// Download OpenSSL.NET (openssl-net) library
// from https://github.com/openssl-net/openssl-net
// In your .NET project, add a reference to the ManagedOpenSsl.dll assembly.

//use headers
//using OpenSSL.Crypto;
//using System.Text;
//using System;

/// <summary>
/// Method for decrypting the message
/// </summary>
/// <param name="msg">encrypted message</param>
/// <returns>decrypted message</returns>

public string DecryptData(string msg)
{
    CipherContext AES_128_ECB = new CipherContext(Cipher.AES_128_ECB);
    string encryptionKey = "d83a91f4596d7a9c199997d24f5bfb52";
    var iv = Encoding.ASCII.GetBytes("12345678");
    var key = Encoding.ASCII.GetBytes(encryptionKey);

    using (var cc = new CipherContext(AES_128_ECB.Cipher))
    {
        var msgBytes = Convert.FromBase64String(msg);
        var ct = cc.Decrypt(msgBytes, key, iv);
        var decryptedMsg = Encoding.ASCII.GetString(ct);
        Console.WriteLine("\"{0}\"", decryptedMsg);
        return decryptedMsg;
    }
}