Introduction

Interesting characteristic (for me!) of .NET the Framework, is the possibility to simulate situations that they were possible
only using the language assembler x86. I refer, in particular, to the STACK (sequence of data). The Stack can be considered
(reductively) as an area of "temporary memory" in which the data they are visible in inverse order to just the insertion.

Background

To emulate an other programming language in order "to confuse" the code against the decompilers !

Using the code

In .NET, therefore, class STACK is present. The main methods exposed from the .NET class STACK are following:

• PUSH (Inserts the value in the stack. Equal instruction is present in the assembler language x86);
• POP (Extracts the value from the stack. Equal instruction is present in the assembler language x86);
• PEEK (Law a value from the stack);
• COUNT (Counts the elements on the stack).

example vb.net code:

Dim st As New Stack

st.Push(1) 
st.Push(2) 
st.Push(3) 

Debug.WriteLine(st.Count) 
Debug.WriteLine(st.Peek) 

st.Pop() 

Debug.WriteLine(st.Count) 
Debug.WriteLine(st.Peek) 

st.Pop() 
st.Pop() 

Debug.WriteLine(st.Count) 

Understood as the stack works, for insertion/extraction of the data (*always* in inverse order), we are ready to implement
our algorithm of cryptography. We want to realize it (relatively simple), effective, it turns out to you always random source
code and that it comes dynamically executed from the compiler... then we want to construct a creative...vb.net...
Poly-Engine Crypter for the strings (...ehila! Who has said like the poly-engines present in the virus code? *yes* is the
answer!). In this tutorial I introduce to you like *only* implementing dynamic code using the functions of: sum, subtraction,
xor (for the nostalgic programmers of the assembler language x86: add, sub, xor) of byte.

We imagine of wanting to hide (crypt) the string:

Hello Word! (hex value: 48 65 6C 6C 6F 20 57 6F 72 64 21)

Dim _myStr As String = "Hello Word!"
Dim rand As New Random 
Dim _count As Integer 
Dim _valCrypt As Integer = 0 
Dim _value As Integer 
Dim ik As Integer

Debug.WriteLine("Dim st As New Stack(" & CStr(_myStr.Length - 1) & ")") 
Debug.WriteLine("Dim bCrypt As Integer = 0") 

For ik = _myStr.Length To 1 Step -1 
    _count = rand.Next(0, 3) 
    _value = Asc(Mid(_myStr, ik, 1))
    
    Debug.WriteLine(PolyEngineWrite(_valCrypt, _count, _value))
    Debug.WriteLine("st.Push(bCrypt)")
Next ik

Poly-Engine (core) Crypter:

Private Function PolyEngineWrite(ByRef valCrypt As Integer, _
                    ByVal count As Integer, _
                    ByVal value As Integer) As String
    Dim tempVal As Integer
    
    Select Case count
        
        Case 0
            tempVal = (valCrypt - value)
        
        Case 1, 3
            tempVal = (valCrypt Xor value)
        
        Case 2
            tempVal = (value - valCrypt)
    End Select
    tempVal = tempVal And 255
    valCrypt = value
    Return ("bCrypt = StackDecrypt(bCrypt, " & CStr(count) & ", &H" & Hex(tempVal) & ")")
End Function 

Generated Source Code

...the generated source code is *always* different!

random output (vb.net source code) example:

Dim st As New Stack(10) 
Dim bCrypt As Integer = 0

bCrypt = StackDecrypt(bCrypt, 2, &H21) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H45) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &HF2) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H1D) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HE8) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HC9) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &HB1) 
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H3)  
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H0)  
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H7)  
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HE3) 
st.Push(bCrypt) 

The bytes comes manipulates from the StackDecrypt procedure and inserted into the stack with PUSH class method (sees
introduction):

Private Function StackDecrypt(ByVal bCrypt As Integer, _
                ByVal iOpCode As Integer, _
                ByVal iSalt As Integer) As Integer     
    Select Case iOpCode
        
        Case 0 
            bCrypt = (bCrypt - iSalt) 
        
        Case 1, 3 
            bCrypt = bCrypt Xor iSalt
        
        Case 2 
            bCrypt = (bCrypt + iSalt)
    End Select 
    bCrypt = bCrypt And 255
    Return bCrypt
End Function

**Now it does not remain that "to recompose" the string from the stack. For this last passage we can use (continuation of
the two examples of output):

Dim str As String = ""
Dim ij As Integer
For ij = 1 To st.Count
    str &= Chr(st.Pop)
Next ij

...the final string obtained from the dynamic process of the code decryption is: Hello Word!

Points of Interest

In a future article I will explain as it is possible to generate dynamic code in assembler language x86 and recalling it with
a callback execution!

With these techniques I try to implement secure code against the decompiler. It must be used in combination with a
obfuscator and a crypter. He will be available, to short, my .NET crypter ;-)

For other information please visits my web site  (in continuos modernization) 

History

July 2004: First Public Release (sorry for my bad english...i'm italian)

About Marcello Cantelmo

from: CodeProject.com