where is the CB file which processes registration?

[PROBLEM]
I need to add a field during registration which requires user to input a 'CD-KEY' sort of thing, which is a unique, one time key for every (paid) user.

[PROPOSED SOLUTION]
I need to look for the php which processes the registration, and modify the code to do a check on the key if it is unique on the database, and validate the CDKEY.

Anyone can point me to where i can process this additional registration field?

Another solution is to use the CB API (see download area for doc) to code a clean new CB field type plugin that does exactly this.

i need to be a paying member to access the documentation?

serverxeon wrote: i need to be a paying member to access the documentation?

CB API documentation is in the FREE download area - did you look?

ok thanks. i'll ask if i have more questions

how do you access the database to search if the CDKEY entered is already used?
the API dont seem to say much SQL stuffs.