FILE *file = fopen("/tmp/signature" , "rb");
fseek(file, 0, SEEK_END);
long buffer_len = ftell(file);
rewind(file);
char *buffer = (char*) malloc (sizeof(char) * buffer_len);
fread(buffer, 1, buffer_len, file);
fclose(file);
HCRYPTMSG msg = CryptMsgOpenToDecode(
    X509_ASN_ENCODING | PKCS_7_ASN_ENCODING, 0, 0, 0, NULL, NULL
);
DWORD data_len = 0;
CryptMsgUpdate(msg, (BYTE *) buffer, (DWORD) buffer_len, 1);
CryptMsgGetParam(msg, CMSG_SIGNER_CERT_INFO_PARAM, 0, NULL, &data_len);
void *data = malloc(data_len);
CryptMsgGetParam(msg, CMSG_SIGNER_CERT_INFO_PARAM, 0, data, &data_len);
PCERT_INFO cert_info = (PCERT_INFO) data;
printf("%d\n", cert_info->Subject.cbData); // >0
printf("%d\n", cert_info->Subject.cbData); //  0
printf("%d\n", cert_info->cExtension);     //  0