substitution cypher with different alphabet length

Question

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

I know how my IDs will look like (combination of upperchase ascii, digits and underscore), and they will be rather long, as they are composed keys. I would like to use a longer alphabet to shorten the resulting codes (I'd like to use upper and lower case ascii letters, digits and nothing else). So my incoming alphabet would be

[A-Z0-9_] (37 chars)

and my outgoing alphabet would be

[A-Za-z0-9] (62 chars)

so a compression of almost 50% would be available.

let's say my URLs look like this:

/my/page/GFZHFFFZFZTFZTF_24_F34

and I want them to look like this instead:

/my/page/Ft32zfegZFV5

Obviously both arrays would be shuffled to bring some random order in.

This does not have to be secure. if someone figures it out: fine, but I don't want the scheme to be obvious.

My desired solution would be to convert the string to an integer representation of radix 37, convert the radix to 62 and use the second alphabet to write out that number. is there any sample code available that does something similar? Integer.parseInt() has some similar logic, but it is hard-coded to use standard digit behavior.

Any ideas?

I am using java to implement this but code or pseudo-code in any other language is of course also helpful

Update: see my own solution further down the page

Answer 1

It's not a substitution cipher at all, but your question is clear enough.

Have a look at Base85: http://en.wikipedia.org/wiki/Ascii85

For Java (as indirectly linked by the Wikipedia article):

• http://java.freehep.org/freehep-io/apidocs/org/freehep/util/io/ASCII85InputStream.html
• http://java.freehep.org/freehep-io/apidocs/org/freehep/util/io/ASCII85OutputStream.html

Answer 2

Inexplicably Character.MAX_RADIX is only 36, but you can always write your own base conversion routine. The following implementation isn't high-performance, but it should be a good starting point:

import java.math.BigInteger;
public class BaseConvert {
    static BigInteger fromString(String s, int base, String symbols) {
        BigInteger num = BigInteger.ZERO;
        BigInteger biBase = BigInteger.valueOf(base);
        for (char ch : s.toCharArray()) {
            num = num.multiply(biBase)
                     .add(BigInteger.valueOf(symbols.indexOf(ch)));
        }
        return num;
    }
    static String toString(BigInteger num, int base, String symbols) {
        StringBuilder sb = new StringBuilder();
        BigInteger biBase = BigInteger.valueOf(base);
        while (!num.equals(BigInteger.ZERO)) {
            sb.append(symbols.charAt(num.mod(biBase).intValue()));
            num = num.divide(biBase);
        }
        return sb.reverse().toString();
    }
    static String span(char from, char to) {
        StringBuilder sb = new StringBuilder();
        for (char ch = from; ch <= to; ch++) {
            sb.append(ch);
        }
        return sb.toString();
    }
}

Then you can have a main() test harness like the following:

public static void main(String[] args) {
    final String SYMBOLS_AZ09_ = span('A','Z') + span('0','9') + "_";
    final String SYMBOLS_09AZ = span('0','9') + span('A','Z');
    final String SYMBOLS_AZaz09 = span('A','Z') + span('a','z') + span('0','9');

    BigInteger n = fromString("GFZHFFFZFZTFZTF_24_F34", 37, SYMBOLS_AZ09_);

    // let's convert back to base 37 first...
    System.out.println(toString(n, 37, SYMBOLS_AZ09_));
    // prints "GFZHFFFZFZTFZTF_24_F34"

    // now let's see what it looks like in base 62...       
    System.out.println(toString(n, 62, SYMBOLS_AZaz09));
    // prints "ctJvrR5kII1vdHKvjA4"

    // now let's test with something we're more familiar with...
    System.out.println(fromString("CAFEBABE", 16, SYMBOLS_09AZ));
    // prints "3405691582"

    n = BigInteger.valueOf(3405691582L);
    System.out.println(toString(n, 16, SYMBOLS_09AZ));
    // prints "CAFEBABE"        
}

Some observations

• BigInteger is probably easiest if the numbers can exceed long
• You can shuffle the char in the symbol String, just stick to one "secret" permutation

Note regarding "50% compression"

You can't generally expect the base 62 string to be around half as short as the base 36 string. Here's Long.MAX_VALUE in base 10, 20, and 30:

    System.out.format("%s%n%s%n%s%n",
        Long.toString(Long.MAX_VALUE, 10), // "9223372036854775807"
        Long.toString(Long.MAX_VALUE, 20), // "5cbfjia3fh26ja7"
        Long.toString(Long.MAX_VALUE, 30)  // "hajppbc1fc207"
    );

Answer 3

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)