcmDecryptFromBase64(byte[] key, string data) {

        const int MAC_BIT_SIZE = 128;

        const int NONCE_BIT_SIZE = 96; // 12 bytes (openssl)

        byte[] EncryptedData = Convert.FromBase64String(data);

        byte[] IV;

        byte[] CipherText;

        byte[] Tag;

        using (MemoryStream MStream = new MemoryStream(EncryptedData))

        using (BinaryReader Binary = new BinaryReader(MStream)) {

            IV = Binary.ReadBytes(NONCE_BIT_SIZE / 8);

            CipherText = Binary.ReadBytes(EncryptedData.Length - IV.Length - (MAC_BIT_SIZE / 8));

            Tag = Binary.ReadBytes((MAC_BIT_SIZE / 8));

        }

        string decryptedtext;

        byte[] associatedData = new byte[0];

        byte[] decryptedData = new byte[CipherText.Length];

        using(var cipher = new AesGcm(key)) {

            cipher.Decrypt(IV, CipherText, Tag, decryptedData, associatedData);

            decryptedtext = Encoding.UTF8.GetString(decryptedData, 0, decryptedData.Length);

            return decryptedtext;

        }

    }

}

​