Question

I would like to know how to reverse the process of the below DecodeBinaryBase64 so that I can have a matching Encode method. In short C# code that if given the output of this method it would return the same string that it took as input.

private static string DecodeBinaryBase64(string stringToDecode)
{
    StringBuilder builder = new StringBuilder();
    foreach (var b in Convert.FromBase64String(stringToDecode))
        builder.Append(string.Format("{0:X2}", b));
    return builder.ToString();
}

Here is an example of an encoded string and its decoded counterpart. The result is a SHA1 hash for a file. The above method is an example of understanding how the decoding works to get to the right string.

ENCODED

/KUGOuoESMWYuDb+BTMK1LaGe7k=

DECODED

FCA5063AEA0448C598B836FE05330AD4B6867BB9

or

0xFCA5063AEA0448C598B836FE05330AD4B6867BB9

Updated to reflect correct SHA1 value thanks to Porges and a fix for hex bug found by Dean ‘codeka’ Hardin.

Implemented Solution

Here is the the implementation I have now, it is from Porges post distilled down to two methods.

private static string EncodeFileDigestBase64(string digest)
{
    byte[] result = new byte[digest.Length / 2];

    for (int i = 0; i < digest.Length; i += 2)
        result[i / 2] = byte.Parse(digest.Substring(i, 2), System.Globalization.NumberStyles.HexNumber);

    if (result.Length != 20)
        throw new ArgumentException("Not a valid SHA1 filedigest.");

    return Convert.ToBase64String(result);
}

private static string DecodeFileDigestBase64(string encodedDigest)
{
    byte[] base64bytes = Convert.FromBase64String(encodedDigest);
    return string.Join(string.Empty, base64bytes.Select(x => x.ToString("X2")));
}  

Answer 1

I don’t believe it’s physically possible. The problem is that string.Format("{0:X}", b) will return either 1 or 2 characters depending on whether the input byte is < 16 or not. And you’ve got no way to know once the string has been joined together.

If you can modify the DecodeBinaryBase64 method so that it always appends two character for each byte, i.e. by using string.Format("{0:X2}", b) then it will be possible by just taking the input string two characters at a time.

If you made that change to your DecodeBinaryBase64, then you can use the following to convert back again:

private static string DecodeBinaryBase64(string stringToDecode)
{
    StringBuilder builder = new StringBuilder();
    foreach (var b in Convert.FromBase64String(stringToDecode))
        builder.Append(string.Format("{0:X2}", b));
    return "0x" + builder.ToString();
}

private static string EncodeBinaryBase64(string stringToEncode)
{
    var binary = new List<byte>();
    for(int i = 2; i < stringToEncode.Length; i += 2)
    {
        string s = new string(new [] {stringToEncode[i], stringToEncode[i+1]});
        binary.Add(byte.Parse(s, NumberStyles.HexNumber));
    }
    return Convert.ToBase64String(binary.ToArray());
}

(Error checking and so on is missing, though)