Simple string encryption/decryption with a small resulting string [closed]

Question

I need to encrypt a string and then be able to decrypt it again.

I implemented the solution here and it works well, but the resulting string is not suitable as it needs to be simple and short enough for a user to use.

I am encrypting incrementing database ID's (from 1) and there won't be more than 500. Ideally I'd like the encrypted string to be not more than 6 characters in length.

Any ideas appreciated..

edit: It's a lengthy form which the user can resume at a later date with this generated string

Answer 1

You can use AES in CTR mode without any padding. In this mode there is a counter that is encrypted and then the result is xor'd with your plaintext which is the number. The result should be small and you will get the encryption of AES which will be better than any substitution cipher you use (which you could probably break by hand). You will have to get the BouncyCastle crypto library however as the Microsoft implementation of Rijndael does not have CTR as an available mode. Below is an example of the AES class you would need to implement. As well as an example of encryption and decryption.

using System;
using System.Text;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;

public class AES
{
    private readonly Encoding encoding;

    private SicBlockCipher mode;


    public AES(Encoding encoding)
    {
        this.encoding = encoding;
        this.mode = new SicBlockCipher(new AesFastEngine());
    }

    public static string ByteArrayToString(byte[] bytes)
    {
        return BitConverter.ToString(bytes).Replace("-", string.Empty);
    }

    public static byte[] StringToByteArray(string hex)
    {
        int numberChars = hex.Length;
        byte[] bytes = new byte[numberChars / 2];

        for (int i = 0; i < numberChars; i += 2)
        {
            bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
        }

        return bytes;
    }


    public string Encrypt(string plain, byte[] key, byte[] iv)
    {
        byte[] input = this.encoding.GetBytes(plain);

        byte[] bytes = this.BouncyCastleCrypto(true, input, key, iv);

        string result = ByteArrayToString(bytes);

        return result;
    }


    public string Decrypt(string cipher, byte[] key, byte[] iv)
    {
        byte[] bytes = this.BouncyCastleCrypto(false, StringToByteArray(cipher), key, iv);

        string result = this.encoding.GetString(bytes);

        return result;
    }


    private byte[] BouncyCastleCrypto(bool forEncrypt, byte[] input, byte[] key, byte[] iv)
    {
        try
        {
            this.mode.Init(forEncrypt, new ParametersWithIV(new KeyParameter(key), iv));

            BufferedBlockCipher cipher = new BufferedBlockCipher(this.mode);

            return cipher.DoFinal(input);
        }
        catch (CryptoException)
        {
            throw;
        }
    }
}

Example Usage

string test = "1";

AES aes = new AES(System.Text.Encoding.UTF8);

RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider();
byte[] key = new byte[32];
byte[] iv = new byte[32];

// Generate random key and IV
rngCsp.GetBytes(key);
rngCsp.GetBytes(iv);

string cipher = aes.Encrypt(test, key, iv);

string plaintext = aes.Decrypt(cipher, key, iv);

Response.Write(cipher + "<BR/>");

Response.Write(plaintext);

Output Example

CB
1 

Answer 2

//encryption   
string output="";   
char[] readChar = yourInput.ToCharArray();   
for (int i = 0; i < readChar.Length; i++)  
{   
    int no = Convert.ToInt32(readChar[i]) + 10;   
    string r = Convert.ToChar(no).ToString();   
    output+=r;   
}  
//decryption  
string output="";   
char[] readChar = yourInput.ToCharArray();   
for (int i = 0; i < readChar.Length; i++)   
{   
    int no = Convert.ToInt32(readChar[i]) - 10;   
    string r = Convert.ToChar(no).ToString();   
    output+=r;   
}