| Question
: 2 (i)
Program in 8086 assembly
language
(i) to convert an input file to
a code(encrypted) form
(ii) to reconvert this encrypted
file back to original
.model small
.data
;Messages for user
s0 db 0dh,0ah,'PROGRAM FOR FILE
ENCRYPTION AND DECRYPTION ..By
Kailas Jagtap$'
s1 db 0dh,0ah,'Do you want to
ENCRYPT(E) or DECRYPT(D) a file
? : $'
s2 db 0dh,0ah,'Enter Name of
File(to be encrypted): $'
s22 db 0dh,0ah,'Enter Name of
File(which will store encrypted
data): $'
s3 db 0dh,0ah,'Enter Name of
encrypted File(to be decrypted):
$'
s33 db 0dh,0ah,'Enter Name of
File(which will store decrypted
data): $'
s4 db 0dh,0ah,'*****************
ERROR!!! - INVALID INPUT ******************
$'
s44 db 0dh,0ah,'*****************
ERROR IN FILE OPERATION !!!!!!
************ $'
s5 db 0dh,0ah,'$'
s6 db 0dh,0ah,'..............
File ENCRYPTED Successfully !
................. $'
s7 db 0dh,0ah,'..............
File DECRYPTED Successfully !
................. $'
;buffer for file(to be encrypted/decrypted)
name read from kb
fname db 80 ;storage for max.no.of
char to read, say 80
db 0 ;storage for no.of char.actually
read
db 80 dup(0) ;storage for string
read from kb terminated by CR(0dh)
;buffer for file(encrypted/decrypted)
name read from kb
newf1 db 80 ;storage for max.no.of
char to read, say 80
db 0 ;storage for no.of char.actually
read
db 80 dup(0) ;storage for string
read from kb terminated by CR(0dh)
save_ax dw 1 dup(0) ;storage
for count of no.of char. actually
read
buffer db 512 dup(0) ;buffer
for storing chars. read from file
endbuffer db '$' ;$ shows end
of buffer
handle dw ? ;storage for handle
of file(to be encrypted/decrypted)
newhandle dw ? ;storage for handle
of file(encrypted/decrypted)
.code
start:
mov ax,@data ;initialise ds with
data seg. addr.
mov ds,ax
call disp_nl ;move cursor to
new line
mov ah,9 ;display initial msg.
lea dx,s0
int 21h
rpt_disp: call disp_nl ;move
cursor to new line
call disp_nl ;move cursor to
new line
call disp_nl ;move cursor to
new line
mov ah,9 ;display encryp/decryp
choice msg.
lea dx,s1
int 21h
mov ah,1 ;get E/D selection from
kb(user)
int 21h
cmp al,'E' ;if E or e entered,
excute encr.pgm
je encr
cmp al,'e'
je encr
cmp al,'D' ;if D or d entered,
excute decr.pgm
je decr2
cmp al,'d'
je decr2
cmp al,'X' ;if X or x entered,
then terminate the pgm
je stop2
cmp al,'x'
je stop2
;if any other char. entered then
call disp_nl ;go to next line
on screen
call disp_nl ;go to next line
on screen
mov ah,9 ;display invalid i/p
msg.
lea dx,s4
int 21h
jmp rpt_disp ;and loop again
for geting correct i/p char.
;
decr2: jmp decr1 ;used because
jump is going out of range(-127
to 128)
stop2: jmp stop1 ; --"--
;---------------------open an
existing file ------
encr: call disp_nl ;go to next
line on screen
mov ah,9 ;display enter file(to
be encr) name msg.
lea dx,s2
int 21h
lea dx,fname ;get start addr.
of file(to be encr.)name storage
call getname ;get file name from
kb
mov ah,3dh ;open existing file
whose name is in fname
lea dx,fname+2 ;get file name
string start addr.
mov al,0 ;ensure al=0, as it
is used further
int 21h ;after opening, ax contains
file handling info.
mov handle,ax ;save it in handle(buf)
jc err_out2 ;if carry set, go
to display err msg.
;-----------------create a new
file(for storing encrypted data)------
call disp_nl ;go to next line
on screen
mov ah,9 ;display enter file(encr)
name msg.
lea dx,s22
int 21h
lea dx,newf1 ;get start addr.
of file(encr.)name storage
call getname ;get file name from
kb
mov ah,3ch ;create new file whose
name is in newf1
lea dx,newf1+2 ;get file name
string start addr.
mov cx,0 ;ensure cx=0, as it
is used further
int 21h ;after creating, ax contains
file handling info.
mov newhandle,ax ;save it in
newhandle(buf)
jc err_out ;if carry set, go
to display err msg.
;----------------------read the
file & transfer the data to
buffer-------
readnext: lea dx,buffer ;get
start addr.of buffer in which
file data will be transfered
mov cx,512 ;no.of chars.to be
read(at a time) is say 512
mov bx,handle ;get start addr.of
file info. storage
mov ah,3fh ;get function no.
for read
int 21h ;read 512 bytes from
file
jc err_out ;if carry set, go
to display err msg.
