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.
Read more about this topic: Challenge-Handshake Authentication Protocol
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