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

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