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Example C Program: Exporting a Session Key

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:

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);
}


 

 

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