using System;

using System.Text;

using System.Security.Cryptography;

using System.IO;

using System.Linq;

​

public class Program

{

    // owasp recommendation.

    public const int DerivationIterations = 600000;

    public const int KeySize = 256;

    public const int BlockSize = 128;

    public static void Main()

    {

        string password = "This is my very secret password";

        string plaintext = "Hello World";

        Console.WriteLine(plaintext);

        string encrypted = Encrypt(plaintext, password);

        Console.WriteLine(encrypted);

        string decrypted = Decrypt(encrypted, password);

        Console.WriteLine(decrypted);

    }

​

    public static string Encrypt(string plainText, string passPhrase)

    {

        var saltStringBytes = GenerateBitsOfRandomEntropy(BlockSize);

        var ivStringBytes = GenerateBitsOfRandomEntropy(BlockSize);

        var plainTextBytes = Encoding.UTF8.GetBytes(plainText);

        using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations, HashAlgorithmName.SHA256))

        {

            var keyBytes = password.GetBytes(KeySize / 8);

            using (var symmetricKey = Aes.Create())

            {

                symmetricKey.BlockSize = BlockSize;

                symmetricKey.Mode = CipherMode.CBC;

                symmetricKey.Padding = PaddingMode.PKCS7;

                using (var encryptor = symmetricKey.CreateEncryptor(keyBytes, ivStringBytes))

                {

                    using (var memoryStream = new MemoryStream())

                    {

                        using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))

                        {

                            cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);

                            cryptoStream.FlushFinalBlock();

                            // Create the final bytes as a concatenation of the random salt bytes, the random iv bytes and the cipher bytes.

                            var cipherTextBytes = saltStringBytes;

                            cipherTextBytes = cipherTextBytes.Concat(ivStringBytes).ToArray();

                            cipherTextBytes = cipherTextBytes.Concat(memoryStream.ToArray()).ToArray();

                            memoryStream.Close();

                            cryptoStream.Close();

                            return Convert.ToBase64String(cipherTextBytes);

                        }

                    }

                }

            }

        }

    }

​

    public static string Decrypt(string cipherText, string passPhrase)

    {

        var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);

        var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(BlockSize / 8).ToArray();

        var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(BlockSize / 8).Take(BlockSize / 8).ToArray();

        var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((BlockSize / 8) * 2).ToArray();

        using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations, HashAlgorithmName.SHA256))

        {

            var keyBytes = password.GetBytes(KeySize / 8);

            using (var symmetricKey = Aes.Create())

            {

                symmetricKey.BlockSize = BlockSize;

                symmetricKey.Mode = CipherMode.CBC;

                symmetricKey.Padding = PaddingMode.PKCS7;

                using (var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes))

                {

                    using (var memoryStream = new MemoryStream(cipherTextBytes))

                    {

                        using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))

                        {

                            var plainTextBytes = new byte[cipherTextBytes.Length];

                            var decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);

                            memoryStream.Close();

                            cryptoStream.Close();

                            return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);

                        }

                    }

                }

            }

        }

    }

​

    private static byte[] GenerateBitsOfRandomEntropy(int size)

    {

        var randomBytes = new byte[size / 8];

        using (var rngCsp = RandomNumberGenerator.Create())

        {

            rngCsp.GetBytes(randomBytes);

        }

​

        return randomBytes;

    }

}