Create unique non-autoincrement key for 700,000 records?

I have a mySQL database with 700,000 records in it, which are presently keyed with an "auto-increment" field. What I'd like to do is create another field with a field where each and every record number has a unique keyvalue. Example: "su5e23vlskd" for records 1, and "34fdfdsglkdj4" for record 2. All that matters is that it's unique, and isn't a number that can be guessed or an "autoincrement" number, where a hacker can just figure out the keyvalue by incrementing numbers. It doesn't matter to me if each keyvalue field is just numbers, or a number/letter combination - - - all that matters is that each keyvalue field is unique. Is there an automatic way that mySQL could do that, or would I need to write a php script to somehow go through each record and create this unique value?