From MSNPiki

CHL

The CHL command is an unsolicited command coming from the server which is used to verify that the client currently connected is an official client (MSN Messenger, Windows Live Messenger, etc.).

CHL TrID challengestring

• TrID: The TransactionID of the command. Always 0.
• challengestring: A string required for the response

A challenge must be replied to using the QRY command.

Challenge Response

The challenge system varies over different MSNP versions.

• For MSNP8, 9, and 10 see MSNP8:Challenges
• For MSNP11 and later, see MSNP11:Challenges
• MSNP13 and 14 uses updated keys, see MSNP13:Challenges
• MSNP15 on later uses updated keys, see MSNP15:Changes