2.2 Interoperation with PAP and CHAP

2.2. Interoperation with PAP and CHAP

For PAP, the NAS takes the PAP ID and password and sends them in an Access-Request packet as the User-Name and User-Password. The NAS MAY include the Attributes Service-Type = Framed-User and Framed-Protocol = PPP as a hint to the RADIUS server that PPP service is expected.

For CHAP, the NAS generates a random challenge (preferably 16 octets) and sends it to the user, who returns a CHAP response along with a CHAP ID and CHAP username. The NAS then sends an Access-Request packet to the RADIUS server with the CHAP username as the User-Name and with the CHAP ID and CHAP response as the CHAP-Password (Attribute 3). The random challenge can either be included in the CHAP-Challenge attribute or, if it is 16 octets long, it can be placed in the Request Authenticator field of the Access-Request packet. The NAS MAY include the Attributes Service-Type = Framed- User and Framed-Protocol = PPP as a hint to the RADIUS server that PPP service is expected.

The RADIUS server looks up a password based on the User-Name, encrypts the challenge using MD5 on the CHAP ID octet, that password, and the CHAP challenge (from the CHAP-Challenge attribute if present, otherwise from the Request Authenticator), and compares that result to the CHAP-Password. If they match, the server sends back an Access-Accept, otherwise it sends back an Access-Reject.

If the RADIUS server is unable to perform the requested authentication it MUST return an Access-Reject. For example, CHAP requires that the user's password be available in cleartext to the server so that it can encrypt the CHAP challenge and compare that to the CHAP response. If the password is not available in cleartext to the RADIUS server then the server MUST send an Access-Reject to the client.