Records the identity 'I' and password 'P' of the authenticating user. The session is incremented to {@link State#STEP_1}.
Argument origin:
- From user: user identity 'I' and password 'P'.
@param userID The identity 'I' of the authenticating user, UTF-8encoded. Must not be {@code null} or empty.
@param password The user password 'P', UTF-8 encoded. Must not be{@code null}.
@throws IllegalStateException If the method is invoked in a stateother than {@link State#INIT}.