C#, encode/decode string to some easy code with checksum inside

Question

I need to implement simple coding and decoding of string inside C#.

I do not need "high secret" encryption, so simple encodings are fine as soon as they got checksum.

I need easy two-way encoding-decoding within one instance of code. There is only one program that does both encoding and decoding, so any "secret keys" can be hardcoded.

Result of encoding should be alphanumeric (text). Binary data (nonletters, nondigits) are to be avoided, because field will participate in files that are text CSV tables.

I need to get a code that can be checked for consistency (so it should include control digit/checksum), and so I can tell that the field is unchanged by third party and is validly decodeable. (Something like control digits inside credit card numbers).

The codelength should be approximately similar to the length of raw string (not 5-10 times longer).

I appreciate if you point me towards the best library within C#, thanks in advance!

Answer 1

RC4 is a simple algorithm that can be implemented easily for encryption/decryption. It is not as secure as AES, but it seems that in your situation you do not need AES security. Below is an implementation of RC4 in .NET from this site

public static class RC4
{
   public static string Encrypt(string key, string data)
   {
      Encoding unicode = Encoding.Unicode;

      return Convert.ToBase64String(Encrypt(unicode.GetBytes(key), unicode.GetBytes(data)));
   }

   public static string Decrypt(string key, string data)
   {
      Encoding unicode = Encoding.Unicode;

      return unicode.GetString(Encrypt(unicode.GetBytes(key), Convert.FromBase64String(data)));
   }

   public static byte[] Encrypt(byte[] key, byte[] data)
   {
      return EncryptOutput(key, data).ToArray();
   }

   public static byte[] Decrypt(byte[] key, byte[] data)
   {
      return EncryptOutput(key, data).ToArray();
   }

   private static byte[] EncryptInitalize(byte[] key)
   {
      byte[] s = Enumerable.Range(0, 256)
        .Select(i => (byte)i)
        .ToArray();

      for (int i = 0, j = 0; i < 256; i++)
      {
         j = (j + key[i % key.Length] + s[i]) & 255;

         Swap(s, i, j);
      }

      return s;
   }

   private static IEnumerable<byte> EncryptOutput(byte[] key, IEnumerable<byte> data)
   {
      byte[] s = EncryptInitalize(key);

      int i = 0;
      int j = 0;

      return data.Select((b) =>
      {
         i = (i + 1) & 255;
         j = (j + s[i]) & 255;

         Swap(s, i, j);

         return (byte)(b ^ s[(s[i] + s[j]) & 255]);
      });
   }

   private static void Swap(byte[] s, int i, int j)
   {
      byte c = s[i];

      s[i] = s[j];
      s[j] = c;
   }
}

Answer 2

I've used this code for encrypt/decrypt with rijndael. It throws a CryptographicException when encrypted string is touched.

Methods from the http://www.codeproject.com/Articles/5719/Simple-encrypting-and-decrypting-data-in-C

public string Encrypt(string clearText, string Password)
    {
        //Convert text to bytes
        byte[] clearBytes =
          System.Text.Encoding.Unicode.GetBytes(clearText);

        //We will derieve our Key and Vectore based on following 
        //password and a random salt value, 13 bytes in size.
        PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,
            new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 
        0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

        byte[] encryptedData = Encrypt(clearBytes,
                 pdb.GetBytes(32), pdb.GetBytes(16));

        return Convert.ToBase64String(encryptedData);
    }

    //Call following function to decrypt data
    public string Decrypt(string cipherText, string Password)
    {
        //Convert base 64 text to bytes
        byte[] cipherBytes = Convert.FromBase64String(cipherText);

        //We will derieve our Key and Vectore based on following 
        //password and a random salt value, 13 bytes in size.
        PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,
            new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 
        0x64, 0x76, 0x65, 0x64, 0x65, 0x76});
        byte[] decryptedData = Decrypt(cipherBytes,
            pdb.GetBytes(32), pdb.GetBytes(16));

        //Converting unicode string from decrypted data
        return Encoding.Unicode.GetString(decryptedData);
    }

    public byte[] Encrypt(byte[] clearData, byte[] Key, byte[] IV)
    {
        byte[] encryptedData;
        //Create stream for encryption
        using (MemoryStream ms = new MemoryStream())
        {
            //Create Rijndael object with key and vector
            using (Rijndael alg = Rijndael.Create())
            {
                alg.Key = Key;
                alg.IV = IV;
                //Forming cryptostream to link with data stream.
                using (CryptoStream cs = new CryptoStream(ms,
                   alg.CreateEncryptor(), CryptoStreamMode.Write))
                {
                    //Write all data to stream.
                    cs.Write(clearData, 0, clearData.Length);
                }
                encryptedData = ms.ToArray();
            }
        }
        return encryptedData;
    }

    public byte[] Decrypt(byte[] cipherData, byte[] Key, byte[] IV)
    {
        byte[] decryptedData;
        //Create stream for decryption
        using (MemoryStream ms = new MemoryStream())
        {
            //Create Rijndael object with key and vector
            using (Rijndael alg = Rijndael.Create())
            {
                alg.Key = Key;
                alg.IV = IV;
                //Forming cryptostream to link with data stream.
                using (CryptoStream cs = new CryptoStream(ms,
                    alg.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    //Write all data to stream.
                    cs.Write(cipherData, 0, cipherData.Length);
                }
                decryptedData = ms.ToArray();
            }
        }
        return decryptedData;
    }