using System;

using System.Security.Cryptography;

using System.Text;

using System.IO;

public class Program

{

        public static string AesDecrypt(string value, string key, string iv = null, int keySize = 128, CipherMode Mode = CipherMode.CBC)

        {

            RijndaelManaged rj = new RijndaelManaged();

            UTF8Encoding encoding = new UTF8Encoding();

​

            byte[] message = Convert.FromBase64String(value);

            byte[] plainTextBytes = new byte[message.Length];

            int byteCount = 0;

            try

            {

                byte[] keyBytes = encoding.GetBytes(key);

                rj.Key = keyBytes;

                // Set or generate IV

                if (String.IsNullOrEmpty(iv))

                    rj.GenerateIV();

                else

                {

                    byte[] ivBytes = encoding.GetBytes(iv);

                    rj.IV = ivBytes;

                }

​

                rj.Mode = Mode;

                rj.KeySize = keySize;

                rj.BlockSize = 128;

                rj.Padding = PaddingMode.PKCS7;

​

                using (ICryptoTransform Encryptor = rj.CreateDecryptor(keyBytes, rj.IV))

                {

                    using (MemoryStream MemStream = new MemoryStream(message))

                    {

                        using (CryptoStream CryptoStream = new CryptoStream(MemStream, Encryptor, CryptoStreamMode.Read))

                        {

                            byteCount = CryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);

                            MemStream.Close();

                            CryptoStream.Close();

                        }

                    }

                }

            }

            finally

            {

                rj.Clear();

            }

​

            return encoding.GetString(plainTextBytes, 0, byteCount);

        }

    public static void Main()

    {

        Console.WriteLine("running example...");

​

        // Encrypted User Id provided by API response.

        string UserID = "5140C8FBECB4E45AC0E1C582662C49169F7B53F171D244C4DA346D864D382B2F";     

        string key = "ggC2H19lkVbQDfakxcrtNMQdd0FloLoo"; // length == 32. Provided by OnMobile

        string iv = "gqLOHUioQ0QjhuvI"; // length == 16. Provided by OnMobile

​

        // Call function with provided parameters.

        string decryptedUserId = AesDecrypt(UserID, key, iv);

        Console.WriteLine(decryptedUserId);

    }

}