public class Base32Encoding
{
    public static byte[] ToBytes(string input)
    {
        if (string.IsNullOrEmpty(input)) throw new ArgumentNullException("input");

        input = input.TrimEnd('='); // remove padding characters
        int byteCount = input.Length * 5 / 8; // this must be TRUNCATED
        byte[] returnArray = new byte[byteCount];
        
        byte curByte = 0, bitsRemaining = 8;
        int mask = 0, arrayIndex = 0;

        foreach (char c in input)
        {
            int cValue = CharToValue(c);
            if (bitsRemaining > 5)
            {
                mask = cValue << (bitsRemaining - 5);
                curByte = (byte)(curByte | mask);
                bitsRemaining -= 5;
            }
            else
            {
                mask = cValue >> (5 - bitsRemaining);
                curByte = (byte)(curByte | mask);
                returnArray[arrayIndex++] = curByte;
                curByte = (byte)(cValue << (3 + bitsRemaining));
                bitsRemaining += 3;
            }
        }
        if (arrayIndex != byteCount) // if we didn't end with a full byte
            returnArray[arrayIndex] = curByte;
        return returnArray;
    }

    public static string ToString(byte[] input)
    {
        if (input == null || input.Length == 0) throw new ArgumentNullException("input");

        int charCount = (int)Math.Ceiling(input.Length / 5d) * 8;
        char[] returnArray = new char[charCount];

        byte nextChar = 0, bitsRemaining = 5;
        int arrayIndex = 0;

        foreach (byte b in input)
        {
            nextChar = (byte)(nextChar | (b >> (8 - bitsRemaining)));
            returnArray[arrayIndex++] = ValueToChar(nextChar);
            
            if (bitsRemaining < 4)
            {
                nextChar = (byte)((b >> (3 - bitsRemaining)) & 31);
                returnArray[arrayIndex++] = ValueToChar(nextChar);
                bitsRemaining += 5;
            }
            bitsRemaining -= 3;
            nextChar = (byte)((b << bitsRemaining) & 31);
        }

        if (arrayIndex != charCount) //if we didn't end with a full char
        {
            returnArray[arrayIndex++] = ValueToChar(nextChar);
            while (arrayIndex != charCount) returnArray[arrayIndex++] = '='; //padding
        }
        return new string(returnArray);
    }

    private static int CharToValue(char c)
    {
        int value = (int)c;
        if (value < 91 && value > 64) return value - 65; //65-90 == uppercase letters
        if (value < 56 && value > 49) return value - 24; //50-55 == numbers 2-7
        if (value < 123 && value > 96) return value - 97; //97-122 == lowercase letters
        throw new ArgumentException("Character is not a Base32 character.", "c");
    }

    private static char ValueToChar(byte b)
    {
        if (b < 26) return (char)(b + 65);
        if (b < 32) return (char)(b + 24);
        throw new ArgumentException("Byte is not a value Base32 value.", "b");
    }
}