PKCS7_get_signed_attribute
cap = PKCS7_get_signed_attribute(si, NID_SMIMECapabilities);
if (PKCS7_get_signed_attribute(si, NID_pkcs9_contentType))
if (!PKCS7_get_signed_attribute(si, NID_pkcs9_signingTime)) {
attr = PKCS7_get_signed_attribute(si, NID_id_smime_aa_signingCertificate);
attr = PKCS7_get_signed_attribute(si, NID_id_smime_aa_signingCertificateV2);
ASN1_TYPE *PKCS7_get_signed_attribute(const PKCS7_SIGNER_INFO *si, int nid);