Receives the server evidence message 'M1'. The session is incremented to {@link State#STEP_3}.
Argument origin:
- From server: evidence message 'M2'.
@param M2 The server evidence message 'M2'. Must not be{@code null}.
@throws IllegalStateException If the method is invoked in a stateother than {@link State#STEP_2}.
@throws SRP6Exception If the session has timed out or theserver evidence message 'M2' is invalid.