Load a Certificate from a File

2017-06-01

This topic describes how to load a certificate from a certificate file.

To load a certificate from a certificate file

Open the certificate file for read access.
Read the contents of the certificate file into the certificate buffer.
Create a certificate using the contents of the buffer.

// In the interest of simplicity, this example
//  uses a fixed-length buffer to hold the certificate. 
// A more robust solution would be to query the size
//  of the certificate file and dynamically
//  allocate a buffer of that size or greater.
#define CERTIFICATE_BUFFER_SIZE 1024

HRESULT  hr                  = S_OK;
BYTE     certEncoded[CERTIFICATE_BUFFER_SIZE] = {0};
DWORD    certEncodedSize     = 0L;
HANDLE   certFileHandle      = NULL;
BOOL     result              = FALSE;

// open the certificate file
certFileHandle = CreateFile(certFile, 
    GENERIC_READ, 
    0, 
    NULL, 
    OPEN_EXISTING, 
    FILE_ATTRIBUTE_NORMAL, 
    NULL);
if (INVALID_HANDLE_VALUE == certFileHandle) {
    hr = HRESULT_FROM_WIN32(GetLastError());
}

if (SUCCEEDED(hr)) {
    // if the buffer is large enough
    //  read the certificate file into the buffer
    if (GetFileSize (certFileHandle, NULL) <= CERTIFICATE_BUFFER_SIZE) {
        result = ReadFile(certFileHandle, 
            certEncoded, 
            CERTIFICATE_BUFFER_SIZE, 
            &certEncodedSize, 
            NULL);
        if (!result) {
            // the read failed, return the error as an HRESULT
            hr = HRESULT_FROM_WIN32(GetLastError());
        } else {
            hr = S_OK;
        }
    } else {
        // The certificate file is larger than the allocated buffer.
        //  To handle this error, you could dynamically allocate
        //  the certificate buffer based on the file size returned or 
        //  use a larger static buffer.
        hr = HRESULT_FROM_WIN32(ERROR_MORE_DATA);
    }    
}
if (SUCCEEDED(hr))
{
    // create a certificate from the contents of the buffer
    *cert = CertCreateCertificateContext(X509_ASN_ENCODING, 
        certEncoded, 
        certEncodedSize);
    if (!(*cert)) {
        hr = HRESULT_FROM_WIN32(GetLastError());
        CloseHandle(certFileHandle);
        hr = E_FAIL;
    } else {
        hr = S_OK;
    }
}
// close the certificate file
if (NULL != certFileHandle) CloseHandle(certFileHandle);

Next Steps

Verify the System Supports a Digest Method

Verify That a Certificate Supports a Signature Method

Embed Certificate Chains in a Document

Used in This Example

CreateFile

ReadFile

CertCreateCertificateContext

For More Information

Cryptography API

Cryptography Functions

XPS Digital Signature API Errors