Delphi DLL

Decrypt File in Chunks using 256-bit AES

See more Encryption Examples

Shows how to decrypt a file chunk-by-chunk using FirstChunk/LastChunk properties and accumulate the results in memory with a Chilkat BinData object. Delphi DLL

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, FileAccess, BinData, Crypt2;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Boolean;
crypt: HCkCrypt2;
fileToDecrypt: PWideChar;
facIn: HCkFileAccess;
chunkSize: Integer;
numChunks: Integer;
bd: HCkBinData;
bdDecrypted: HCkBinData;
i: Integer;

begin
success := False;

// This example assumes the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// This example decrypts the file previously encrypted in this example:
// Encrypt File in Chunks using AES CBC

crypt := CkCrypt2_Create();

CkCrypt2_putCryptAlgorithm(crypt,'aes');
CkCrypt2_putCipherMode(crypt,'cbc');
CkCrypt2_putKeyLength(crypt,256);

CkCrypt2_SetEncodedKey(crypt,'000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F','hex');
CkCrypt2_SetEncodedIV(crypt,'000102030405060708090A0B0C0D0E0F','hex');

fileToDecrypt := 'c:/temp/qa_output/hamlet.enc';
facIn := CkFileAccess_Create();
success := CkFileAccess_OpenForRead(facIn,fileToDecrypt);
if (success <> True) then
  begin
    Memo1.Lines.Add('Failed to open file to be decrytped.');
    Exit;
  end;

// Let's decrypt in 32000 byte chunks.
chunkSize := 32000;
numChunks := CkFileAccess_GetNumBlocks(facIn,chunkSize);

CkCrypt2_putFirstChunk(crypt,True);
CkCrypt2_putLastChunk(crypt,False);

bd := CkBinData_Create();
bdDecrypted := CkBinData_Create();

i := 0;
while i < numChunks do
  begin
    i := i + 1;
    if (i = numChunks) then
      begin
        CkCrypt2_putLastChunk(crypt,True);
      end;

    // Read the next chunk from the file.
    // The last chunk will be whatever amount remains in the file..
    CkBinData_Clear(bd);
    CkFileAccess_FileReadBd(facIn,chunkSize,bd);

    // Decrypt this chunk.
    CkCrypt2_DecryptBd(crypt,bd);
    // Accumulate the decrypted chunks.
    CkBinData_AppendBd(bdDecrypted,bd);

    CkCrypt2_putFirstChunk(crypt,False);
  end;

// Make sure both FirstChunk and LastChunk are restored to True after
// encrypting or decrypting in chunks.  Otherwise subsequent encryptions/decryptions
// will produce unexpected results.
CkCrypt2_putFirstChunk(crypt,True);
CkCrypt2_putLastChunk(crypt,True);

CkFileAccess_FileClose(facIn);

// The fully decrypted file is contained in bdDecrypted.
// You can save to a file if desired, or use the decrypted data in your application directly from bdDecrypted.
CkBinData_WriteFile(bdDecrypted,'c:/temp/qa_output/hamlet_decrypted.xml');

CkCrypt2_Dispose(crypt);
CkFileAccess_Dispose(facIn);
CkBinData_Dispose(bd);
CkBinData_Dispose(bdDecrypted);

end;