BN_swap_ct
int BN_swap_ct(BN_ULONG swap, BIGNUM *a, BIGNUM *b, size_t nwords);
if (!BN_swap_ct(c, (a)->X, (b)->X, w) || \
!BN_swap_ct(c, (a)->Y, (b)->Y, w) || \
!BN_swap_ct(c, (a)->Z, (b)->Z, w)) \
if (!BN_swap_ct(kbit, k, lambda, group_top + 2))