CRYPT_SIGN_MESSAGE_PARA SignPara;
const BYTE * rgpbToBeSigned[] = {pbContent}; //pbContent -- собственно сообщение
DWORD rgcbToBeSigned[] = {cbContent}; //cbContent -- размер сообщения
DWORD cbSignedBlob;
BYTE * pbSignedBlob;

memset(&SignPara,0,sizeof(CRYPT_SIGN_MESSAGE_PARA));
SignPara.cbSize = sizeof(CRYPT_SIGN_MESSAGE_PARA);
SignPara.dwMsgEncodingType = X509_ASN_ENCODING | PKCS_7_ASN_ENCODING;
SignPara.pSigningCert = pUserCert; //указатель на CERT_CONTEXT с сертификатом подписанта
SignPara.HashAlgorithm.pszObjId = szOID_RSA_SHA1RSA;  //я не уверен, что здесь именно такой алгоритм надо использовать
SignPara.HashAlgorithm.Parameters.cbData = 0;
SignPara.pvHashAuxInfo = 0;
SignPara.cMsgCert = 1;
SignPara.rgpMsgCert = &pUserCert; 
SignPara.cMsgCrl = 0;
SignPara.rgpMsgCrl = 0;
SignPara.cAuthAttr = 0;
SignPara.rgAuthAttr = 0;
SignPara.cUnauthAttr = 0;
SignPara.rgUnauthAttr = 0;
SignPara.dwFlags = NULL;
SignPara.dwInnerContentType = NULL;

if( !CryptSignMessage(&SignPara,TRUE,1,rgpbToBeSigned,rgcbToBeSigned,NULL,&cbSignedBlob) )
{
	puts("Error in CryptSignMessage 1");
	printf("%X\n",GetLastError());
	return -100;
}

pbSignedBlob = (BYTE *)malloc(sizeof(BYTE));

if( !CryptSignMessage(&SignPara,TRUE,1,rgpbToBeSigned,rgcbToBeSigned,pbSignedBlob,&cbSignedBlob) )
{
	puts("Error in CryptSignMessage 2");
	printf("%X\n",GetLastError());
	return -100;
}

FILE * tmp = fopen("out.p7b","wb");
fwrite(pbSignedBlob,cbSignedBlob,sizeof(BYTE),tmp);
fclose(tmp);