type    
    PArray = ^TArray;
    TArray = array[0..0] of Byte;

const
    xA = $EC;
    xB = $C3;
    xC = $DA;
...
procedure Crypt(fname : string; Key : Byte);
var
    f : file;
    data : PArray;
    size, i : Integer;
begin
    AssignFile(f, fname);
    Reset(f, 1);
    size := FileSize(f);
    GetMem(data, size);
    BlockRead(f, data[0], size);
    CloseFile(f);
    for i := 0 to size - 1 do
      begin
        data[i] := data[i] xor Key;
        Key := (Key * xA + xB) xor xC;
      end;
    Rewrite(f, 1);
    BlockWrite(f, data[0], size);
    CloseFile(f);
    FreeMem(data);
end;