The subscribe and unsubscribe requests to WebSocket private feeds require a signed challenge
message with the user api_secret.

The challenge is obtained as is shown in Section WebSocket API Public (using the api_key).

Authenticated requests must include both the original challenge message (original_challenge) and the signed (signed_challenge) in JSON format.

The challenge is a UUID string.

The steps to sign the challenge are the same as the steps to generate an authenticated REST request
except for step 1 which now is just the challenge string:

The result of the step 4 is the signed challenge which will be included in the subscribe request.

The table below shows the expected output from example inputs: