Remko Weijnen's Blog (Remko's Blog)

About Virtualization, VDI, SBC, Application Compatibility and anything else I feel like

Archive for May, 2008

Encrypting RDP Passwords in VB.NET

Justin Shepard converted my code to encrypt RPD passwords to VB.NET:

Working with bitfields in Delphi

Recently I needed to convert a C header file to Delphi which contained bitfields. Let’s take a look at a sample structure that contains bitfields:

It means that there is a DWORD (Cardinal) dwValue1 followed by a bitfield with the size of a ULONG (32 bits). In this bitfield 4 values are defined (BitValue1..4) which are used as boolean’s because the value can offcourse be 0 or 1. Since Delphi doesn’t know a bitfield type the question is how to translate it. Usually it would mean that we simply treat the whole bitfield value as a ULONG and extract the required properties by applying a bitmask (shl/shr). Starting from BDS2006 we can define a record with propertes and use getters and setters. Using this technique we can present boolean values to the user:

Code completion shows that the record has one DWORD Value and 4 Boolean Values which is just what we want!

Offcourse we need to implement the Getters and Setters:

We can even add a constructor to it, this can be used to e.g. initialize the record (in the example below we fill with zeroes). Note that only a constructor with at least one argument can be used:

So why not use a class instead of record? The answer is that a class is just a pointer we can never pass this to a function, procedure or api call that expects a record. But if we want to support older Delphi versions, like Delphi 6 or Delphi 7 and even Delphi 2005, which are still used a lot we need to find another solution. I came up with (ab)using sets to emulate bitfields, we can do this because a set is actually a set of bits (limited to 256 bits). The example structure could look like this if we use sets:

We can use normal set operations to get and set bitvalues:

Settings like minimal enum size and record alignment are important because we need to asssure that te record size matches the C structure’s size (especially when using structures with a lot of bitfields. I choose to do this with a litte trick, first I declare some constants:

We use these constants to force the correct size, in the example the bitfield was a ULONG which is 32 bits. We add the al32Bit constant to the bitfield:

So I thought I had it figured out… until I came to this line in the C header file:

So we have a bitfield consisting off multiple bits! This gave me some headaches but I finally came up with the following approach

We need a helper function to retreive the numeric value of ColorDepth:

The helper function is used like this:

Some limitations remain, although I don’t think you are likely to encouter these:

  • A Delphi Set can contain at most 256 values.
  • The ValueFromBitSet function returns an Int64, so values that do not fit in an Int64 cannot be returned.
  • Values in a Set need a unique name.