As we increasingly rely upon our computer information systems to store and operate on sensitive information, the methods we use to authenticate user identity also become more important. One of the most important such methods is the password. However, passwords that provide better security also tend to be more difficult to remember. They also tend to be difficult to type, and typing errors can have negative consequences, such as being locked out of a critical information system. We present a computational cognitive model of password rehearsal and a typing extension to the ACT-R cognitive architecture intended to study human-computer interaction issues in the usable security domain.