;otherwise read is successful,ax=no.of
bytes read
;check for end of file
mov save_ax,ax ;save the count(no.of
bytes read)
cmp ax,0 ;end of file? i.e. no.of
bytes read is 0?
je done1 ;if yes, go to close
both files and come out
;------------------------convert
the buffer in coded(encrypted)
form-----
call currupt ;encrypt the data
in buf
;------------------write buffer
into the new file------------------
lea dx,buffer ;get start addr.
of buf
mov ah,40h ;get function no.
for write
mov bx,newhandle ;get sart addr.of
file(encr) info buf
mov cx,save_ax ;get saved read
count for writing the same no.
of bytes
int 21h ;write
jc err_out ;if write is not successful,
disp.err msg & come out
jmp readnext ;otherwise(successful),
go to read next 512 bytes
;
err_out2: jmp err_out
decr1: jmp decr ;used because
jump is going out of range(-127
to 128)
stop1: jmp stop ; ----"-----
;
done1: lea dx,s6 ;display encryp-successful
msg.
jmp done3
done2: lea dx,s7 ;display decryp-successful
msg.
done3: mov ah,9
int 21h ;display msg
jmp done ;go to close both files
;
stop: mov ah,4ch ;Hand over control
to dos
int 21h
;
getname proc near
mov ah,0ah ;read file name from
kb
int 21h ;after read,dx remains
start addr.of file name buf
mov si,dx ;use si for indexing
first 3 byte locations in buf
mov bl,[si+1] ;get no.of char.read
count(2nd byte)
add si,02 ;point to 3rd byte,where
actual name string starts
sub bh,bh ;clear bh, as used
further
mov BYTE PTR[si+bx],0 ;replace
cr(0dh) at end of string with
0
ret ;to make it useful to other
functions
getname endp
;---------------------------------------------
err_out: mov ah,9 ;display err
msg.
lea dx,s44
int 21h
jmp stop ;come out
;close both files
done: mov ah,3eh ;get function
code for closing file
mov bx,handle ;get start addr.
of file info.(1st file)
int 21h ;close 1st file
mov bx,newhandle ;get start addr.
of file info.(2nd file)
int 21h ;close 2nd file
jmp stop ;come out
;
done4: jmp done2 ;used because
jump is going out of range(-127
to 128)
;------------------------------------------------------------------------------
currupt proc near ;procedure
for encr.(encoding) data
lea si,buffer ;get start addr.of
buf containing bytes to be encrypted
mov cx,save_ax ;get no.of bytes
to be encr.
again: mov al,[si] ;get a byte
from buf
add al,07h ;encr. it
mov [si],al ;restore it at same
location in buf
inc si ;point to next byte in
buf
loop again ;reapeat until all
bytes in buf are encr.
ret
currupt endp
;
err_out1: jmp err_out
;------------------open an existing
currupted file c:\masm51\f1cod.txt------
decr: call disp_nl ;go to next
line on screen
mov ah,9 ;display enter file(to
be decr) name msg.
lea dx,s3
int 21h
lea dx,fname ;read file(to be
decr) name from kb
call getname ;further comments
are same as that for encr.
mov ah,3dh
lea dx,fname+2
mov al,0
int 21h
mov handle,ax
jc err_out
;-----------------create a new
file(for storing decrypted data)
c:\masm61\f1dec.txt------
call disp_nl ;go to next line
on screen
mov ah,9 ;display enter file(decr)
name msg.
lea dx,s33
int 21h
lea dx,newf1 ;read file(encr)
name from kb
call getname ;further comments
are same as that for encr.
mov ah,3ch
lea dx,newf1+2
mov cx,0
int 21h
mov newhandle,ax
jc err_out
;--------------read the file
& transfer the corrupted data
to buffer-------
readnext1: lea dx,buffer ;get
start addr.of buffer in which
file data will be transfered
mov cx,512 ;further comments
are same as that for encr.
mov bx,handle
mov ah,3fh
int 21h
jc err_out1
;check for end of file
mov save_ax,ax ;save the count(no.of
bytes read)
cmp ax,0 ;end of file? i.e. no.of
bytes read is 0?
je done4 ;if yes, go to close
both files and come out
;------------------convert the
buffer in decoded form-----
call decurrupt ;get data decrypted
;------------------write into
the new file------------------
lea dx,buffer ;get start addr.
of buf
mov ah,40h ;further comments
are same as that for encr.
mov bx,newhandle
mov cx,save_ax
int 21h
jc err_out1 ;if write is not
successful, disp.err msg &
come out
jmp readnext1 ;otherwise(successful),
go to read next 512 bytes
;-----------------------------------------------------------------------
decurrupt proc near ;procedure
for decr.(decoding) data
lea si,buffer ;further comments
are same as that for encr.
mov cx,512
again1: mov al,[si]
sub al,07h
mov [si],al
inc si
loop again1
ret
decurrupt endp
disp_nl PROC NEAR ;move cursor
to new line on screen
mov ah,9 ;get function code for
disp. string
lea dx,s5 ;get start addr. of
new line msg string
int 21h ;display string(cr &
lf)
ret
disp_nl ENDP
end start
end
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