NID_SMIMECapabilities
r = CMS_signed_add1_attr_by_NID(si, NID_SMIMECapabilities,
{"SMIME-CAPS", "S/MIME Capabilities", NID_SMIMECapabilities, 9, &so[982]},
if (!PKCS7_add_signed_attribute(si, NID_SMIMECapabilities,
cap = PKCS7_get_signed_attribute(si, NID_SMIMECapabilities);