Question

I’m trying to understand how to get a public key imported from PEM format (sample included in the code below) across XP, Vista and Windows 7. The sample code will import the key on both XP and Windows Vista/7, but not the same way.

On Windows XP, the string “(Prototype)” is required in the cryptographic provider’s name, and allows the call to CryptImportPublicKeyInfo to pass.

On Windows 7, the “(Prototype)” provider is apparently present, but does not support the call to CryptImportPublicKeyInfo, which is confusing.

What might a correct implementation look like between these operating systems? Is it necessary to detect XP and request the name with “(Prototype)”, and without it for other operating systems? Is it possible that that will still fail on some XP systems?

Or, is there a way to detect this confusing behavior and select whichever cryptographic provider will support the necessary call?

Output on Windows 7:

ANALYZING CRYPTOGRAPHIC SUPPORT FOR:
     "Microsoft Enhanced RSA and AES Cryptographic Provider"
     CryptAcquireContext success.
     CryptAcquireContext.1 success.
     CryptStringToBinary.2 success.
     CryptDecodeObjectEx success.
     CryptImportPublicKeyInfo success.
     SUCCESS.
ANALYZING CRYPTOGRAPHIC SUPPORT FOR:
     "Microsoft Enhanced RSA and AES Cryptographic Provider (Prototype)"
     CryptAcquireContext success.
     CryptAcquireContext.1 success.
     CryptStringToBinary.2 success.
     CryptDecodeObjectEx success.
     CryptImportPublicKeyInfo FAILED****.

Output on Windows XP:

ANALYZING CRYPTOGRAPHIC SUPPORT FOR:
     "Microsoft Enhanced RSA and AES Cryptographic Provider"
     CryptAcquireContext success.
     CryptAcquireContext.1 success.
     CryptStringToBinary.2 success.
     CryptDecodeObjectEx success.
     CryptImportPublicKeyInfo FAILED****.
ANALYZING CRYPTOGRAPHIC SUPPORT FOR:
     "Microsoft Enhanced RSA and AES Cryptographic Provider (Prototype)"
     CryptAcquireContext success.
     CryptAcquireContext.1 success.
     CryptStringToBinary.2 success.
     CryptDecodeObjectEx success.
     CryptImportPublicKeyInfo success.
     SUCCESS.

C++ source code which produces that output: (requires crypt32.lib)

#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <wincrypt.h>

bool windowsAcquireProviderContext(HCRYPTPROV *pHandleProv, LPCTSTR pProviderName);
bool analyzeCryptographicSupport(LPCTSTR pProviderName);

int _tmain(int argc, _TCHAR* argv[])
{
    analyzeCryptographicSupport(MS_ENH_RSA_AES_PROV);
    analyzeCryptographicSupport(L"Microsoft Enhanced RSA and AES Cryptographic Provider (Prototype)");
    return 0;
}

bool windowsAcquireProviderContext(HCRYPTPROV *pHandleProv, LPCTSTR pProviderName) {
    WCHAR *pContainerName = L"blah blah blah";
    if(!CryptAcquireContext(pHandleProv, pContainerName, pProviderName, PROV_RSA_AES, CRYPT_SILENT)) {
        if(GetLastError() == NTE_BAD_KEYSET) {
            if(CryptAcquireContext(pHandleProv, pContainerName, pProviderName, PROV_RSA_AES, CRYPT_NEWKEYSET|CRYPT_SILENT)) {
                return true;
            }
        } 
    }
    return true;
}

LPCWSTR pwszPemPublicKey = 
    L"-----BEGIN PUBLIC KEY-----\r\n"
    L"MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC6GUVcbn92bahlwOskKi8XkG9q\r\n"
    L"Vq863+C4cOWC6HzJojc011pJFFIBu8/pG1EI8FZJdBmTrFaJTriYw1/SpbOH0QqE\r\n"
    L"eHanT8qWn+S5m9xgDJoWTBJKcnu3OHOvJJU3c8jOHQQnRWLfghJH4vnwStdiwUUY\r\n"
    L"SMWpwuHObsNelGBgEQIDAQAB\r\n"
    L"-----END PUBLIC KEY-----\r\n";
int pemPublicKeySize = wcslen(pwszPemPublicKey);

bool analyzeCryptographicSupport(LPCTSTR pProviderName) {

    printf("ANALYZING CRYPTOGRAPHIC SUPPORT FOR:\r\n");
    wprintf(L"\t \"%s\"\r\n", pProviderName);

