Poly-Engine Crypt String

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) 'insert 1 into the stack

st.Push(2) 'insert 2...

st.Push(3) 'insert 3...

'+

Debug.WriteLine(st.Count) 'return: 3

Debug.WriteLine(st.Peek) 'return last value onto the stack: 3

'+

st.Pop() 'extract value 3

'+

Debug.WriteLine(st.Count) 'return: 2

Debug.WriteLine(st.Peek) 'return last value onto the stack: 2

'+

st.Pop() 'extract value 2

st.Pop() 'extract value 1

'+

Debug.WriteLine(st.Count) 'return 0

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 'used for randomize operations on the byte

Dim _count As Integer '0=sub/1=xor/2=add/3=xor ...only your creativity :-)))

Dim _valCrypt As Integer = 0 'temporary value

Dim _value As Integer 'single char value

Dim ik As Integer
'<<<

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

'<<<

For ik = _myStr.Length To 1 Step -1 'from last char (stack inverse order)

    _count = rand.Next(0, 3) 'random value

    _value = Asc(Mid(_myStr, ik, 1))
    '+ generation

    Debug.WriteLine(PolyEngineWrite(_valCrypt, _count, _value))
    Debug.WriteLine("st.Push(bCrypt)")
Next ik
'...end procedure poly-generation

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
        'SUB

        Case 0
            tempVal = (valCrypt - value)
        'XOR

        Case 1, 3
            tempVal = (valCrypt Xor value)
        'ADD

        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) 'lenght string - 1

Dim bCrypt As Integer = 0
'<<<

bCrypt = StackDecrypt(bCrypt, 2, &H21) '01. 00h  +  21h = 21h (!)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H45) '02. 21h Xor 45h = 64h (d)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &HF2) '03. 64h  -  F2h = 72h (r)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H1D) '04. 72h Xor 1Dh = 6Fh (o)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HE8) '05. 6Fh  +  E8h = 57h (W)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HC9) '06. 57h  +  C9h = 20h (space)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &HB1) '07. 20h  -  B1h = 6Fh (o)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H3)  '08. 6Fh  -  03h = 6Ch (l)

st.Push(b)
bCrypt = StackDecrypt(bCrypt, 0, &H0)  '09. 6Ch  -  00h = 6Ch (l)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H7)  '0A. 6Ch  -  07h = 65h (e)

st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HE3) '0B. 65h  +  E3h = 48h (H)

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
        'SUB 

        Case 0 
            bCrypt = (bCrypt - iSalt) 
        'XOR 

        Case 1, 3 
            bCrypt = bCrypt Xor iSalt
        'ADD 

        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)'it composes final string

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)