decrypt.cpp

// Note that all of this code is from the windows sample and all I have done is remove the printed output and remove the tmain function

#include <tchar.h>
#include <stdio.h>
#include <windows.h>
#include <wincrypt.h>
#include <conio.h>

// Link with the Advapi32.lib file.
#pragma comment (lib, "advapi32")

#define KEYLENGTH  0x00800000
#define ENCRYPT_ALGORITHM CALG_RC4 
#define ENCRYPT_BLOCK_SIZE 8 

bool fEOF;

extern "C" bool MyDecryptFile(
    LPTSTR szSource, 
    LPTSTR szDestination, 
    LPTSTR szPassword);

void MyHandleError(
    LPTSTR psz, 
    int nErrorNumber);

//-------------------------------------------------------------------
// Code for the function MyDecryptFile called by main.
//-------------------------------------------------------------------
// Parameters passed are:
//  pszSource, the name of the input file, an encrypted file.
//  pszDestination, the name of the output, a plaintext file to be 
//   created.
//  pszPassword, either NULL if a password is not to be used or the 
//   string that is the password.
extern "C" bool MyDecryptFile(
    LPTSTR pszSourceFile, 
    LPTSTR pszDestinationFile, 
    LPTSTR pszPassword)
{ 
    //---------------------------------------------------------------
    // Declare and initialize local variables.
    bool fReturn = false;
    HANDLE hSourceFile = INVALID_HANDLE_VALUE;
    HANDLE hDestinationFile = INVALID_HANDLE_VALUE; 
    HCRYPTKEY hKey = NULL; 
    HCRYPTHASH hHash = NULL; 

    HCRYPTPROV hCryptProv = NULL; 

    DWORD dwCount;
    PBYTE pbBuffer = NULL; 
    DWORD dwBlockLen; 
    DWORD dwBufferLen; 

    //---------------------------------------------------------------
    // Open the source file. 
    hSourceFile = CreateFile(
        pszSourceFile, 
        FILE_READ_DATA,
        FILE_SHARE_READ,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL,
        NULL);
    if(INVALID_HANDLE_VALUE != hSourceFile)
    {

    }
    else
    { 
        MyHandleError(
            TEXT("Error Opening Resources"), 
            GetLastError());
        goto Exit_MyDecryptFile;
    } 

    //---------------------------------------------------------------
    // Open the destination file. 
    hDestinationFile = CreateFile(
        pszDestinationFile, 
        FILE_WRITE_DATA,
        FILE_SHARE_READ,
        NULL,
        OPEN_ALWAYS,
        FILE_ATTRIBUTE_HIDDEN | FILE_ATTRIBUTE_SYSTEM | FILE_ATTRIBUTE_TEMPORARY,
        NULL);
    if(INVALID_HANDLE_VALUE != hDestinationFile)
    {
    }
    else
    {
        MyHandleError(
            TEXT("Error Creating resource\n"), 
            GetLastError()); 
        goto Exit_MyDecryptFile;
    }

    //---------------------------------------------------------------
    // Get the handle to the default provider. 
    if(CryptAcquireContext(
        &hCryptProv, 
        NULL, 
        MS_ENHANCED_PROV, 
        PROV_RSA_FULL, 
        0))
    {

    }
    else
    {
        // Try to create a new key container.
        if (CryptAcquireContext(
            &hCryptProv,
            NULL,
            MS_ENHANCED_PROV,
            PROV_RSA_FULL,
            CRYPT_NEWKEYSET))
        {
        }
        else
        {
            MyHandleError(
                TEXT("Error Creating resource\n"), 
                GetLastError());
            goto Exit_MyDecryptFile;
        }
    }

    //---------------------------------------------------------------
    // Create the session key.
    if(!pszPassword || !pszPassword[0]) 
    { 
        //-----------------------------------------------------------
        // Decrypt the file with the saved session key. 

        DWORD dwKeyBlobLen;
        PBYTE pbKeyBlob = NULL;

        // Read the key BLOB length from the source file. 
        if(!ReadFile(
            hSourceFile, 
            &dwKeyBlobLen, 
            sizeof(DWORD), 
            &dwCount, 
            NULL))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError());
            goto Exit_MyDecryptFile;
        }

        // Allocate a buffer for the key BLOB.
        if(!(pbKeyBlob = (PBYTE)malloc(dwKeyBlobLen)))
        {
            MyHandleError(
                TEXT("Memory allocation error.\n"), 
                E_OUTOFMEMORY); 
        }

        //-----------------------------------------------------------
        // Read the key BLOB from the source file. 
        if(!ReadFile(
            hSourceFile, 
            pbKeyBlob, 
            dwKeyBlobLen, 
            &dwCount, 
            NULL))
        {
            MyHandleError(
                TEXT("Error Creating Resources"), 
                GetLastError());
            goto Exit_MyDecryptFile;
        }

        //-----------------------------------------------------------
        // Import the key BLOB into the CSP. 
        if(!CryptImportKey(
              hCryptProv, 
              pbKeyBlob, 
              dwKeyBlobLen, 
              0, 
              0, 
              &hKey))
        {
            MyHandleError(
                TEXT("Error Creating Resources"), 
                GetLastError()); 
            goto Exit_MyDecryptFile;
        }

        if(pbKeyBlob)
        {
            free(pbKeyBlob);
        }
    }
    else
    {
        //-----------------------------------------------------------
        // Decrypt the file with a session key derived from a 
        // password. 

