using System;

using System.Text;

using System.Security.Cryptography;

using System.IO;

using System.Linq;

​

public static class StringCipher

    {

        // This constant is used to determine the keysize of the encryption algorithm in bits.

        // We divide this by 8 within the code below to get the equivalent number of bytes.

        private const int Keysize = 256;

​

        // This constant determines the number of iterations for the password bytes generation function.

        private const int DerivationIterations = 1000;

​

        public static string Encrypt(string plainText, string passPhrase = "")

        {

            if(string.IsNullOrEmpty(passPhrase))

             passPhrase = "TPESMS";

            // Salt and IV is randomly generated each time, but is preprended to encrypted cipher text

            // so that the same Salt and IV values can be used when decrypting.  

            var saltStringBytes = Generate256BitsOfRandomEntropy();

            var ivStringBytes = Generate256BitsOfRandomEntropy();

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

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

            {

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

                using (var symmetricKey = new RijndaelManaged())

                {

                    symmetricKey.BlockSize = 256;

                    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 = "")

        {

            if(string.IsNullOrEmpty(passPhrase))

             passPhrase = "TPESMS";

            // Get the complete stream of bytes that represent:

            // [32 bytes of Salt] + [32 bytes of IV] + [n bytes of CipherText]

            var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);

            // Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes.

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

            // Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes.

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

            // Get the actual cipher text bytes by removing the first 64 bytes from the cipherText string.

            var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((Keysize / 8) * 2).Take(cipherTextBytesWithSaltAndIv.Length - ((Keysize / 8) * 2)).ToArray();

​

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

            {

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

                using (var symmetricKey = new RijndaelManaged())

                {

                    symmetricKey.BlockSize = 256;

                    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[] Generate256BitsOfRandomEntropy()

        {

            var randomBytes = new byte[32]; // 32 Bytes will give us 256 bits.

            using (var rngCsp = new RNGCryptoServiceProvider())

            {

                // Fill the array with cryptographically secure random bytes.

                rngCsp.GetBytes(randomBytes);

            }

            return randomBytes;

        }

​

}

​

namespace EncryptStringSample

{

    public class Program

    {

        public static void Main(string[] args)

        {

            Console.WriteLine("Please enter a password to use:");

            string password = Console.ReadLine();

            Console.WriteLine("Please enter a string to encrypt:");

            string plaintext = Console.ReadLine();

            Console.WriteLine("");

​

            Console.WriteLine("Your encrypted string is:");

            string encryptedstring = StringCipher.Encrypt(plaintext);

            Console.WriteLine(encryptedstring);

            Console.WriteLine("");

​

            Console.WriteLine("Your decrypted string is:");

            string decryptedstring = StringCipher.Decrypt(encryptedstring);

            Console.WriteLine(decryptedstring);

            Console.WriteLine("");

​

            Console.WriteLine("Press any key to exit...");

            Console.ReadLine();

        }

    }

}