Encrypt a number to another number of the same length

Question

I need a way to take a 12 digit number and encrypt it to a different 12 digit number (no characters other than 0123456789). Then at a later point I need to be able to decryp

Answer 1

I ended up solving this thanks to you guys using "FPE from a prefix cipher" from the wikipedia page http://en.wikipedia.org/wiki/Format-preserving_encryption. I'll give the basic steps below to hopefully be helpful for someone in the future.

NOTE - I'm sure any expert will tell you this is a hack. The numbers seemed random and it was secure enough for what I needed, but if security is a big concern use something else. I'm sure experts can point to holes in what I did. My only goal for posting this is because I would have found it useful when doing my search for an answer to the problem. Also only use this in situations where it couldn't be decompiled.

I was going to post steps, but its too much to explain. I'll just post my code. This is my proof of concept code I still need to clean up, but you'll get the idea. Note my code is specific to a 12 digit number, but adjusting for others should be easy. Max is probably 16 with the way I did it.

public static string DoEncrypt(string unencryptedString)
{
    string encryptedString = "";
    unencryptedString = new string(unencryptedString.ToCharArray().Reverse().ToArray());
    foreach (char character in unencryptedString.ToCharArray())
    {
        string randomizationSeed = (encryptedString.Length > 0) ? unencryptedString.Substring(0, encryptedString.Length) : "";
        encryptedString += GetRandomSubstitutionArray(randomizationSeed)[int.Parse(character.ToString())];
    }

    return Shuffle(encryptedString);
}

public static string DoDecrypt(string encryptedString)
{
    // Unshuffle the string first to make processing easier.
    encryptedString = Unshuffle(encryptedString);

    string unencryptedString = "";
    foreach (char character in encryptedString.ToCharArray().ToArray())
        unencryptedString += GetRandomSubstitutionArray(unencryptedString).IndexOf(int.Parse(character.ToString()));

    // Reverse string since encrypted string was reversed while processing.
    return new string(unencryptedString.ToCharArray().Reverse().ToArray());
}

private static string Shuffle(string unshuffled)
{
    char[] unshuffledCharacters = unshuffled.ToCharArray();
    char[] shuffledCharacters = new char[12];
    shuffledCharacters[0] = unshuffledCharacters[2];
    shuffledCharacters[1] = unshuffledCharacters[7];
    shuffledCharacters[2] = unshuffledCharacters[10];
    shuffledCharacters[3] = unshuffledCharacters[5];
    shuffledCharacters[4] = unshuffledCharacters[3];
    shuffledCharacters[5] = unshuffledCharacters[1];
    shuffledCharacters[6] = unshuffledCharacters[0];
    shuffledCharacters[7] = unshuffledCharacters[4];
    shuffledCharacters[8] = unshuffledCharacters[8];
    shuffledCharacters[9] = unshuffledCharacters[11];
    shuffledCharacters[10] = unshuffledCharacters[6];
    shuffledCharacters[11] = unshuffledCharacters[9];
    return new string(shuffledCharacters);
}

private static string Unshuffle(string shuffled)
{
    char[] shuffledCharacters = shuffled.ToCharArray();
    char[] unshuffledCharacters = new char[12];
    unshuffledCharacters[0] = shuffledCharacters[6];
    unshuffledCharacters[1] = shuffledCharacters[5];
    unshuffledCharacters[2] = shuffledCharacters[0];
    unshuffledCharacters[3] = shuffledCharacters[4];
    unshuffledCharacters[4] = shuffledCharacters[7];
    unshuffledCharacters[5] = shuffledCharacters[3];
    unshuffledCharacters[6] = shuffledCharacters[10];
    unshuffledCharacters[7] = shuffledCharacters[1];
    unshuffledCharacters[8] = shuffledCharacters[8];
    unshuffledCharacters[9] = shuffledCharacters[11];
    unshuffledCharacters[10] = shuffledCharacters[2];
    unshuffledCharacters[11] = shuffledCharacters[9];
    return new string(unshuffledCharacters);
}

public static string DoPrefixCipherEncrypt(string strIn, byte[] btKey)
{
    if (strIn.Length < 1)
        return strIn;

    // Convert the input string to a byte array 
    byte[] btToEncrypt = System.Text.Encoding.Unicode.GetBytes(strIn);
    RijndaelManaged cryptoRijndael = new RijndaelManaged();
    cryptoRijndael.Mode =
    CipherMode.ECB;//Doesn't require Initialization Vector 
    cryptoRijndael.Padding =
    PaddingMode.PKCS7;


    // Create a key (No IV needed because we are using ECB mode) 
    ASCIIEncoding textConverter = new ASCIIEncoding();

    // Get an encryptor 
    ICryptoTransform ictEncryptor = cryptoRijndael.CreateEncryptor(btKey, null);


    // Encrypt the data... 
    MemoryStream msEncrypt = new MemoryStream();
    CryptoStream csEncrypt = new CryptoStream(msEncrypt, ictEncryptor, CryptoStreamMode.Write);


    // Write all data to the crypto stream to encrypt it 
    csEncrypt.Write(btToEncrypt, 0, btToEncrypt.Length);
    csEncrypt.Close();


    //flush, close, dispose 
    // Get the encrypted array of bytes 
    byte[] btEncrypted = msEncrypt.ToArray();


    // Convert the resulting encrypted byte array to string for return 
    return (Convert.ToBase64String(btEncrypted));
}

private static List GetRandomSubstitutionArray(string number)
{
    // Pad number as needed to achieve longer key length and seed more randomly.
    // NOTE I didn't want to make the code here available and it would take too longer to clean, so I'll tell you what I did. I basically took every number seed that was passed in and prefixed it and  postfixed it with some values to make it 16 characters long and to get a more unique result. For example:
    // if (number.Length = 15)
    //    number = "Y" + number;
    // if (number.Length = 14)
    //    number = "7" + number + "z";
    // etc - hey I already said this is a hack ;)

    // We pass in the current number as the password to an AES encryption of each of the
    // digits 0 - 9. This returns us a set of values that we can then sort and get a 
    // random order for the digits based on the current state of the number.
    Dictionary prefixCipherResults = new Dictionary();
    for (int ndx = 0; ndx < 10; ndx++)
        prefixCipherResults.Add(DoPrefixCipherEncrypt(ndx.ToString(), Encoding.UTF8.GetBytes(number)), ndx);

    // Order the results and loop through to build your int array.
    List group = new List();
    foreach (string key in prefixCipherResults.Keys.OrderBy(k => k))
        group.Add(prefixCipherResults[key]);

    return group;
}