Question

I am working on an OSX application using Objective-C, and one of the things I need to do is read text/xml files which were encrypted on a Windows machine using a simple bit-shift algorithm. The encryption code on the Windows side is fairly simple, in Delphi:

const
  EncryptKey : word = ????;
var
  InMS  : TMemoryStream;
  cnt   : Integer;
  c     : byte;
begin
  InMS    := TMemoryStream.Create;
  result  := TMemoryStream.Create;
  try
    InMS.LoadFromFile( FileName );
    InMS.Position := 0;
    for cnt := 0 to InMS.Size - 1 do
      begin
      InMS.Read( c, 1 );
      c := ( c xor not ( ord( EncryptKey shr cnt ) ) );
      result.Write( c, 1 );
      end;
  finally
    InMS.Free;
  end;
end;

The problem is I can’t figure out how to properly read and decrypt this on the Mac side. I’ve tried various approaches to using NSData with no success whatsoever.

Any help or suggestions would be greatly appreciated.

Answer 1

May be this would help you (simple xor encrypting):

-(void) decryptData :(NSMutableData *) data{
    unsigned char *bytes = (unsigned char *)malloc([data length]); 
    unsigned char magic[4] = {(currentCipher >> 24) & 0xff,(currentCipher >> 16) & 0xff,(currentCipher >> 8) & 0xff,(currentCipher) & 0xff};
    [data getBytes:bytes];
    int magic_pointer = 0;
    for (int i = 16; i < [data length]; i++) {
        bytes[i] ^= magic[magic_pointer];        
        if (magic_pointer == 3) magic_pointer = 0; else magic_pointer++;
    }
    free(bytes);
    [data setData:[NSMutableData dataWithBytes : bytes length: [data length] ]];
}

here:
currentCipher is your EcrytpKey, ^ xor. also shift right in C is >>, not operator is !.