There exists the 'crypt' function, but no inverse version.

It's used primarily for password stuff, where you only need to see if the user knows the right source, and no one (sort of) can figure it out from the resultant version. I doubt this is what you want. I suppose you are trying to mask the url in a link or something, that's relatively easy (but off topic). But any user with half a brain can still figure out what the link points to just by following it, or bookmarking it and then checking the bookmark file (or numerous other methods)... So I don't think I understand the reason for your query. I suppose you could use Perl to drive a PKI program to do your encryption/decryption. Or something of the like.