Challenge-Handshake Authentication Protocol

In computing, the Challenge-Handshake Authentication Protocol (CHAP) authenticates a user or network host to an authenticating entity. That entity may be, for example, an Internet access provider.

RFC 1994: PPP Challenge Handshake Authentication Protocol (CHAP) defines the protocol.

CHAP is an authentication scheme used by Point to Point Protocol (PPP) servers to validate the identity of remote clients. CHAP periodically verifies the identity of the client by using a three-way handshake. This happens at the time of establishing the initial link, and may happen again at any time afterwards. The verification is based on a shared secret (such as the client user's password).

1. After the completion of the link establishment phase, the authenticator sends a "challenge" message to the peer.
2. The peer responds with a value calculated using a one-way hash function, such as an MD5 checksum hash.
3. The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authenticator acknowledges the authentication; otherwise it should terminate the connection.
4. At random intervals the authenticator sends a new challenge to the peer and repeats steps 1 through 3.

CHAP provides protection against playback attack by the peer through the use of an incrementally changing identifier and of a variable challenge-value. CHAP requires that both the client and server know the plaintext of the secret, although it is never sent over the network.

Microsoft has implemented a variant of the Challenge-handshake authentication protocol, called MS-CHAP, which does not require either peer to know the plaintext.

Working Cycle

• Challenge Packet (System to User)
• Response Packet (User to System)
• Success or failure packet (System to User)

CHAP Packets

Description	1 byte	1 byte	2 bytes	1 byte	Variable	variable
Challenge	Code = 1	ID	Length	Challenge length	Challenge value	Name
Response	Code = 2	ID	Length	Response Length	Response value	Name
Success	Code = 3	ID	Length		Message
Failure	Code = 4	ID	Length		Message

CHAP packet embedded in a PPP frame. The protocol field has a value of C223(hex)

Flag	Address	Control	Protocol (C223(hex))	Payload (table above)	FCS	Flag

See also

• Password Authentication Protocol
• Challenge-response test
• Cryptographic hash function

References

• RFC 1994