The CryptExportKey function is used to export cryptographic keys out of a cryptographic service provider in a secure manner.
A handle to the key to be exported is passed into the function and the function returns a key blob to the caller. This key blob can be sent over a nonsecure transport or stored in a nonsecure storage location. The key blob is useless until the intended recipient uses the CryptImportKey function on it, which will then import the key into the recipient’s CSP.
BOOL CRYPTFUNC CryptExportKey(
HCRYPTKEY hKey, | |
HCRYPTKEY hExpKey, | |
DWORD dwBlobType, | |
DWORD dwFlags, | |
BYTE *pbData, | |
DWORD *pdwDataLen | |
); |
Most often, this will be the key exchange public key of the destination user. However, certain protocols require that a session key belonging to the destination user be used for this purpose.
If the key blob type specified by dwBlobType is PUBLICKEYBLOB, then this parameter is unused and should be set to zero.
If the key blob specified by dwBlobType is PRIVATEKEYBLOB, then this is
typically a handle to a session key that is to be used to encrypt the key
blob. Some CSPs allow this parameter to be zero, in which case the application
should encrypt the private key blob manually so as to protect it.
As a rule, SIMPLEBLOBs will be 256 bytes or less, PUBLICKEYBLOBs will be 1000
bytes or less, and PRIVATEKEYBLOBS will be 5000 bytes or less.
If the buffer specified by pbData is not large enough to hold the data, the function returns the ERROR_MORE_DATA error code (through GetLastError) and stores the required buffer size, in bytes, into the variable pointed to by pdwDataLen.
If pbData is NULL, then no error is returned and the function stores the size of the data, in bytes, in the variable pointed to by pdwDataLen.
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To retrieve extended error information, use the GetLastError function.
The following table lists the error codes most commonly returned by the GetLastError function. The error codes prefaced by “NTE” are generated by the particular CSP you are using.
Error |
Description |
ERROR_INVALID_HANDLE |
One of the parameters specifies an invalid handle. |
ERROR_INVALID_PARAMETER |
One of the parameters contains an invalid value. This is most often an illegal pointer. |
NTE_BAD_FLAGS |
The dwFlags parameter is nonzero. |
NTE_BAD_KEY |
One or both of the keys specified by hKey and hExpKey are invalid. |
NTE_BAD_KEY_STATE |
You do not have permission to export the key. That is, when the hKey key was created, the CRYPT_EXPORTABLE flag was not specified. |
NTE_BAD_PUBLIC_KEY |
The key blob type specified by dwBlobType is PUBLICKEYBLOB, but hExpKey does not contain a public key handle. |
NTE_BAD_TYPE |
The dwBlobType parameter specifies an unknown blob type. |
NTE_BAD_UID |
The CSP context that was specified when the hKey key was created cannot be found. |
NTE_NO_KEY |
A session key is being exported and the hExpKey parameter does not specify a public key. |
#include <wincrypt.h> HCRYPTPROV hProv; // Handle to CSP HCRYPTKEY hKey; // Handle to session key HCRYPTKEY hXchgKey; // Handle to receiver’s exchange public key BYTE *pbKeyBlob = NULL; DWORD dwBlobLen; ... // Determine size of key blob and allocate memory. if(!CryptExportKey(hKey, hXchgKey, SIMPLEBLOB, 0, NULL, &dwBlobLen)) { printf("Error %x computing blob length!\n", GetLastError()); ... } if((pbKeyBlob = malloc(dwBlobLen)) == NULL) { printf("Out of memory!\n"); ... } // Export key into a simple key blob. if(!CryptExportKey(hKey, hXchgKey, SIMPLEBLOB, 0, pbKeyBlob, &dwBlobLen)) { printf("Error %x during CryptExportKey!\n", GetLastError()); ... }
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