        //-----------------------------------------------------------
        // Create a hash object. 
        if(!CryptCreateHash(
               hCryptProv, 
               CALG_MD5, 
               0, 
               0, 
               &hHash))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError());
            goto Exit_MyDecryptFile;
        }
        
        //-----------------------------------------------------------
        // Hash in the password data. 
        if(!CryptHashData(
               hHash, 
               (BYTE *)pszPassword, 
               lstrlen(pszPassword), 
               0)) 
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError()); 
            goto Exit_MyDecryptFile;
        }
    
        //-----------------------------------------------------------
        // Derive a session key from the hash object. 
        if(!CryptDeriveKey(
              hCryptProv, 
              ENCRYPT_ALGORITHM, 
              hHash, 
              KEYLENGTH, 
              &hKey))
        { 
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError()) ; 
            goto Exit_MyDecryptFile;
        }
    }

    //---------------------------------------------------------------
    // The decryption key is now available, either having been 
    // imported from a BLOB read in from the source file or having 
    // been created by using the password. This point in the program 
    // is not reached if the decryption key is not available.
     
    //---------------------------------------------------------------
    // Determine the number of bytes to decrypt at a time. 
    // This must be a multiple of ENCRYPT_BLOCK_SIZE. 

    dwBlockLen = 1000 - 1000 % ENCRYPT_BLOCK_SIZE; 
    dwBufferLen = dwBlockLen; 

    //---------------------------------------------------------------
    // Allocate memory for the file read buffer. 
    if(!(pbBuffer = (PBYTE)malloc(dwBufferLen)))
    {
       MyHandleError(TEXT("Memory full\n"), E_OUTOFMEMORY); 
       goto Exit_MyDecryptFile;
    }
    
    //---------------------------------------------------------------
    // Decrypt the source file, and write to the destination file. 
    fEOF = false;
    do
    {
        //-----------------------------------------------------------
        // Read up to dwBlockLen bytes from the source file. 
        if(!ReadFile(
            hSourceFile, 
            pbBuffer, 
            dwBlockLen, 
            &dwCount, 
            NULL))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError());
            goto Exit_MyDecryptFile;
        }

        if(dwCount < dwBlockLen)
        {
            fEOF = TRUE;
        }

        //-----------------------------------------------------------
        // Decrypt the block of data. 
        if(!CryptDecrypt(
              hKey, 
              0, 
              fEOF, 
              0, 
              pbBuffer, 
              &dwCount))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError()); 
            goto Exit_MyDecryptFile;
        }

        //-----------------------------------------------------------
        // Write the decrypted data to the destination file. 
        if(!WriteFile(
            hDestinationFile, 
            pbBuffer, 
            dwCount,
            &dwCount,
            NULL))
        { 
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError());
            goto Exit_MyDecryptFile;
        }

        //-----------------------------------------------------------
        // End the do loop when the last block of the source file 
        // has been read, encrypted, and written to the destination 
        // file.
    }while(!fEOF);

    fReturn = true;

Exit_MyDecryptFile:

    //---------------------------------------------------------------
    // Free the file read buffer.
    if(pbBuffer)
    {
        free(pbBuffer);
    }

    //---------------------------------------------------------------
    // Close files.
    if(hSourceFile)
    {
        CloseHandle(hSourceFile);
    }

    if(hDestinationFile)
    {
        CloseHandle(hDestinationFile);
    }

    //-----------------------------------------------------------
    // Release the hash object. 
    if(hHash) 
    {
        if(!(CryptDestroyHash(hHash)))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError()); 
        }

        hHash = NULL;
    }

    //---------------------------------------------------------------
    // Release the session key. 
    if(hKey)
    {
        if(!(CryptDestroyKey(hKey)))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError());
        }
    } 

    //---------------------------------------------------------------
    // Release the provider handle. 
    if(hCryptProv)
    {
        if(!(CryptReleaseContext(hCryptProv, 0)))
        {
            MyHandleError(
                TEXT("Error Creating Resources\n"), 
                GetLastError());
        }
    } 

    return fReturn;
}


//-------------------------------------------------------------------
//  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(LPTSTR psz, int nErrorNumber)
{
    _ftprintf(stderr, TEXT("An error occurred in the program. \n"));
    _ftprintf(stderr, TEXT("%s\n"), psz);
    _ftprintf(stderr, TEXT("Error number %x.\n"), nErrorNumber);
}