Challenge Hand Shake Authentication Protocol is used when a remote client needs to authenticate himself to a network server, or for 2 routers to begin a point-to-point protocol (PPP) session.
CHAP
1. Network Access Server (NAS) sends a challenge (session ID, random string of data)
2. Client uses MD5 to return:
a. Username
b. Encrypted challenge
c. Session ID
d. Password