Introduction
Interesting characteristic (for me!) of the .NET framework, is the possibility to simulate situations that 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 are visible in the inverse order to insertion.
Background
To emulate another 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 the 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
(Returns last 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 simply), effectively. It turns out to your always getting random source code and that it comes dynamically executed from the compiler... We want to construct a creative...VB.NET app...
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 *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(b)
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 come manipulated from the StackDecrypt
procedure and inserted into the stack with PUSH
class method (see 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. They will be available, to short, my .NET crypter ;-)
For other information please visit my web site (in continuous modernization).
History
July 2004: First public release. (Sorry for my bad English...I'm Italian)
President of "Cantelmo Software" (micro-ISV situated in Lizzanello (Lecce) - Italy): Development Software and Professional Component for .NET Platform. Author of Goliath .NET Obfuscator