Reversing a md5 hash algorithm in C# [duplicate]

This question already exists: Closed 11 years ago.

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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.GetS开发者_高级运维tring(encodedDataAsBytes);
    return returnValue;
}

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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();
}

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A hash is derived from the original information, but it does not contain the original information. If you want a shorter value that hides the original information, but can be resolved to the original value your options are fairly limited:

• Compress the original information. If you need a string, then your original information would have to be fairly large in order for the compressed, base-64-encoded version to not be even bigger than the original data.
• Encrypt the original information - this is more secure than just compressing it, and can be combined with compression, but it's also probably going to be larger than the original information.
• Store the original information somewhere and return a lookup key.

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If you want to be able to get the data back, you want compression, not hashing.

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What you want to do is Encrypt and Decrypt....

Not Hash and Unhash which, as @Thomas pointed out, is impossible. Hashes are typically defeated using rainbow tables or some other data set which includes something which produces the same hash... not guaranteed to be the input value, just some value which produces the same output in the hashing algorithm.

Jeff Atwood has some good code for understanding encryption here:
http://www.codeproject.com/KB/security/SimpleEncryption.aspx

If that's useful to you

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A cryptographic hash is by definition not reversible with typical amounts of computation power. It's usually not even possible to find any input which has the same hash as your original input.

Getting back the original input is mathematically impossible if there are more than 2^n different inputs. With n being the bitlength of the hash(128 for md5). Look up the pidgeonhole principle.

A hash is no lossless compression function.

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A cryptographic hash, like MD5, is designed to be a one-way function, that is, it is computationally infeasable to derive the source data from which a given hash was computed. MD5, though, hasn't considered to be secure for some time, due to weaknesses that have been discovered:

Wikipedia on MD5 Security MD5 Considered Harmful

Another weakness in MD5 is that due to its relative small size, large rainbow tables have been published that let you look up a given MD5 hash to get a source input that will collide with the specified hash value.