verify_signer_xcs
if ((pk = verify_signer_xcs(&xcs[u], 1, &cn, 1, &trust_anchors)) != NULL) {
pk = verify_signer_xcs(xcs, num, elts, num_elts, &forbidden_anchors);
pk = verify_signer_xcs(xcs, num, elts, num_elts, &trust_anchors);