Delphi ActiveX

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 ActiveX

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

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Integer;
crypt: TChilkatCrypt2;
fileToDecrypt: WideString;
facIn: TCkFileAccess;
chunkSize: Integer;
numChunks: Integer;
bd: TChilkatBinData;
bdDecrypted: TChilkatBinData;
i: Integer;

begin
success := 0;

// 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 := TChilkatCrypt2.Create(Self);

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 := TCkFileAccess.Create(Self);
success := facIn.OpenForRead(fileToDecrypt);
if (success <> 1) then
  begin
    Memo1.Lines.Add('Failed to open file to be decrytped.');
    Exit;
  end;

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

crypt.FirstChunk := 1;
crypt.LastChunk := 0;

bd := TChilkatBinData.Create(Self);
bdDecrypted := TChilkatBinData.Create(Self);

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

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

    crypt.FirstChunk := 0;
  end;

// Make sure both FirstChunk and LastChunk are restored to 1 after
// encrypting or decrypting in chunks.  Otherwise subsequent encryptions/decryptions
// will produce unexpected results.
crypt.FirstChunk := 1;
crypt.LastChunk := 1;

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');
end;