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Example C Program: ASN.1 Encoding and Decoding

Example C Program: ASN.1 Encoding and Decoding

The following example shows using CryptEncodeObjectEx and CryptDecodeObjectEx. This example can easily be modified to use CryptEncodeObject and CryptDecodeObject.

This example also uses a modified version of the function ByteToStr to print an Abstract Syntax Notation One (ASN.1) encoded series of octets. It also uses MyHandleError. Code for these functions is included with the sample.


#include <stdio.h>
#include <windows.h>
#include "Wincrypt.h"
//-------------------------------------------------------------------
//  Copyright (C) Microsoft.  All rights reserved.
//  Only X509_ASN_ENCODING is needed with these
//  functions.
#pragma comment(lib, "crypt32.lib")

#define MY_TYPE   X509_ASN_ENCODING

//--------------------------------------------------------------------
//  Declare function ByteToString.

void ByteToStr(
     DWORD cb, 
     void* pv, 
     LPSTR sz);
//-------------------------------------------------------------------
//   Declare function MyHandleError.
void MyHandleError(char *s); 

//-------------------------------------------------------------------
//   Begin main.

void main(void)
{

//-------------------------------------------------------------------
//   Declare and initialize local variables.

char *Cert_Sub_Name ="Test User Name";

//-------------------------------------------------------------------
// Initialize a single RDN structure.

CERT_RDN_ATTR rgNameAttr = 
{
   szOID_COMMON_NAME,                // the OID
   CERT_RDN_PRINTABLE_STRING,        // type of string
   (DWORD)strlen(Cert_Sub_Name)+1,   // string length including
                                     // the terminating null 
                                     // character
   (BYTE *)Cert_Sub_Name             // pointer to the string
};
//-------------------------------------------------------------------
// Declare and initialize a structure to include
// the array of RDN structures.

CERT_RDN rgRDN[] = 
{
   1,               // the number of elements in the array
   &rgNameAttr      // pointer to the array
};

//-------------------------------------------------------------------
//  Declare and initialize a CERT_NAME_INFO 
//  structure that includes a CERT_RND.

CERT_NAME_INFO CertName = 
{
    1,          // number of elements in the CERT_RND's array
    rgRDN
};

//-------------------------------------------------------------------
//  Declare additional variables.

CERT_NAME_INFO *pDecodeName;  // point variable to hold
                              // the address of the decoded
                              // buffer
DWORD cbEncoded;              // variable to hold the
                              // length of the encoded string
DWORD cbDecoded;              // variable to hold the 
                              // length of the decoded buffer

BYTE *pbEncoded;              // variable to hold a pointer to the 
                              // encoded buffer
BYTE *pbDecoded;              // variable to hold a pointer to the
                              // decoded buffer
LPSTR sz;

//-------------------------------------------------------------------
//    Allocate memory for a large buffer.

if(sz=(char *)malloc(512))
{
     printf("Memory for sz allocated\n");
}
else
{
    MyHandleError("Memory allocation failed.");
}

//-------------------------------------------------------------------
// Call CrypteEncodeObjectEx to get 
// length to allocate for pbEncoded.

if( CryptEncodeObjectEx(
     MY_TYPE,        // the encoding/decoding type
     X509_NAME,    
     &CertName,
     0,                 
     NULL, 
     NULL,
     &cbEncoded))    // fill in the length needed for
                     // the encoded buffer
{
     printf("The number of bytes needed is %d \n",cbEncoded);
}
else
{
     MyHandleError("The first call to the function failed.\n");
}

if( pbEncoded = (BYTE*)malloc(cbEncoded))
{
     printf("Memory for pvEncoded has been allocated.\n");
}
else
{
    MyHandleError("Memory allocation failed.");
}

if(CryptEncodeObjectEx(
     MY_TYPE,
     X509_NAME,
     &CertName,
     0,
     NULL, 
     pbEncoded,
     &cbEncoded))
{
     ByteToStr(cbEncoded, pbEncoded,sz);
     printf("The Encoded octets are \n%s\n",sz);
}
else
{
     MyHandleError("Encoding failed.");
}

//-------------------------------------------------------------------
// Decode the encoded buffer.

//-------------------------------------------------------------------
//  Get the length needed for the decoded buffer.

if(CryptDecodeObjectEx(
     MY_TYPE,
     X509_NAME,
     pbEncoded,     // the buffer to be decoded
     cbEncoded,
     CRYPT_DECODE_NOCOPY_FLAG,
     NULL, 
     NULL,
     &cbDecoded))
{
     printf("The needed buffer length is %d\n",cbDecoded);
}
else
{
    MyHandleError("The first decode pass failed.");
}

//-------------------------------------------------------------------
// Allocate memory for the decoded information.

if(!(pbDecoded = (BYTE*)malloc(cbDecoded)))
{
     MyHandleError("Decode buffer memory allocation failed.");
}

//-------------------------------------------------------------------
// Decode the encoded buffer.

if(CryptDecodeObjectEx(
     MY_TYPE,
     X509_NAME,
     pbEncoded,     // the buffer to be decoded
     cbEncoded,
     CRYPT_DECODE_NOCOPY_FLAG,   
     NULL, 
     pbDecoded,
     &cbDecoded))
{
    pDecodeName = ( CERT_NAME_INFO     *)pbDecoded;
    printf("The cRDN is -> %d \n",pDecodeName->cRDN);
    printf("The OID is -> ");
    printf("%s\n",pDecodeName->rgRDN->rgRDNAttr->pszObjId);
    printf("The string is ->");
    printf(" %s\n",pDecodeName->rgRDN->rgRDNAttr->Value.pbData);}
else
{
     MyHandleError("Decode failed.");
}

// Clean up memory.

if(sz)
   free(sz);
if(pbEncoded)
   free(pbEncoded);
if(pbDecoded)
   free(pbDecoded);

printf("Processing completed without error.\n");
}  // end of main()

//-------------------------------------------------------------------
//  Define ByteToStr.

void ByteToStr(
     DWORD cb, 
     void* pv, 
     LPSTR sz)

//-------------------------------------------------------------------
// Parameters passed are:
//    pv -- the Array of BYTES to be converted.
//    cb -- the number of BYTEs in the array.
//    sz -- a pointer to the string to be returned.

{

//-------------------------------------------------------------------
//  Declare and initialize local variables.

BYTE* pb = (BYTE*) pv; // local pointer to a BYTE in the BYTE array
DWORD i;               // local loop counter
int b;                 // local variable

//  Ensure that sz is large enough to hold pv.
if(strlen(sz) < cb)
{
    MyHandleError("The array of bytes is too long for the "
        "allocated string.");
}

//-------------------------------------------------------------------
//  Begin processing loop.

for (i=0; i<cb; i++)
{
   b = (*pb & 0xF0) >> 4;
   *sz++ = (b <= 9) ? b + '0' : (b - 10) + 'A';
   b = *pb & 0x0F;
   *sz++ = (b <= 9) ? b + '0' : (b - 10) + 'A';
   pb++;
   *sz++=' ';
}
*sz++ = 0;
}  // end of ByteToStr
 
void MyHandleError(char *s) 
{ 
printf("An error occurred in running the program.\n"); 
    printf("%s\n",s); 
printf("Program terminating.\n"); 
exit(1); 
} // end of MyHandleError


 

 

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