Question

I need to load a PEM encoded X.509 certificate into a Windows Crypto API context to use with C++. They are the ones that have -----BEGIN RSA XXX KEY----- and -----END RSA XXX KEY-----. I found examples for Python and .NET but they use specific functions I can’t relate to the plain Windows Crypto API.

I understand how to encrypt/decrypt once I’ve got a HCRYPTKEY.
BUT, I just don’t get how to import the Base64 blob in the .PEM file(s) and get a HCRYPTKEY that I can use out of it.

I have that strange feeling that there is more to it than simply calling CryptDecodeObject().

Any pointers that can put me on track? I’ve already lost two days doing “trial & error” programming and getting nowhere.

Answer 1

KJKHyperion said in his answer:

I discovered the “magic” sequence of calls to import a RSA public key in PEM format. Here you go:

1. decode the key into a binary blob with CryptStringToBinary; pass CRYPT_STRING_BASE64HEADER in dwFlags
2. decode the binary key blob into a CERT_PUBLIC_KEY_INFO with CryptDecodeObjectEx; pass X509_ASN_ENCODING in dwCertEncodingType and X509_PUBLIC_KEY_INFO in lpszStructType
3. decode the PublicKey blob from the CERT_PUBLIC_KEY_INFO into a RSA key blob with CryptDecodeObjectEx; pass X509_ASN_ENCODING in dwCertEncodingType and RSA_CSP_PUBLICKEYBLOB in lpszStructType
4. import the RSA key blob with CryptImportKey

This sequence really helped me understand what’s going on, but it didn’t work for me as-is. The second call to CryptDecodeObjectEx gave me an error:
“ASN.1 bad tag value met”.
After many attempts at understanding Microsoft documentation, I finally realized that the output of the fist decode cannot be decoded as ASN again, and that it is actually ready for import. With this understanding I found the answer in the following link:

http://www.ms-news.net/f2748/problem-importing-public-key-4052577.html

Following is my own program that imports a public key from a .pem file to a CryptApi context:

int main()
{
    char           pemPubKey[2048];
    int            readLen;
    char           derPubKey[2048];
    size_t         derPubKeyLen = 2048;
    CERT_PUBLIC_KEY_INFO *publicKeyInfo;
    int            publicKeyInfoLen;
    HANDLE         hFile;
    HCRYPTPROV     hProv = 0;
    HCRYPTKEY      hKey = 0;

    /*
     * Read the public key cert from the file
     */
    hFile = CreateFileA( "c:\\pub.pem", GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL );
    if ( hFile == INVALID_HANDLE_VALUE )
    {
        fprintf( stderr, "Failed to open file. error: %d\n", GetLastError() );
    }

    if ( !ReadFile( hFile, pemPubKey, 2048, &readLen, NULL ) )
    {
        fprintf( stderr, "Failed to read file. error: %d\n", GetLastError() );
    }

    /*
     * Convert from PEM format to DER format - removes header and footer and decodes from base64
     */
    if ( !CryptStringToBinaryA( pemPubKey, 0, CRYPT_STRING_BASE64HEADER, derPubKey, &derPubKeyLen, NULL, NULL ) )
    {
        fprintf( stderr, "CryptStringToBinary failed. Err: %d\n", GetLastError() );
    }

    /*
     * Decode from DER format to CERT_PUBLIC_KEY_INFO
     */
    if ( !CryptDecodeObjectEx( X509_ASN_ENCODING, X509_PUBLIC_KEY_INFO, derPubKey, derPubKeyLen, 
                               CRYPT_ENCODE_ALLOC_FLAG, NULL, &publicKeyInfo, &publicKeyInfoLen ) )
    {
        fprintf( stderr, "CryptDecodeObjectEx 1 failed. Err: %p\n", GetLastError() );
        return -1;
    }

    /*
     * Acquire context 
     */
    if( !CryptAcquireContext(&hProv, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT) )
    {
        {
            printf( "CryptAcquireContext failed - err=0x%x.\n", GetLastError() );
            return -1;
        }
    }

    /*
     * Import the public key using the context
     */
    if ( !CryptImportPublicKeyInfo( hProv, X509_ASN_ENCODING, publicKeyInfo, &hKey ) )
    {
        fprintf( stderr, "CryptImportPublicKeyInfo failed. error: %d\n", GetLastError() );
        return -1;
    }
    LocalFree( publicKeyInfo );

    /*
     * Now use hKey to encrypt whatever you need.
     */

    return 0;
}