    HCRYPTPROV hProv;
    if(!windowsAcquireProviderContext(&hProv, pProviderName)) {
        wprintf(L"\t CryptAcquireContext FAILED.\r\n");
        return false;
    }
    wprintf(L"\t CryptAcquireContext success.\r\n");

    DWORD blobSize;

    if(!CryptStringToBinary(pwszPemPublicKey, pemPublicKeySize, CRYPT_STRING_BASE64_ANY,  NULL, &blobSize, NULL, NULL)) {
        CryptReleaseContext(hProv, 0);
        wprintf(L"\t CryptStringToBinary.1 FAILED****.\r\n");
        return false;
    }
    wprintf(L"\t CryptAcquireContext.1 success.\r\n");

    BYTE *pBlob = (BYTE *)malloc(blobSize);

    if(!CryptStringToBinary(pwszPemPublicKey, pemPublicKeySize, CRYPT_STRING_BASE64_ANY,  pBlob, &blobSize, NULL, NULL)) {
        free(pBlob);
        CryptReleaseContext(hProv, 0);
        wprintf(L"\t CryptStringToBinary.2 FAILED****.\r\n");
        return false;
    }
    wprintf(L"\t CryptStringToBinary.2 success.\r\n");

    CERT_PUBLIC_KEY_INFO *publicKeyInfo;
    DWORD publicKeyInfoLen;
    HCRYPTKEY hPublicKey;

    if(!CryptDecodeObjectEx(X509_ASN_ENCODING|PKCS_7_ASN_ENCODING, X509_PUBLIC_KEY_INFO, pBlob, blobSize, CRYPT_DECODE_ALLOC_FLAG, NULL, &publicKeyInfo, &publicKeyInfoLen)) {
        free(pBlob);
        CryptReleaseContext(hProv, 0);
        wprintf(L"\t CryptDecodeObjectEx FAILED****.\r\n");
        return false;
    }
    wprintf(L"\t CryptDecodeObjectEx success.\r\n");

    if(!CryptImportPublicKeyInfo(hProv, X509_ASN_ENCODING|PKCS_7_ASN_ENCODING, publicKeyInfo, &hPublicKey)) {
        LocalFree(publicKeyInfo);
        free(pBlob);
        CryptReleaseContext(hProv, 0);
        wprintf(L"\t CryptImportPublicKeyInfo FAILED****.\r\n");
        return false;
    }
    wprintf(L"\t CryptImportPublicKeyInfo success.\r\n");

    CryptDestroyKey(hPublicKey);
    LocalFree(publicKeyInfo);
    free(pBlob);
    CryptReleaseContext(hProv, 0);

    wprintf(L"\t SUCCESS.\r\n");
    return true;
}

Answer 1

The reason of the problem which you describes is very easy: Microsoft renamed AES Cryptographic Provider from

• "Microsoft Enhanced RSA and AES Cryptographic Provider (Prototype)" in Windows XP to
• "Microsoft Enhanced RSA and AES Cryptographic Provider" in the later versions of the operation systems.

In WinCrypt.h are defined the corresponding constants as MS_ENH_RSA_AES_PROV and MS_ENH_RSA_AES_PROV_XP which you can use.

If you don’t want to test the version of the operation system you can just use CryptAcquireContext with NULL as pszProvider (and continue to use PROV_RSA_AES as the dwProvType). In your code you can include analyzeCryptographicSupport(NULL);.

You can also examine the “Name” value of the registry key

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography\Defaults\Provider Types\Type 024

to see the name of the default PROV_RSA_AES provider.