Challenge-Handshake Authentication Protocol - CHAP Packets

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

The ID chosen for the random challenge is also used in the corresponding response, success, and failure packets. A new challenge with a new ID must be different from the last challenge with another ID. If the success or failure is lost the same response can be sent again, and triggers the same success or failure indication. For MD5 as hash the response value is MD5(ID||secret||challenge), the MD5 for the concatenation of ID, secret, and challenge.

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