CHAP Challenge and Response Frames

CHAP Challenge frames (code 0x01) are used to start the authentication negotiation, and are transmitted by the authenticator. They contain the CHAP name and a challenge value, which is calculated from the CHAP secret using a one-way hash algorithm. Up to ten CHAP Challenge frames are sent without receiving a Response frame before the authentication phase fails.

A CHAP Response frame (code 0x02) is sent on receipt of a recognized CHAP Challenge frame. It contains a response value, which is calculated using the CHAP secret, the challenge value received, and the same one-way hash algorithm.

CHAP Challenge and Response frames have the format shown in Figure A-7:

Figure A-7 CHAP Challenge and Response Frame Format

The CHAP name is one or more octets in length and contains the character string specified by the send_chap_name parameter in the file ppp.conf.