Question

Possible Duplicate:
Reversing an MD5 Hash

Given this method in c#

public string CalculateFileHash(string filePaths) {
    var csp = new MD5CryptoServiceProvider();
    var pathBytes = csp.ComputeHash(Encoding.UTF8.GetBytes(filePaths));
    return BitConverter.ToUInt64(pathBytes, 0).ToString();
}

how would one reverse this process with a “DecodeFileHash” method?

var fileQuery = "fileone.css,filetwo.css,file3.css";
var hashedQuery = CalculateFileHash(fileQuery); // e.g. "23948759234"
var decodedQuery = DecodeFileHash(hashedQuery); // "fileone.css,filetwo.css,file3.css"

where decodedQuery == fileQuery in the end.

Is this even possible? If it isn’t possible, would there by any way to generate a hash that I could easily decode?

Edit: So just to be clear, I just want to compress the variable “fileQuery” and decompress fileQuery to determine what it originally was. Any suggestions for solving that problem since hashing/decoding is out?

Edit Again: just doing a base64 encode/decode sounds like the optimal solution then.

public string EncodeTo64(string toEncode) {
    var toEncodeAsBytes = Encoding.ASCII.GetBytes(toEncode);
    var returnValue = System.Convert.ToBase64String(toEncodeAsBytes);
    return returnValue;
}

public string DecodeFrom64(string encodedData) {
    var encodedDataAsBytes  = System.Convert.FromBase64String(encodedData);
    var returnValue = Encoding.ASCII.GetString(encodedDataAsBytes);
    return returnValue;
}

Answer 1

Impossible. By definition and design hashes cannot be reverted to plain text or their original input.

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It sounds like you are actually trying to compress the files. If that is the case, here is a simple method to do so using GZip:

public static byte[] Compress( byte[] data )
{
    var output = new MemoryStream();
    using ( var gzip = new GZipStream( output, CompressionMode.Compress, true ) )
    {
        gzip.Write( data, 0, data.Length );
        gzip.Close();
    }
    return output.ToArray();
}