Lazarus Pascal

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. Lazarus Pascal

program ChilkatDemo;

// Demonstrates using the Chilkat Pascal wrapper via the C bridge DLL.
// Builds as a console application under Lazarus (FPC) or Delphi.

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}
{$APPTYPE CONSOLE}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  SysUtils,
  CkDllLoader,
  Chilkat.FileAccess,
  Chilkat.BinData,
  Chilkat.Crypt2;

// ---------------------------------------------------------------------------

procedure RunDemo;
var
  success: Boolean;
  crypt: TCrypt2;
  fileToDecrypt: string;
  facIn: TFileAccess;
  chunkSize: Integer;
  numChunks: Integer;
  bd: TBinData;
  bdDecrypted: TBinData;
  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 := TCrypt2.Create;

  crypt.CryptAlgorithm := 'aes';
  crypt.CipherMode := 'cbc';
  crypt.KeyLength := 256;

  crypt.SetEncodedKey('000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F','hex');
  crypt.SetEncodedIV('000102030405060708090A0B0C0D0E0F','hex');

  fileToDecrypt := 'c:/temp/qa_output/hamlet.enc';
  facIn := TFileAccess.Create;
  success := facIn.OpenForRead(fileToDecrypt);
  if (success <> True) then
    begin
      WriteLn('Failed to open file to be decrytped.');
      Exit;
    end;

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

  crypt.FirstChunk := True;
  crypt.LastChunk := False;

  bd := TBinData.Create;
  bdDecrypted := TBinData.Create;

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

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

      //  Decrypt this chunk.
      crypt.DecryptBd(bd);
      //  Accumulate the decrypted chunks.
      bdDecrypted.AppendBd(bd);

      crypt.FirstChunk := 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.
  crypt.FirstChunk := True;
  crypt.LastChunk := True;

  facIn.FileClose();

  //  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.
  bdDecrypted.WriteFile('c:/temp/qa_output/hamlet_decrypted.xml');


  crypt.Free;
  facIn.Free;
  bd.Free;
  bdDecrypted.Free;

end;

// ---------------------------------------------------------------------------

begin

  try
    RunDemo;
  except
    on E: Exception do
      WriteLn('Unhandled exception: ', E.ClassName, ': ', E.Message);
  end;

  WriteLn;
  {$IFDEF MSWINDOWS}
  WriteLn('Press Enter to exit...');
  ReadLn;
  {$ENDIF}
end.