URI/Email Question

[eluser]ramabodhi[/eluser]
well right now i'm setting the activation key as an encryption of one of the unique user fields, versus my original idea of a random string, which has a potential for duplication(albet small)?

is this a bad idea, or a security hole? if not.. right now my only issue is the == at the end of my encrypted strings, which isn't registering in the url, which can be changed by perhaps using a different codex? i dont know a whole lot about encryption, but it seems like a solid way to get an activation key

[edit]How do i get a strictly alpha numeric encryption?