Pascal (Lazarus/Delphi)

Dropbox Content Hash

See more Dropbox Examples

Demonstrates how to compute the Dropbox content_hash of a file. (This is the "content_hash" found in the Dropbox FileMetadata object.

Note: This example requires Chilkat v9.5.0.79 or greater because it uses the fac.ReadBlockBd method.

Pascal (Lazarus/Delphi)

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;
  fac: TFileAccess;
  bd: TBinData;
  bdHashes: TBinData;
  blockSize: Integer;
  numBlocks: Integer;
  i: Integer;
  hashHex: string;
  content_hash: string;

begin
  success := False;

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

  crypt := TCrypt2.Create;

  //  To calculate the content_hash of a file:
  //  Split the file into blocks of 4 MB (4,194,304 or 4 * 1024 * 1024 bytes). The last block (if any) may be smaller than 4 MB.
  //  Compute the hash of each block using SHA-256.
  //  Concatenate the hash of all blocks in the binary format to form a single binary string.
  //  Compute the hash of the concatenated string using SHA-256. Output the resulting hash in hexadecimal format.

  crypt.HashAlgorithm := 'sha256';
  crypt.EncodingMode := 'hex_lower';

  //  We're going to calculate the content_hash for the Milky Way JPG image linked here:  https://www.dropbox.com/developers/reference/content-hash

  fac := TFileAccess.Create;
  bd := TBinData.Create;
  bdHashes := TBinData.Create;

  success := fac.OpenForRead('qa_data/jpg/milky-way-nasa.jpg');
  if (success = False) then
    begin
      WriteLn(fac.LastErrorText);
      Exit;
    end;

  blockSize := 4194304;
  numBlocks := fac.GetNumBlocks(blockSize);

  i := 0;
  while (i < numBlocks) do
    begin
      //  Read the next 4MB block into bd.
      bd.Clear();
      success := fac.ReadBlockBd(i,blockSize,bd);

      hashHex := crypt.HashBdENC(bd);
      WriteLn(i + ': ' + hashHex);
      bdHashes.AppendEncoded(hashHex,'hex_lower');

      i := i + 1;
    end;

  fac.FileClose();

  //  Hash the concatenated SHA-256 hashes.
  content_hash := crypt.HashBdENC(bdHashes);
  WriteLn('content_hash = ' + content_hash);


  crypt.Free;
  fac.Free;
  bd.Free;
  bdHashes.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.