;ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
;                   Black Wolf's File Protection Utilities 2.1s
;
;EN_COM - Decryption Code for COM file encryption protection in EncrCOM.
;         If modified, convert to data bytes and re-instate program into
;         EncrCOM.ASM, then recompile EncrCOM.
;      
;         Basically, this code is attached to a .COM file and, when executed,
;         decrypts the .COM file and continues execution.
;
;LISCENSE:
;    Released As Freeware - These files may be distributed freely.
;
;Any modifications made to this program should be listed below the solid line,
;along with the name of the programmer and the date the file was changed.
;Also - they should be commented where changed.
;
;NOTE THAT MODIFICATION PRIVILEDGES APPLY ONLY TO THIS VERSION (2.1s)!  
;I'd appreciate notification of any modifications if at all possible, 
;reach me through the address listed in the documentation file (bwfpu21s.doc).
;
;DISCLAIMER:  The author takes ABSOLUTELY NO RESPONSIBILITY for any damages
;resulting from the use/misuse of this program/file.  The user agrees to hold
;the author harmless for any consequences that may occur directly or 
;indirectly from the use of this program by utilizing this program/file
;in any manner.
;ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
;Modifications:
;       None as of 08/05/93 - Initial Release.

.model tiny
.radix 16
.code
        org 100
start:
        call    Get_Offset
Displaced:
        lea     si,[Decrypt_COM+bp]  
        lea     di,[Get_Offset+bp]   
        mov     cx,(Get_Offset-Decrypt_COM)/2
        cld
        jmp     short EncrLoop      ;EncrLoop here isn't really much of an
        db      0ea                 ;encryption, but disassemblers should
                                    ;love it - it flips the code in memory.

EncrLoop:                           ;This part must be stepped through and
        lodsb                       ;re-saved before using the resulting 
        dec     di                  ;.COM as data for EncrCOM, otherwise
        mov     ah,[di]             ;it will simply flip the memory image
        mov     [si-1],ah           ;of the program and crash when run...
        mov     [di],al
        loop    EncrLoop
        cmp     cs:[Counter+bp],4       ;Check if last Anti-debug trick was
        jb      Yip                     ;skipped, if so, mess up user....
        
        sub     word ptr [bp+Decrypt_COM+1],100 ;another prefetch trick...
Decrypt_COM:
        mov     si,100
        mov     di,si
        mov     cx,bp
        shr     cx,1
        inc     cx

Decrypt_Loop:                   ;This could be LOTS better....
        lodsw                   ;Suggestions on the algorithm?
        xor     ax,[Key1+bp]
        add     ax,[Key2+bp]
        ror     ax,1
        xor     ax,[Key3+bp]
        sub     ax,[Key4+bp]
        rol     ax,1
        stosw
        loop    Decrypt_Loop

        mov     byte ptr [bp+RestoreFile],0c3
RestoreFile:
        mov     di,100
        push    di
        lea     si,bp+Storage_Bytes      ;Restore control to COM.
        movsw
        movsw

        mov     ax,[Key1+bp]
        xor     ds:[100h],ax
        mov     ax,[Key2+bp]
        xor     ds:[102h],ax

        xor     ax,ax
        call    KillInt3
        mov     si,ax
        mov     di,ax
        jmp     RestoreFile


Yip:                            ;If you end up here... you messed up.
        push    ax bx cx        
        mov     ax,ss
        mov     bx,sp           ;reverse words in stack - probably end up
        push    ax              ;somewhere in BIOS ROM....
        xchg    bx,ax
        mov     ss,ax
        mov     sp,bx
        pop     ax
        pop     cx bx ax
        mov     byte ptr cs:[Thinker+bp],0c3    ;move a return w/prefetch
Thinker:
        db      0ea,0,0,0ff,0ff ;if they skipped prefetch... reboot cold.

;------------------------------------------------------------------------
Get_Offset:
        pop     bp
        jmp     short confuzzled
        db      0ea
confuzzled:
        push    bp
        sub     bp,offset Displaced
        call    killInt3
        ret

Killint3:                       ;Xor first 4 interrupts with 0d00dh...
        push    ax es ds        ;not nice on debuggers if traced....
        xor     ax,ax
        mov     ds,ax
        mov     es,ax
        xor     si,si
        xor     di,di
        mov     cx,8

killints:        
        lodsw
        xor     ax,0d00dh
        stosw
        inc     word ptr cs:[Counter+bp]
        loop    killints

        pop     ds es ax
        ret

Counter dw      0
;------------------------------------------------------------------------
Storage_Bytes   db      90,90,0cdh,20
Key1            dw      0
Key2            dw      0
Key3            dw      0
Key4            dw      0
;------------------------------------------------------------------------
end_prog:
end start