verifying valid A/B values

[rodspade]

The bnGreaterOrEqual function is broken. It should use >= instead of >

Also, I think that client_getS and server_getS do not validate their inputs correctly:
if((zero.compareTo(B_num) > 0) && (N.compareTo(B_num) < 0))
throw new Error("invalid server-supplied 'B', must be 1..N-1");

This will error only if B is both less than 0 AND greater than N! That will never happen. I believe it should be:
zero.compareTo(B_num) >= 0 || N.compareTo(B_num) <= 0