Editing Challenge-Handshake Authentication Protocol (section)

==CHAP packets==
{| class="wikitable"
!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 it triggers the same success or failure indication. For [[MD5]] as hash the response value is <code>MD5(ID||secret||challenge)</code>, the MD5 for the concatenation of ID, secret, and challenge.<ref>{{cite web
|url=http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a00800b4131.shtml 
|title=Understanding and Configuring PPP CHAP Authentication
|publisher=[[Cisco Systems|Cisco tech note]]
|year=2005
|access-date=2011-08-14}}</ref>