Extract Certificate Chain from SChannel with C++ and CryptoApi/SChannel

Question

Is it possible to extract the certificate chain sent by the remote computer in TLS 1.0 Handshake?

The API QueryContextAttributes with SECPKG_ATTR_REMOTE_CERT_CONTEXT value, returns only the end certificate.

Is possible to extract all the chain certificates using some methods? Environment Windows and C++ using CryptoApi and SChannel.

Thanks!

Answer 1

Yes, it is.

Use QueryContextAttributes() with SECPKG_ATTR_REMOTE_CERT_CONTEXT and server certificate returned will have hCertStore member set to a certificate store containing all server's intermediate CA certificates. (See remarks at MSDN.)

See the code snippet below (Source: WebClient.c, Microsoft Platform SDK) how you can parse the chain:

static 
void
DisplayCertChain(
    PCCERT_CONTEXT  pServerCert,
    BOOL            fLocal)
{
    CHAR szName[1000];
    PCCERT_CONTEXT pCurrentCert;
    PCCERT_CONTEXT pIssuerCert;
    DWORD dwVerificationFlags;

    printf("\n");

    // display leaf name
    if(!CertNameToStr(pServerCert->dwCertEncodingType,
                      &pServerCert->pCertInfo->Subject,
                      CERT_X500_NAME_STR | CERT_NAME_STR_NO_PLUS_FLAG,
                      szName, sizeof(szName)))
    {
        printf("**** Error 0x%x building subject name\n", GetLastError());
    }
    if(fLocal)
    {
        printf("Client subject: %s\n", szName);
    }
    else
    {
        printf("Server subject: %s\n", szName);
    }
    if(!CertNameToStr(pServerCert->dwCertEncodingType,
                      &pServerCert->pCertInfo->Issuer,
                      CERT_X500_NAME_STR | CERT_NAME_STR_NO_PLUS_FLAG,
                      szName, sizeof(szName)))
    {
        printf("**** Error 0x%x building issuer name\n", GetLastError());
    }
    if(fLocal)
    {
        printf("Client issuer: %s\n", szName);
    }
    else
    {
        printf("Server issuer: %s\n\n", szName);
    }


    // display certificate chain
    pCurrentCert = pServerCert;
    while(pCurrentCert != NULL)
    {
        dwVerificationFlags = 0;
        pIssuerCert = CertGetIssuerCertificateFromStore(pServerCert->hCertStore,
                                                        pCurrentCert,
                                                        NULL,
                                                        &dwVerificationFlags);
        if(pIssuerCert == NULL)
        {
            if(pCurrentCert != pServerCert)
            {
                CertFreeCertificateContext(pCurrentCert);
            }
            break;
        }

        if(!CertNameToStr(pIssuerCert->dwCertEncodingType,
                          &pIssuerCert->pCertInfo->Subject,
                          CERT_X500_NAME_STR | CERT_NAME_STR_NO_PLUS_FLAG,
                          szName, sizeof(szName)))
        {
            printf("**** Error 0x%x building subject name\n", GetLastError());
        }
        printf("CA subject: %s\n", szName);
        if(!CertNameToStr(pIssuerCert->dwCertEncodingType,
                          &pIssuerCert->pCertInfo->Issuer,
                          CERT_X500_NAME_STR | CERT_NAME_STR_NO_PLUS_FLAG,
                          szName, sizeof(szName)))
        {
            printf("**** Error 0x%x building issuer name\n", GetLastError());
        }
        printf("CA issuer: %s\n\n", szName);

        if(pCurrentCert != pServerCert)
        {
            CertFreeCertificateContext(pCurrentCert);
        }
        pCurrentCert = pIssuerCert;
        pIssuerCert = NULL;
    }
}