I don't know much about QR codes, but from what I know this would be one of the best solutions:
You embed a login token in the QR code's url, ie:
https://www.KurtSite.com/QRLogin?LoginToken=[Big number]
You can read in this number using p_web.GetValue('LoginToken')
You would need a LoginToken as a column in your login table, with a unique key so you can look up who is logging in and handle your login login as normal.
This method protects the user name and password. Though anyone with a QR reader could learn the login token. Any method chosen would have this problem.
Matthew