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Example C Program: Converting Names from Certificates to ASN.1 and Back

The following example enumerates the certificates in a certificate store, displays the subject and user of each certificate, and converts the subject name from each certificate into its Abstract Syntax Notation One (ASN.1) encoded form, and then back in to its decoded form.

This example shows the following tasks and CryptoAPI functions:


#pragma comment(lib, "crypt32.lib")



//-------------------------------------------------------------------
// Example C Program: 
// Converting a name from a certificate to an ASN.1 encoded string
// and back.

#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <Wincrypt.h>

#define MY_ENCODING_TYPE (PKCS_7_ASN_ENCODING | X509_ASN_ENCODING)
#define MY_STRING_TYPE (CERT_OID_NAME_STR)

//-------------------------------------------------------------------
// Declare auxiliary functions

void MyHandleError(LPTSTR);
void Local_wait();

void main(void)
{
    HCERTSTORE hCertStore;
    PCCERT_CONTEXT pCertContext;

    //---------------------------------------------------------------
    // Begin Processing by opening a certificate store.

    if(!(hCertStore = CertOpenStore(
        CERT_STORE_PROV_SYSTEM,
        MY_ENCODING_TYPE,
        NULL,
        CERT_SYSTEM_STORE_CURRENT_USER,
        L"MY")))
    {
        MyHandleError(TEXT("The MY system store did not open."));
    }

    //---------------------------------------------------------------
    //       Loop through the certificates in the store. 
    //       For each certificate,
    //             get and print the name of the 
    //                  certificate subject and issuer.
    //             convert the subject name from the certificate
    //                  to an ASN.1 encoded string and print the
    //                  octets from that string.
    //             convert the encoded string back into its form 
    //                  in the certificate.

    pCertContext = NULL;
    while(pCertContext = CertEnumCertificatesInStore(
        hCertStore,
        pCertContext))
    {
        LPTSTR pszString;
        LPTSTR pszName;
        DWORD cbSize;
        CERT_BLOB blobEncodedName;

        //-----------------------------------------------------------
        //        Get and display 
        //        the name of subject of the certificate.

        if(!(cbSize = CertGetNameString(   
            pCertContext,   
            CERT_NAME_SIMPLE_DISPLAY_TYPE,   
            0,
            NULL,   
            NULL,   
            0)))
        {
            MyHandleError(TEXT("CertGetName 1 failed."));
        }

        if(!(pszName = (LPTSTR)malloc(cbSize * sizeof(TCHAR))))
        {
            MyHandleError(TEXT("Memory allocation failed."));
        }

        if(CertGetNameString(
            pCertContext,
            CERT_NAME_SIMPLE_DISPLAY_TYPE,
            0,
            NULL,
            pszName,
            cbSize))

        {
            _tprintf(TEXT("\nSubject -> %s.\n"), pszName);

            //-------------------------------------------------------
            //       Free the memory allocated for the string.
            free(pszName);
        }
        else
        {
            MyHandleError(TEXT("CertGetName failed."));
        }

        //-----------------------------------------------------------
        //        Get and display 
        //        the name of Issuer of the certificate.

        if(!(cbSize = CertGetNameString(   
            pCertContext,   
            CERT_NAME_SIMPLE_DISPLAY_TYPE,   
            CERT_NAME_ISSUER_FLAG,
            NULL,   
            NULL,   
            0)))
        {
            MyHandleError(TEXT("CertGetName 1 failed."));
        }

        if(!(pszName = (LPTSTR)malloc(cbSize * sizeof(TCHAR))))
        {
            MyHandleError(TEXT("Memory allocation failed."));
        }

        if(CertGetNameString(   
            pCertContext,   
            CERT_NAME_SIMPLE_DISPLAY_TYPE,   
            CERT_NAME_ISSUER_FLAG,
            NULL,   
            pszName,   
            cbSize))
        {
            _tprintf(TEXT("Issuer  -> %s.\n"), pszName);

            //-------------------------------------------------------
            //       Free the memory allocated for the string.
            free(pszName);
        }
        else
        {
            MyHandleError(TEXT("CertGetName failed."));
        }

        //-----------------------------------------------------------
        //       Convert the subject name to an ASN.1 encoded
        //       string and print the octets in that string.

