Example C Program: Exporting a Session Key

The following example creates a random session key and creates an exportable key BLOB. The example illustrates the use of CryptGetUserKey, CryptExportKey, and related functions.

This example illustrates the following tasks and CryptoAPI functions:

• Acquiring a CSP context using CryptAcquireContext.
• Gaining access to two different pairs of public/private keys using CryptGetUserKey.
• Generating an exportable session key using CryptGenKey.
• Creating a simple key BLOB containing a session key using CryptExportKey.
• Destroying a session key and access to the two pairs of public/private keys using CryptDestroyKey.
• Releasing the CSP context using CryptReleaseContext.

This example uses the function MyHandleError. The code for this function is included with the sample. Code for this and other auxiliary functions is also listed under General Purpose Functions.

//--------------------------------------------------------------------
// Copyright (C) Microsoft. All rights reserved.
// This example program creates a session key and a simple key 
// BLOB holding that session key. The key BLOB can be written to disk 
// and later read from a disk file.
#pragma comment(lib, "crypt32.lib")

#include <stdio.h>
#include <windows.h>
#include <Wincrypt.h>
#define MY_ENCODING_TYPE (PKCS_7_ASN_ENCODING | X509_ASN_ENCODING)
void MyHandleError(char *s);

void main(void)
{
//--------------------------------------------------------------------
// Declare and initialize variables.

HCRYPTPROV hProv; // CSP handle
HCRYPTKEY hSignKey; // Signature key pair handle
HCRYPTKEY hXchgKey; // Exchange key pair handle
HCRYPTKEY hKey; // Session key handle
BYTE *pbKeyBlob; // Pointer to a simple key BLOB
DWORD dwBlobLen; // The length of the key BLOB

//--------------------------------------------------------------------
// Acquire a cryptographic provider context handle.

if(CryptAcquireContext(
 &hProv, 
 NULL, 
 NULL, 
 PROV_RSA_FULL, 
 0)) 
{
 printf("The CSP has been acquired. \n");
}
else
{
 MyHandleError("Error during CryptAcquireContext.");
}
//--------------------------------------------------------------------
// Get a handle to the signature key.
// The signature key must exist before it can be retrieved. For more
// information, see the CryptGetUserKey documentation.

if(CryptGetUserKey(
 hProv, 
 AT_SIGNATURE, 
 &hSignKey)) 
{
 printf("The signature key has been acquired. \n");
}
else
{
 MyHandleError("Error during CryptGetUserKey for signkey.");
}
//--------------------------------------------------------------------
// Get a handle to the key exchange key.
// The key must exist before it can be retrieved. For more
// information, see the CryptGetUserKey documentation.

if(CryptGetUserKey(
 hProv, 
 AT_KEYEXCHANGE, 
 &hXchgKey)) 
{
 printf("The key exchange key has been acquired. \n");
}
else
{
 printf("Error during CryptGetUserKey exchange key.");
}
// hSignKey may be used to verify a signature. hXchgKey will be used
// to export a session key.

//--------------------------------------------------------------------
// Generate a session key.

if (CryptGenKey( 
 hProv, 
 CALG_RC4, 
 CRYPT_EXPORTABLE, 
 &hKey))
{ 
 printf("Original session key is created. \n");
}
else
{
 MyHandleError("ERROR -- CryptGenKey.");
}
// Determine the size of the key BLOB and allocate memory.

if(CryptExportKey(
 hKey, 
 hXchgKey, 
 SIMPLEBLOB, 
 0, 
 NULL, 
 &dwBlobLen)) 
{
 printf("Size of the BLOB for the session key determined. \n");
}
else
{
 MyHandleError("Error computing BLOB length.");
}

if(pbKeyBlob = (BYTE*)malloc(dwBlobLen)) 
{
 printf("Memory has been allocated for the BLOB. \n");
}
else
{
 MyHandleError("Out of memory. \n");
}
//--------------------------------------------------------------------
// Export the key into a simple key BLOB.

if(CryptExportKey(
 hKey, 
 hXchgKey, 
 SIMPLEBLOB, 
 0, 
 pbKeyBlob, 
 &dwBlobLen))
{
 printf("Contents have been written to the BLOB. \n");
}
else
{
 MyHandleError("Error during CryptExportKey.");
}
//--------------------------------------------------------------------
// At this point, other processing such as writing the key BLOB to
// a file could be done.

//--------------------------------------------------------------------
// After all processing, clean up.

//--------------------------------------------------------------------
// Free the memory used by the key BLOB.

free(pbKeyBlob);

// Destroy the session key.
if(hKey)
 CryptDestroyKey(hKey);

// Destroy the signature key handle.
if(hSignKey)
 CryptDestroyKey(hSignKey);

// Destroy the key exchange key handle.
if(hXchgKey)
 CryptDestroyKey(hXchgKey);

// Release the provider handle.
if(hProv) 
 CryptReleaseContext(hProv, 0);
printf("The program ran to completion without error. \n");

}// End of main 

//--------------------------------------------------------------------
// This example uses the function MyHandleError, a simple error
// handling function, to print an error message and exit 
// the program. 
// For most applications, replace this function with one 
// that does more extensive error reporting.

void MyHandleError(char *s)
{
 printf("An error occurred in running the program.\n");
 printf("%s\n",s);
 printf("Error number %x\n.",GetLastError());
 printf("Program terminating.\n");
 exit(1);
}