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List:       ms-cryptoapi
Subject:    Re: Extension data definition
From:       "Rivard, Dennis" <dennis.rivard () INTEL ! COM>
Date:       2000-07-19 16:50:06
[Download RAW message or body]


this code fragment should help...
When you get the extension,m the info is encoded, so you need to decode...
Here is exampe code for getting the CRL distribution point...

hope the formatting doesn't get mangled...

// begin code fragment...

        HCERTSTORE       hCertStore = pCertContext->hCertStore;


        DWORD cBufferSize = 0;

        PCERT_EXTENSION  pCertExtInfo = NULL;
        PCRL_DIST_POINTS_INFO pCRLDist=NULL;  // use the struct for the ifo
you need

        // search for the dist points extension
        pCertExtInfo = CertFindExtension(szOID_CRL_DIST_POINTS/* your OID
here */, pCertContext->pCertInfo->cExtension,
pCertContext->pCertInfo->rgExtension);



        if (NULL != pCertExtInfo)
        {
                TRACE(_T("objid = %s\r\n"),pCertExtInfo->pszObjId);

                // convert the BLOB info into something useful for decoding
                const BYTE* ptemp = pCertExtInfo->Value.pbData;
                DWORD cbCount = pCertExtInfo->Value.cbData;


                if(CryptDecodeObject(
                         MY_ENCODING_TYPE,
                         X509_CRL_DIST_POINTS, // your OID goes here
                         ptemp ,     // The buffer to be decoded.
                         cbCount,
                         CRYPT_DECODE_NOCOPY_FLAG,
                         //0,
                         NULL,
                         &cBufferSize))
                {
                         TRACE(_T("The needed buffer length is
%d\n"),cBufferSize);
                }
                else
                {
                        HandleError(_T("The first decode pass failed."));
                }

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

                if(!(pCRLDist = new CRL_DIST_POINTS_INFO[cBufferSize*2]))
                {
                         HandleError(_T("Decode buffer memory allocation
failed."));
                }

                if(CryptDecodeObject(
                         MY_ENCODING_TYPE,
                         X509_CRL_DIST_POINTS,  // your OID goes here
                         ptemp,     // The buffer to be decoded.
                         cBufferSize,
                         CRYPT_DECODE_NOCOPY_FLAG,
                         pCRLDist,
                         &cBufferSize)) // FRANK: the bug was here
                {



                        TRACE(_T("Decode object OK!"));

                // do what you need to do...
                }
-----Original Message-----
From: J Michael Workman [mailto:jworkman@MITRE.ORG]
Sent: Wednesday, July 19, 2000 11:47 AM
To: CryptoAPI@DISCUSS.MICROSOFT.COM
Subject: Re: Extension data definition


I cannot determine what the values are supposed to be. A quick study of the
function, CertFindExtension, and it's members and sub-members:

CertFindExtension - finds the first extension in the CERT_EXTENSION array,
as identified by its OID. A CERT_INFO structure is derived from a decoded
certificate. That structure's rgExtension array is passed to
CertFindExtension which returns a pointer to the extension if it is in the
array.

Parameters
   pszObjId     [in] Pointer to the Object Identifier
                (OID) to use in the search.
   cExtensions  Number of extensions in the rgExtensions
                array.
   rgExtensions Array of CERT_EXTENSION structures.

        CERT_EXTENSION Members
           pszObjId  OID specifies the structure
                     of the extension data contained
                     in the Value member. See X.509
                     Certificate Extension Structures
                     for OIDs.
           fCritical Specifies if extension is necessary.
           Value     A BLOB structure that contains
                     encoded extension data.

               Value members
                  cbData  Count of bytes in the buffer
                          pointed to by pbData.
                  pbData Pointer to a block of data bytes.

After using this function, I get a successful return of data in Value.cbData
and Value.pbData. Unfortunately, I do not know what to do with the returned
values. I have examined the pbData byte by byte (4 bytes) and get returned
values that look like (in hex):

    03 02 05 20

and also

    03 02 06 c0

Does anybody know what I am talking about? I don't...


On Mon, 17 Jul 2000 16:42:59 -0500, Suhail Ansari <sansari@MEDNA.COM> wrote:
>See MSDN it has a good explanation of the return value by this function. It
>returns pointer to CERT_EXTENSION structure if found. Then you can get the
>value of the extension.
>
>-----Original Message-----
>From: J Michael Workman [mailto:jworkman@MITRE.ORG]
>>I am using the following CAPI function call to get the key usage extension
>>from a x.509 certificate:
>>
>>    ext=CertFindExtension(szOID_KEY_USAGE ,
>>              cExtension,
>>              rgExtension);
>>
>>This call is successful (no error returned) and I have data to examine at
>>ext->Value.pbData. But I don't know what this data means. I am looking for
>>key encypherment or digital signing bits being set. How is this data
>>formatted. What do the values mean? Is there a definition someplace of the
>>data returned by this function?

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