Example C Program: Using CryptProtectData

The following example encrypts and decrypts a data BLOB using CryptProtectData and CryptUnprotectData.

This example illustrates the following tasks and CryptoAPI functions:

• Initializing a CRYPTPROTECT_PROMPTSTRUCT data structure.
• Using CryptProtectData to encrypt a data BLOB.
• Using CryptUnprotectData to decrypt the data.
• Using LocalFree to release allocated memory.

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

The following example shows protecting data.

#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()
{

// Copyright (C) Microsoft.  All rights reserved.
// Encrypt data from DATA_BLOB DataIn to DATA_BLOB DataOut.
// Then decrypt to DATA_BLOB DataVerify.

//-------------------------------------------------------------------
// Declare and initialize variables.

DATA_BLOB DataIn;
DATA_BLOB DataOut;
DATA_BLOB DataVerify;
BYTE *pbDataInput =(BYTE *)"Hello world of data protection.";
DWORD cbDataInput = strlen((char *)pbDataInput)+1;
DataIn.pbData = pbDataInput;    
DataIn.cbData = cbDataInput;
CRYPTPROTECT_PROMPTSTRUCT PromptStruct;
LPWSTR pDescrOut = NULL;

//-------------------------------------------------------------------
//  Begin processing.

printf("The data to be encrypted is: %s\n",pbDataInput);

//-------------------------------------------------------------------
//  Initialize PromptStruct.

ZeroMemory(&PromptStruct, sizeof(PromptStruct));
PromptStruct.cbSize = sizeof(PromptStruct);
PromptStruct.dwPromptFlags = CRYPTPROTECT_PROMPT_ON_PROTECT;
PromptStruct.szPrompt = L"This is a user prompt.";

//-------------------------------------------------------------------
//  Begin protect phase.

if(CryptProtectData(
     &DataIn,
     L"This is the description string.", // A description string. 
     NULL,                               // Optional entropy
                                         // not used.
     NULL,                               // Reserved.
     &PromptStruct,                      // Pass a PromptStruct.
     0,
     &DataOut))
{
     printf("The encryption phase worked. \n");
}
else
{
    MyHandleError("Encryption error!");
}
//-------------------------------------------------------------------
//   Begin unprotect phase.

if (CryptUnprotectData(
        &DataOut,
        &pDescrOut,
        NULL,                 // Optional entropy
        NULL,                 // Reserved
        &PromptStruct,        // Optional PromptStruct
        0,
        &DataVerify))
{
     printf("The decrypted data is: %s\n", DataVerify.pbData);
     printf("The description of the data was: %S\n",pDescrOut);
}
else
{
    MyHandleError("Decryption error!");
}
//-------------------------------------------------------------------
// At this point, memcmp could be used to compare DataIn.pbData and 
// DataVerify.pbDate for equality. If the two functions worked
// correctly, the two byte strings are identical. 

//-------------------------------------------------------------------
//  Clean up.

LocalFree(pDescrOut);
LocalFree(DataOut.pbData);
LocalFree(DataVerify.pbData);
} // End of main

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

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