Starts a user authentication session.

Use this API to start the user authentication session to authenticate the user against CyberArk Identity.

• The API gets invoked when the user wants to authenticate to CyberArk Identity and the user enters username on the login form.
• If the user exists in CyberArk Identity cloud directory, the server returns an array of security challenges to the client, that the user must fulfill for each challenge.
• The user’s responses to these mechanisms are processed via the AdvanceAuthentication endpoint.