I need to generate a random alpha/numeric to give to users that they come to the site to enter. I dont' know much about random numbers and such, I know there are seeding issues and such, but I'm not sure what they are.

So, I used this:

select substrING(md5(concat_ws('-',md5(username_usr), MD5(zip_usr), MD5(id_usr), MD5(created_usr))),-12) from users_usr

Is this safe? I used concat_ws because sometimes zip is null, but the others never are.

And yes, I know this is kinda short, but 1. They have to enter the last 4 of their social, 2. It's 1 time use, 3. There's no private data displayed back in the application and 4. I may use captcha, but since there's no private data, thats probably overkill.

THanks