How to convert a byte array (MD5 hash) into a string (36 chars)?

Question

I\'ve got a byte array that was created using a hash function. I would like to convert this array into a string. So far so good, it will give me hexadecimal string.

Answer 1

Using BigInteger (needs the System.Numerics reference)

Using BigInteger (needs the System.Numerics reference)

const string chars = "0123456789abcdefghijklmnopqrstuvwxyz";

// The result is padded with chars[0] to make the string length
// (int)Math.Ceiling(bytes.Length * 8 / Math.Log(chars.Length, 2))
// (so that for any value [0...0]-[255...255] of bytes the resulting
// string will have same length)
public static string ToBaseN(byte[] bytes, string chars, bool littleEndian = true, int len = -1)
{
    if (bytes.Length == 0 || len == 0)
    {
        return String.Empty;
    }

    // BigInteger saves in the last byte the sign. > 7F negative, 
    // <= 7F positive. 
    // If we have a "negative" number, we will prepend a 0 byte.
    byte[] bytes2;

    if (littleEndian)
    {
        if (bytes[bytes.Length - 1] <= 0x7F)
        {
            bytes2 = bytes;
        }
        else
        {
            // Note that Array.Resize doesn't modify the original array,
            // but creates a copy and sets the passed reference to the
            // new array
            bytes2 = bytes;
            Array.Resize(ref bytes2, bytes.Length + 1);
        }
    }
    else
    {
        bytes2 = new byte[bytes[0] > 0x7F ? bytes.Length + 1 : bytes.Length];

        // We copy and reverse the array
        for (int i = bytes.Length - 1, j = 0; i >= 0; i--, j++)
        {
            bytes2[j] = bytes[i];
        }
    }

    BigInteger bi = new BigInteger(bytes2);

    // A little optimization. We will do many divisions based on 
    // chars.Length .
    BigInteger length = chars.Length;

    // We pre-calc the length of the string. We know the bits of 
    // "information" of a byte are 8. Using Log2 we calc the bits of 
    // information of our new base. 
    if (len == -1)
    {
        len = (int)Math.Ceiling(bytes.Length * 8 / Math.Log(chars.Length, 2));
    }

    // We will build our string on a char[]
    var chs = new char[len];
    int chsIndex = 0;

    while (bi > 0)
    {
        BigInteger remainder;
        bi = BigInteger.DivRem(bi, length, out remainder);

        chs[littleEndian ? chsIndex : len - chsIndex - 1] = chars[(int)remainder];
        chsIndex++;

        if (chsIndex < 0)
        {
            if (bi > 0)
            {
                throw new OverflowException();
            }
        }
    }

    // We append the zeros that we skipped at the beginning
    if (littleEndian)
    {
        while (chsIndex < len)
        {
            chs[chsIndex] = chars[0];
            chsIndex++;
        }
    }
    else
    {
        while (chsIndex < len)
        {
            chs[len - chsIndex - 1] = chars[0];
            chsIndex++;
        }
    }

    return new string(chs);
}

public static byte[] FromBaseN(string str, string chars, bool littleEndian = true, int len = -1)
{
    if (str.Length == 0 || len == 0)
    {
        return new byte[0];
    }

    // This should be the maximum length of the byte[] array. It's 
    // the opposite of the one used in ToBaseN.
    // Note that it can be passed as a parameter
    if (len == -1)
    {
        len = (int)Math.Ceiling(str.Length * Math.Log(chars.Length, 2) / 8);
    }

    BigInteger bi = BigInteger.Zero;
    BigInteger length2 = chars.Length;
    BigInteger mult = BigInteger.One;

    for (int j = 0; j < str.Length; j++)
    {
        int ix = chars.IndexOf(littleEndian ? str[j] : str[str.Length - j - 1]);

        // We didn't find the character
        if (ix == -1)
        {
            throw new ArgumentOutOfRangeException();
        }

        bi += ix * mult;

        mult *= length2;
    }

    var bytes = bi.ToByteArray();

    int len2 = bytes.Length;

    // BigInteger adds a 0 byte for positive numbers that have the
    // last byte > 0x7F
    if (len2 >= 2 && bytes[len2 - 1] == 0)
    {
        len2--;
    }

    int len3 = Math.Min(len, len2);

    byte[] bytes2;

    if (littleEndian)
    {
        if (len == bytes.Length)
        {
            bytes2 = bytes;
        }
        else
        {
            bytes2 = new byte[len];
            Array.Copy(bytes, bytes2, len3);
        }
    }
    else
    {
        bytes2 = new byte[len];

        for (int i = 0; i < len3; i++)
        {
            bytes2[len - i - 1] = bytes[i];
        }
    }

    for (int i = len3; i < len2; i++)
    {
        if (bytes[i] != 0)
        {
            throw new OverflowException();
        }
    }

    return bytes2;
}

Be aware that they are REALLY slow! REALLY REALLY slow! (2 minutes for 100k). To speed them up you would probably need to rewrite the division/mod operation so that they work directly on a buffer, instead of each time recreating the scratch pads as it's done by BigInteger. And it would still be SLOW. The problem is that the time needed to encode the first byte is O(n) where n is the length of the byte array (this because all the array needs to be divided by 36). Unless you want to work with blocks of 5 bytes and lose some bits. Each symbol of Base36 carries around 5.169925001 bits. So 8 of these symbols would carry 41.35940001 bits. Very near 40 bytes.

Note that these methods can work both in little-endian mode and in big-endian mode. The endianness of the input and of the output is the same. Both methods accept a len parameter. You can use it to trim excess 0 (zeroes). Note that if you try to make an output too much small to contain the input, an OverflowException will be thrown.