(Delphi DLL) Dropbox Content Hash

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.

For more information, see https://www.dropbox.com/developers/reference/content-hash

Chilkat for Delphi Downloads Chilkat non-ActiveX DLL for Delphi Chilkat ActiveX DLL for Delphi * The examples here use the non-ActiveX 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
crypt: HCkCrypt2;
fac: HCkFileAccess;
bd: HCkBinData;
bdHashes: HCkBinData;
success: Boolean;
blockSize: Integer;
numBlocks: Integer;
i: Integer;
hashHex: PWideChar;
content_hash: PWideChar;

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

crypt := CkCrypt2_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.

CkCrypt2_putHashAlgorithm(crypt,'sha256');
CkCrypt2_putEncodingMode(crypt,'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 := CkFileAccess_Create();
bd := CkBinData_Create();
bdHashes := CkBinData_Create();

success := CkFileAccess_OpenForRead(fac,'qa_data/jpg/milky-way-nasa.jpg');
if (success = False) then
  begin
    Memo1.Lines.Add(CkFileAccess__lastErrorText(fac));
    Exit;
  end;

blockSize := 4194304;
numBlocks := CkFileAccess_GetNumBlocks(fac,blockSize);

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

    hashHex := CkCrypt2__hashBdENC(crypt,bd);
    Memo1.Lines.Add(IntToStr(i) + ': ' + hashHex);
    CkBinData_AppendEncoded(bdHashes,hashHex,'hex_lower');

    i := i + 1;
  end;

CkFileAccess_FileClose(fac);

// Hash the concatenated SHA-256 hashes.
content_hash := CkCrypt2__hashBdENC(crypt,bdHashes);
Memo1.Lines.Add('content_hash = ' + content_hash);

CkCrypt2_Dispose(crypt);
CkFileAccess_Dispose(fac);
CkBinData_Dispose(bd);
CkBinData_Dispose(bdHashes);

end;