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Security Fundamentals & Crypto · Intermediate

Challenge-Handshake Authentication Protocol (CHAP)

PPP authentication protocol using a three-way challenge-response with a hash of a shared secret.

Definition

CHAP sends a random challenge; the peer replies with an MD5 hash of the challenge, identifier, and shared secret, so the password never crosses the link. It can reauthenticate periodically during a session. Microsoft's MS-CHAPv2 variant is cryptographically broken and should be protected inside a TLS tunnel (as in PEAP).

Example

Two routers authenticate a PPP link with 'ppp authentication chap' and a shared secret.

Exam tip

CHAP is better than PAP; MS-CHAPv2 alone is crackable.

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