using System;

using System.IO;

using System.Security.Cryptography;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Encrypt("5qXFEjyUN","?50*belgium*EXCEPT*galaxy*EXPLAIN*87?"));

        Console.WriteLine(Decrypt("86nLieK72Mj1pZvv8mwrBVodbXRA5HqHnlKbcLo/kDI=","?50*belgium*EXCEPT*galaxy*EXPLAIN*87?"));

    }

    public static string Encrypt(string text, string key) {

        if (string.IsNullOrEmpty(key))

            throw new ArgumentException("Key must have valid value.", nameof(key));

        if (string.IsNullOrEmpty(text))

            throw new ArgumentException("The text must have valid value.", nameof(text));

​

        var buffer = Encoding.UTF8.GetBytes(text);

        var hash = new SHA512CryptoServiceProvider();

        var aesKey = new byte[24];

        Buffer.BlockCopy(hash.ComputeHash(Encoding.UTF8.GetBytes(key)), 0, aesKey, 0, 24);

        using (var aes = Aes.Create()) {

            if (aes == null)

                throw new ArgumentException("Parameter must not be null.", nameof(aes));

​

            aes.Key = aesKey;

            using (var encryptor = aes.CreateEncryptor(aes.Key, aes.IV))

                using (var resultStream = new MemoryStream()) {

                using (var aesStream = new CryptoStream(resultStream, encryptor, CryptoStreamMode.Write))

                    using (var plainStream = new MemoryStream(buffer)) {

                    plainStream.CopyTo(aesStream);

                }

​

                var result = resultStream.ToArray();

                var combined = new byte[aes.IV.Length + result.Length];

                Array.ConstrainedCopy(aes.IV, 0, combined, 0, aes.IV.Length);

                Array.ConstrainedCopy(result, 0, combined, aes.IV.Length, result.Length);

​

                return Convert.ToBase64String(combined);

            }

        }

    }   

    public static string Decrypt(string encryptedText, string key) {

        if (string.IsNullOrEmpty(key))

            throw new ArgumentException("Key must have valid value.", nameof(key));

        if (string.IsNullOrEmpty(encryptedText))

            throw new ArgumentException("The encrypted text must have valid value.", nameof(encryptedText));

​

        try {

            var combined = Convert.FromBase64String(encryptedText);

            var buffer = new byte[combined.Length];

            var hash = new SHA512CryptoServiceProvider();

            var aesKey = new byte[24];

            Buffer.BlockCopy(hash.ComputeHash(Encoding.UTF8.GetBytes(key)), 0, aesKey, 0, 24);

​

            using (var aes = Aes.Create()) {

                if (aes == null)

                    throw new ArgumentException("Parameter must not be null.", nameof(aes));

​

                aes.Key = aesKey;

​

                var iv = new byte[aes.IV.Length];

                var ciphertext = new byte[buffer.Length - iv.Length];

​

                Array.ConstrainedCopy(combined, 0, iv, 0, iv.Length);

                Array.ConstrainedCopy(combined, iv.Length, ciphertext, 0, ciphertext.Length);

​

                aes.IV = iv;

​

                using (var decryptor = aes.CreateDecryptor(aes.Key, aes.IV))

                    using (var resultStream = new MemoryStream()) {

                    using (var aesStream = new CryptoStream(resultStream, decryptor, CryptoStreamMode.Write))

                        using (var plainStream = new MemoryStream(ciphertext)) {

                        plainStream.CopyTo(aesStream);

                    }

​

                    return Encoding.UTF8.GetString(resultStream.ToArray());

                }

            }

        } catch {

            return encryptedText;

        }

    }   

}