encyrpt/decrypt problem

I want to encyrpt a buffer into a file and decyrpt a file into a buffer using microsofts encrypt and decrypt functions but I don't know how I would go about doing it. How would I set it up so it reads a block of encrypted/decrypted data and then encrypts/decrypts it then appends it to a buffer or writes it to a file? Here are some examples on msdn how they ecrypt and decrypt files. Encrypting



dwBlockLen = 1000 - 1000 % ENCRYPT_BLOCK_SIZE;

if(ENCRYPT_BLOCK_SIZE > 1) 
    dwBufferLen = dwBlockLen + ENCRYPT_BLOCK_SIZE; 
else 
    dwBufferLen = dwBlockLen; 
    
//-------------------------------------------------------------------
// Allocate memory. 
if(pbBuffer = (BYTE *)malloc(dwBufferLen))
{
    printf("Memory has been allocated for the buffer. \n");
}
else
{ 
    MyHandleError("Out of memory. \n"); 
}
//-------------------------------------------------------------------
// In a do loop, encrypt the source file, 
// and write to the source file. 

do 
{ 

//-------------------------------------------------------------------
// Read up to dwBlockLen bytes from the source file. 
dwCount = fread(pbBuffer, 1, dwBlockLen, hSource); 
if(ferror(hSource))
{ 
    MyHandleError("Error reading plaintext!\n");
}
 
//-------------------------------------------------------------------
// Encrypt data. 
if(!CryptEncrypt(
     hKey, 
     0, 
     feof(hSource), 
     0, 
     pbBuffer, 
     &dwCount, 
     dwBufferLen))
{ 
   MyHandleError("Error during CryptEncrypt. \n"); 
} 

//-------------------------------------------------------------------
// Write data to the destination file. 

fwrite(pbBuffer, 1, dwCount, hDestination); 
if(ferror(hDestination))
{ 
    MyHandleError("Error writing ciphertext.");
}

} 
while(!feof(hSource));

Decrypting



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

//--------------------------------------------------------------------
// Allocate memory. 

if(!(pbBuffer = (BYTE *)malloc(dwBufferLen)))
{
   MyHandleError("Out of memory!\n"); 
}
//--------------------------------------------------------------------
// Decrypt source file, and write to destination file. 

do { 
//--------------------------------------------------------------------
// Read up to dwBlockLen bytes from source file. 

dwCount = fread(
     pbBuffer, 
     1, 
     dwBlockLen, 
     hSource); 
if(ferror(hSource))
{
    MyHandleError("Error reading ciphertext!");
}
//--------------------------------------------------------------------
// Decrypt data. 
if(!CryptDecrypt(
      hKey, 
      0, 
      feof(hSource), 
      0, 
      pbBuffer, 
      &dwCount))
{
   MyHandleError("Error during CryptDecrypt!"); 
}
//--------------------------------------------------------------------
// Write data to destination file. 

fwrite(
    pbBuffer, 
    1, 
    dwCount, 
    hDestination); 
if(ferror(hDestination))
{
   MyHandleError("Error writing plaintext!"); 
}
} 
while(!feof(hSource)); 
status = TRUE;

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