Problem - Kevin Tafuro

exportable from key objects, but the data must be encrypted with another key to prevent
accidental disclosure of the raw key data.
To extract the raw key data from a CryptoAPI key, you must first export the key
using the CryptoAPI function CryptoExportKey( ). The key data obtained from this
function will be encrypted with another key, which you can then use to decrypt the
encrypted key data to obtain the raw key data that another API, such as OpenSSL,
can use.
Discussion
To export a key using the CryptoExportKey( ) function, you must provide the function
with another key that will be used to encrypt the key data that’s to be exported.
Recipe 5.26 includes a function, SpcGetExportableContext( ), that obtains a handle to
a CSP context object suitable for exporting keys created with it. The CSP context
object uses a “container” to store public key pairs. Every public key container can
have a special public key pair known as an exchange key, which is the key that we’ll
use to decrypt the exported key data.
The function CryptGetUserKey( ) is used to obtain the exchange key. If it doesn’t
exist, SpcExportKeyData( ), listed later in this section, will create a 1,024-bit exchange
key, which will be stored as the exchange key in the public key container so future
attempts to get the key will succeed. The special algorithm identifier AT_KEYEXCHANGE
is used to reference the exchange key.
Symmetric keys are always exported via CryptExportKey( ) in “simple blob” format,
specified by the SIMPLEBLOB constant passed to CryptExportKey( ). The data returned
in the buffer from CryptExportKey( ) will have a BLOBHEADER structure, followed by an
ALG_ID for the algorithm used to encrypt the key data. The raw key data will follow
the BLOBHEADER and ALG_ID header information. For extracting the raw key data from
a CryptoAPI key, the data in the BLOBHEADER structure and the ALG_ID are of no interest,
but you must be aware of their existence so that you can skip over them to find
the encrypted key data.
Finally, the encrypted key data can be decrypted using CryptDecrypt( ) and the
exchange key. The CryptDecrypt( ) function is described in more detail in Recipe 5.25.
The decrypted data is the raw key data that can now be passed off to other APIs or
used in protocols that already provide their own protection for the key. The return
from SpcExportKeyData( ) will be a buffer allocated with LocalAlloc( ) that contains
the unencrypted symmetric key if no errors occur; otherwise, NULL will be returned.
#include
#include
BYTE *SpcExportKeyData(HCRYPTPROV hProvider, HCRYPTKEY hKey, DWORD *cbData) {
BOOL bResult = FALSE;
BYTE *pbData = 0, *pbKeyData;
Extracting Raw Key Data from a CryptoAPI Key Object | 247
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