Delphi ActiveX

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.

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;
fac: TCkFileAccess;
bd: TChilkatBinData;
bdHashes: TChilkatBinData;
blockSize: Integer;
numBlocks: Integer;
i: Integer;
hashHex: WideString;
content_hash: WideString;

begin
success := 0;

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

crypt := TChilkatCrypt2.Create(Self);

// 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 := TCkFileAccess.Create(Self);
bd := TChilkatBinData.Create(Self);
bdHashes := TChilkatBinData.Create(Self);

success := fac.OpenForRead('qa_data/jpg/milky-way-nasa.jpg');
if (success = 0) then
  begin
    Memo1.Lines.Add(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.ControlInterface);

    hashHex := crypt.HashBdENC(bd.ControlInterface);
    Memo1.Lines.Add(IntToStr(i) + ': ' + hashHex);
    bdHashes.AppendEncoded(hashHex,'hex_lower');

    i := i + 1;
  end;

fac.FileClose();

// Hash the concatenated SHA-256 hashes.
content_hash := crypt.HashBdENC(bdHashes.ControlInterface);
Memo1.Lines.Add('content_hash = ' + content_hash);
end;