Increments this SRP-6a authentication session to {@link State#STEP_2}.
Argument origin:
- From client: public value 'A' and evidence message 'M1'.
@param A The client public value. Must not be {@code null}.
@param M1 The client evidence message. Must not be {@code null}.
@return The server evidence message 'M2'.
@throws SRP6Exception If the session has timed out, the client publicvalue 'A' is invalid or the user credentials are invalid.
@throws IllegalStateException If the method is invoked in a stateother than {@link State#STEP_1}.