# Introduction
`CHL` is a command introduced with [MSNP6](../versions/msnp6.md).

It is a Notification Server command, without a request or response payload.

A request to generate a challenge. Respond with QRY on a new transaction to continue your connection.

# Client/Request
`CHL TrID`

# Server/Response
`CHL TrID challenge`

If this command is sent asynchronously, the Transaction ID will be `0` instead.

Where `challenge` is usually a 20-character numeric value (but can be any valid string)
that is concatenated with the client's private key.

# Examples

## From server at any time
```
S: CHL 0 12345678901234567890
```

## Requested challenge
```
C: CHL 1
S: CHL 1 12345678901234567890
```

## Challenge timeout
```
S: CHL 0 12345678901234567890
```
... some time passes ...
```
S: OUT
```

Server disconnects client.

# Known changes
* [MSNP11](../versions/msnp11.md): Challenge response (QRY commands)'s
algorithm changed to be more formidable.