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Assembly Source File | 1991-06-24 | 12.9 KB | 270 lines

;**************************************************************************** ; ENCRYPT encrypts a data file using a variation on the Vernam cipher ; technique, which XORs each character of a message with each character ; of a password. Its syntax is: ; ; ENCRYPT [d:][path]filename "password" ; ; where "file1" is the name of the file to be encrypted and "password" is ; the password used to encrypt the file. The password must be enclosed in ; quotation marks and may be as long as the command line permits. ; ; Files are unencrypted by passing them through ENCRYPT again with the ; same password. If a password is forgotten, there is no way to recover ; the original file short of manually breaking the code. ;**************************************************************************** code segment assume cs:code,ds:code org 100h begin: jmp main helpmsg db "Encrypts a file for data security.",13,10,13,10 db "ENCRYPT [d:][path]filename ",22h,"password",22h db 13,10,13,10 db " filename Name of the file to be encrypted",13,10 db " password Password used for encryption",13,10 db 13,10 db "To unencrypt a file, run it through ENCRYPT again " db "with the same password.",13,10,"$" errmsg1 db "Syntax: ENCRYPT [d:][path]filename " db 22h,"password",22h,13,10,"$" errmsg2 db "File not found or path invalid",13,10,"$" errmsg3 db "File could not be opened for reading",13,10,"$" errmsg4 db "File could not be opened for writing",13,10,"$" errmsg5 db "File read error",13,10,"$" errmsg6 db "File write error",13,10,"$" errmsg7 db "Disk full error",13,10,"$" errmsg8 db "Not enough memory",13,10,"$" filename dw ? ;Address of file name r_handle dw ? ;File handle for reading w_handle dw ? ;File handle for writing password dw ? ;Password address count dw ? ;Password length blocksize dw ? ;Block size for file I/O bytesread dw ? ;Bytes read from input file ;**************************************************************************** ; Procedure MAIN ;**************************************************************************** main proc near cld ;Clear direction flag mov si,81h ;Point SI to command line call scanhelp ;Scan for "/?" switch jnc checkmem ;Branch if not found mov ah,09h ;Display help text and exit mov dx,offset helpmsg ; with ERRORLEVEL=0 int 21h mov ax,4C00h int 21h checkmem: mov dx,offset errmsg8 ;Make sure there's enough cmp sp,0A000h ; memory to run the ja parse ; program error: mov ah,09h ;Display error message and int 21h ; exit with ERRORLEVEL=1 mov ax,4C01h int 21h ; ; Parse the command line. ; parse: call findchar ;Find first parameter jnc parse2 ;Branch if found error1: mov dx,offset errmsg1 ;Error if not jmp error parse2: cmp byte ptr [si],22h ;Error if quotation mark je error1 mov filename,si ;Save address call finddelim ;Find end of string jc error1 ;Error if end of line mov byte ptr [si],0 ;Convert to ASCIIZ inc si ;Advance SI call findchar ;Find next parameter jc error1 ;Error if end of line cmp byte ptr [si],22h ;Error if NOT quotation jne error1 ; mark this time inc si ;Skip quotation mark mov password,si ;Save address sub cx,cx ;Initialize counter readnext: lodsb ;Find the closing quotation cmp al,0Dh ; mark and exit on error je error1 ; if end of line is cmp al,22h ; encountered first je quotefound inc cx jmp readnext quotefound: jcxz error1 ;Error if length is 0 mov count,cx ;Save password length ; ; Open the file with separate handles for reading and writing. ; mov ax,3D00h ;Open the file for reading mov dx,filename ; using DOS function 3DH int 21h mov r_handle,ax ;Save file handle jnc open ;Continue if no error mov dx,offset errmsg3 ;Determine what the error cmp ax,2 ; was and point DX to the jb goto_error ; appropriate error cmp ax,3 ; message ja goto_error mov dx,offset errmsg2 