Username registration validation?

[bigrover]

I am trying to figure out how vB validates a new username during registration. I want to use the same logic or routines to validate a loginname field I have created in the user profile. I haven't found where the username validate routines are being called. I would appreciate a pointer in the right direction.

The hack I am working on allows a user to log in with a different name than the username. The reason is to deal with some denial of service issues related to people targeting certain members of a forum. A perp can either saturate the login process with repeated login attacks/failures, or engage a user lockout for repeated login failures. In either case, users have experienced denial of access to their forum. I have been requested to provide a bit of security via obfuscation. I have read a number of threads discussing mods to add a display name similar to other products. After some consideration, I have concluded it would be preferable to not attempt to disrupt the user name functionality in vB and the many mods I may want to take advantage of. A better solution is to authenticate logins against another value linked to the standard user name and userid. I have this working with a loginname stored in field5 of the userprofile table. What I need to do now is validate the loginname is unique during the registration process.