        //       First : Get the number of bytes that must 
        //       be allocated for the string.

        cbSize = CertNameToStr(
            pCertContext->dwCertEncodingType,
            &(pCertContext->pCertInfo->Subject),
            MY_STRING_TYPE,
            NULL,
            0);

        //-----------------------------------------------------------
        //  The function CertNameToStr returns the number
        //  of bytes needed for a string to hold the
        //  converted name, including the null terminator. 
        //  If it returns one, the name is an empty string.

        if (1 == cbSize)
        {
            MyHandleError(TEXT("Subject name is an empty string."));
        }

        //-----------------------------------------------------------
        //        Allocated the needed buffer. Note that this
        //        memory must be freed inside the loop or the 
        //        application will leak memory.

        if(!(pszString = (LPTSTR)malloc(cbSize * sizeof(TCHAR))))
        {
            MyHandleError(TEXT("Memory allocation failed."));
        }

        //-----------------------------------------------------------
        //       Call the function again to get the string. 

        cbSize = CertNameToStr(
            pCertContext->dwCertEncodingType,
            &(pCertContext->pCertInfo->Subject),
            MY_STRING_TYPE,
            pszString,
            cbSize);

        //-----------------------------------------------------------
        //  The function CertNameToStr returns the number
        //  of bytes in the string, including the null terminator.
        //  If it returns 1, the name is an empty string.

        if (1 == cbSize)
        {
            MyHandleError(TEXT("Subject name is an empty string."));
        }

        //-----------------------------------------------------------
        //    Get the length needed to convert the string back 
        //    back into the name as it was in the certificate.

        if(!(CertStrToName(
            MY_ENCODING_TYPE,
            pszString, 
            MY_STRING_TYPE,
            NULL,
            NULL,        // NULL to get the number of bytes 
                            // needed for the buffer.          
            &cbSize,     // Pointer to a DWORD to hold the 
                            // number of bytes needed for the 
                            // buffer
            NULL )))     // Optional address of a pointer to
                            // old the location for an error in the 
                            // input string.
        {
            MyHandleError(
                TEXT("Could not get the length of the BLOB."));
        }

        if (!(blobEncodedName.pbData = (LPBYTE)malloc(cbSize)))
        {
            MyHandleError(
                TEXT("Memory Allocation for the BLOB failed."));
        }
        blobEncodedName.cbData = cbSize;

        if(CertStrToName(
            MY_ENCODING_TYPE,
            pszString, 
            MY_STRING_TYPE,
            NULL,
            blobEncodedName.pbData,
            &blobEncodedName.cbData,
            NULL))
        {
            _tprintf(TEXT("CertStrToName created the BLOB.\n"));
        }
        else
        {
            MyHandleError(TEXT("Could not create the BLOB."));
        }

        //-----------------------------------------------------------
        //       Free the memory.

        free(blobEncodedName.pbData);
        free(pszString);

        //-----------------------------------------------------------
        //       Pause before information on the next certificate
        //       is displayed.

        Local_wait();

    } // End of while loop
     
    _tprintf(
        TEXT("\nThere are no more certificates in the store. \n"));

    //---------------------------------------------------------------
    //   Close the MY store.

    if(CertCloseStore(
        hCertStore,
        CERT_CLOSE_STORE_CHECK_FLAG))
    {
        _tprintf(TEXT("The store is closed. ")
            TEXT("All certificates are released.\n"));
    }
    else
    {
        _tprintf(TEXT("The store was closed, ")
            TEXT("but certificates still in use.\n"));
    }

    _tprintf(TEXT("This demonstration program ran to completion ")
        TEXT("without error.\n"));

    Local_wait();

} // End of main

//-------------------------------------------------------------------
//    This example uses the function MyHandleError, a simple error
//    handling function, to print an error message to the standard  
//    error (stderr) file and exit the program. 
//    For most applications, replace this function with one 
//    that does more extensive error reporting.

void MyHandleError(LPTSTR psz)
{
    _ftprintf(stderr,
        TEXT("An error occurred in running the program. \n"));
    _ftprintf(stderr, TEXT("%s\n"), psz);
    _ftprintf(stderr, TEXT("Error number %x.\n"), GetLastError());
    _ftprintf(stderr, TEXT("Program terminating. \n"));
    exit(1);
} // End of MyHandleError

void Local_wait()
{
//-------------------------------------------------------------------
//  This function prints a prompt string 
//  and wait for the user to hit enter.
//  It provides a pause with its length controlled by the user.

     _tprintf(TEXT("Hit Enter to continue : "));
     getchar();
}


 

 

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