Example Code for User Defined Functions

The first function code is for a subroutine that enumerates all the certificates in a store until a particular certificate is found with an issuer name that matches a string passed as a parameter. This code demonstrates:

• The enumeration of certificates.
• Getting the issuer name from each enumerated certificate.
• Comparing the retrieved name with the desired name for each enumerated certificate.
• Returning a pointer to the CERT_CONTEXT created when a match is made.

// GetSubjectCert() - A function to enumerate certificates
// in the store and get a handle to one of them based on issuer
// name.

#define _WIN32_WINNT 0x0400
#include <stdio.h>
#include <windows.h>
#include "wincrypt.h"
#define MY_ENCODING_TYPE  (PKCS_7_ASN_ENCODING | X509_ASN_ENCODING)
PCCERT_CONTEXT GetSubjectCert(
     HCERTSTORE hCertStore,       // the store to search
     LPCTSTR pszSearchName)       // the string to match
{

//********************************************************************
// Declare variables.
PCCERT_CONTEXT   pCertContext = NULL;
BOOL             fFound = FALSE;
BOOL             fMore = TRUE;
DWORD            dwSize = 0;
LPSTR            pszSubjectName;
LPSTR            pszNameFound;
CERT_NAME_BLOB   NameBlob;

//********************************************************************
//  Enumerate the certificates in the store until a match is found.
while(fMore && !fFound)
{     
    if(!(pCertContext = CertEnumCertificatesInStore(
      hCertStore,
      pCertContext)))
    {
        // The end of the store has been reached. Get out of the loop.
        fMore=FALSE;
        break;
     }
     NameBlob = pCertContext->pCertInfo->Issuer;
     // Call CertNameToStr to get dwSize, the length of the 
     // CertNameString
     dwSize = CertNameToStr(
         MY_ENCODING_TYPE,        // Encoding type
         &NameBlob,               // the subject from the pCertInfo
         CERT_SIMPLE_NAME_STR,    // Type of string
         NULL,                    // Place to return string.
                                  // In the first pass, NULL since 
                                  // space for the name string has not
                                  // yet been allocated.
         0);                      // Size of the name string 
     if (dwSize<2)        // If the string length returned is 
                          // less than 2, the function could not
                          // determine a valid name string length.
                          // The function failed. 
      {     
          // The call to the function failed. Get out of the loop.
          printf("Error First pass of getting Name String1");
          fMore = FALSE;
          break;
      }           
      // Allocate memory for the subject name string.
      if(!(pszSubjectName = (LPSTR)malloc(dwSize)))
      {  
          // Memory allocation failed. Get out of the loop.
          printf("Error Allocating Memory");
          fMore = FALSE;
          break;
      }   
      //  Make second call to CertNameToStr to get the string.
     dwSize = CertNameToStr(
          MY_ENCODING-TYPE,       // Encoding type
          &NameBlob,              // CERT_NAME_BLOB
          CERT_SIMPLE_NAME_STR,   // Type
          pszSubjectName,         // Place to return string
          dwSize);                // Size of string (chars)
     if(dwSize<2)
     {  
           // If the length returned is less than 2, 
           // the function failed. Get out of the loop.
           printf("Error in second pass.\n");
           fMore = FALSE;
           break;
      }
      pszNameFound = strstr(pszSubjectName,                   
                             pszSearchName);
      if(pszNameFound)
      {
          // The string searched for was matched. Get out of the loop.
          printf("Target Certificate %s Found \n", pszNameFound);
          fFound = TRUE;
          break;
}  // end while  
free(pszSubjectName);
if(!fFound)
{      
   printf("Target Cert Not Found\n");
   CertFreeCertificateContext(pCertContext);
   return NULL;
}   
else
    return (pCertContext);   // return the whole context that included
                             // the subject name string.
}       // end of function

The following function is used to print an error message and exit a program. It is used the example code for several CryptoAPI functions.

void handle_error(char *s){
    printf("An error occured in running the program.\n");
    printf("%s\n\n",s);
    printf("Program terminating.\n");
    exit(1);
}