substitution cipher with different alphabet length

Question

I would like to implement a simple substitution cipher to mask private ids in URLs.

I know how my IDs will look like (combination of uppercase ASCII letters, digits

Answer 1

I now have a working solution which you can find here:

http://pastebin.com/Mctnidng

The problem was that a) I was losing precision in long codes through this part:

value = value.add(//
    BigInteger.valueOf((long) Math.pow(alphabet.length, i)) // error here
        .multiply(
            BigInteger.valueOf(ArrayUtils.indexOf(alphabet, c))));

(long just wasn't long enough)

and b) whenever I had a text that started with the character at offset 0 in the alphabet, this would be dropped, so I needed to add a length character (a single character will do fine here, as my codes will never be as long as the alphabet)