Stack Poly-Engine Crypter for Secure String

Introduction

Interesting characteristic (for me!) of the .NET Framework is the possibility to simulate situations that were possible 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 is visible in inverse order to that of the 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 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:

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

Understanding how the stack works in 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 and that it comes dynamically executed from the compiler... then we want to construct a creative... 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 0-3

_value = Asc(Mid(_myStr, ik, 1))
 
Debug.WriteLine(PolyEngineWrite(_valCrypt, _count, _value))
Debug.WriteLine("st.Push(bCrypt)")
Next ik
 
'...end procedure

 
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

Two output examples:

Dim st As New Stack(10) 'lenght string

Dim bCrypt As Integer = 0
 
'options: 0=sub/1=xor/2=add/3=xor

 
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)
 
'...continue**

Dim st As New Stack(10) 'lenght string

Dim bCrypt As Integer = 0
 
'options: 0=sub/1=xor/2=add/3=xor

 
bCrypt = StackDecrypt(bCrypt, 0, &HDF)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H45)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H16)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H3)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H18)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 0, &H37)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H4F)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HFD)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &H0)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 2, &HF9)
st.Push(bCrypt)
bCrypt = StackDecrypt(bCrypt, 1, &H2D)
st.Push(bCrypt)
 
'...continue**

01. 00h  +  21h = 21h (!)
02. 21h xor 45h = 64h (d)
03. 64h  -  F2h = 72h (r)
04. 72h xor 1Dh = 6Fh (o)
05. 6Fh  +  E8h = 57h (W)
06. 57h  +  C9h = 20h (space)
07. 20h  -  B1h = 6Fh (o)
08. 6Fh  -  03h = 6Ch (l)
09. 6Ch  -  00h = 6Ch (l)
10. 6Ch  -  07h = 65h (e)
11. 65h  +  E3h = 48h (H)

01. 00h  -  DFh = 21h (!)
02. 21h xor 45h = 64h (d)
03. 64h xor 16h = 72h (r)
04. 72h  -  03h = 6Fh (o)
05. 6Fh  -  18h = 57h (W)
06. 57h  -  37h = 20h (space)
07. 20h xor 4Fh = 6Fh (o)
08. 6Fh  +  FDh = 6Ch (l)
09. 6Ch  +  00h = 6Ch (l)
10. 6Ch  +  F9h = 65h (e)
11. 65h xor 2Dh = 48h (H)

...the code result is always different!

The bytes comes manipulated from the StackDecrypt procedure and inserted into the stack with PUSH 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 one execution callback!

With these techniques, I try to implement secure code against the decompiler. It must be used in combination with an obfuscator and a crypter. It will be available soon, 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.)