goto_error: jmp error ;Exit on error open: mov ax,3D01h ;Open the file for writing mov dx,filename ; using DOS function 3Dh int 21h mov w_handle,ax ;Save file handle mov dx,offset errmsg4 ;Branch on error jc error ; ; Read the file, encrypt it, and write it back out to disk. ; mov ax,8000h ;Compute block size for sub dx,dx ; file I/O div count ;Quotient in AX mul count ;Product in AX (<8000H) mov blocksize,ax ;Save block size readblock: mov ah,3Fh ;Read one block using mov bx,r_handle ; DOS function 3FH mov cx,blocksize mov dx,offset buffer int 21h mov dx,offset errmsg5 jc goto_error ;Error if call failed jcxz exit ;Done if zero bytes read mov bytesread,ax ;Save bytes read count call encrypt ;Encrypt the block mov ah,40h ;Write one block using mov bx,w_handle ; DOS function 40H mov cx,bytesread mov dx,offset buffer int 21h mov dx,offset errmsg6 jc goto_error ;Error if call failed cmp ax,bytesread ;Proceed if bytes written je check ; equals bytes read mov dx,offset errmsg7 ;Queue up "Disk full" jmp error ; message and exit check: mov ax,bytesread ;Loop back for more if cmp ax,blocksize ; bytes read is less jnb readblock ; than bytes requested ; ; Close file handles and exit. ; exit: mov ah,3Eh ;Close the read handle mov bx,r_handle int 21h mov ah,3Eh ;Close the write handle mov bx,w_handle int 21h mov ax,4C00h ;Exit with ERRORLEVEL=0 int 21 main endp ;**************************************************************************** ; FINDCHAR advances SI to the next non-space or non-comma character. ; On return, carry set indicates EOL was encountered. ;**************************************************************************** findchar proc near lodsb ;Get the next character cmp al,20h ;Loop if space je findchar cmp al,2Ch ;Loop if comma je findchar dec si ;Point SI to the character cmp al,0Dh ;Exit with carry set if end je eol ; of line is reached clc ;Clear carry and exit ret eol: stc ;Set carry and exit ret findchar endp ;**************************************************************************** ; FINDDELIM advances SI to the next space or comma character. On return, ; carry set indicates EOL was encountered. ;**************************************************************************** finddelim proc near lodsb ;Get the next character cmp al,20h ;Exit if space je fd_exit cmp al,2Ch ;Exit if comma je fd_exit cmp al,0Dh ;Loop back for more if end jne finddelim ; of line isn't reached dec si ;Set carry and exit stc ret fd_exit: dec si ;Clear carry and exit clc ret finddelim endp ;**************************************************************************** ; SCANHELP scans the command line for a /? switch. If found, carry returns ; set and SI contains its offset. If not found, carry returns clear. ;**************************************************************************** scanhelp proc near push si ;Save SI scanloop: lodsb ;Get a character cmp al,0Dh ;Exit if end of line je scan_exit cmp al,"?" ;Loop if not "?" jne scanloop cmp byte ptr [si-2],"/" ;Loop if not "/" jne scanloop add sp,2 ;Clear the stack sub si,2 ;Adjust SI stc ;Set carry and exit ret scan_exit: pop si ;Restore SI clc ;Clear carry and exit ret scanhelp endp ;**************************************************************************** ; ENCRYPT encrypts a block of data using an XOR algorithm. ;**************************************************************************** encrypt proc near mov di,offset buffer ;Point DI to buffer mov si,password ;Point SI to password mov cx,bytesread ;Initialize CX with count enloop: lodsb ;Get password character cmp al,22h ;Branch if not a quotation jne notquote ; mark mov si,password ;Reset BX to start of lodsb ; start of password notquote: xor [di],al ;Encrypt the character inc di ;Advance buffer pointer loop enloop ;Loop until done ret encrypt endp buffer = $ ;File I/O buffer code ends end begin