ct_x509_get_ext
preidx = ct_x509_get_ext(presigner, NID_authority_key_identifier,
certidx = ct_x509_get_ext(cert, NID_authority_key_identifier,
int poison_idx = ct_x509_get_ext(cert, NID_ct_precert_poison, &poison_ext_is_dup);
idx = ct_x509_get_ext(cert, NID_ct_precert_scts, &sct_ext_is_dup);