You can code a file then (if you remember your codeing-key!) decode it.

There is a file 'file.cod'. You can code this file, and only that
person can decode it, how has this program and know your codeing-key

                                    ` This code was downloaded from The Game Creators
                                    ` It is reproduced here with full permission
                                    ` http://www.thegamecreators.com
                                    
                                    rem This program (codeing system 0.2) is created by Attila Ihász
rem Sory, my english is not perfect. I am hungaryan.
hide mouse
0:
dim ASCII(size)
for reset=1 to size
ASCII(reset)=0
next reset
undim ASCII(0)

coded$=""
decoded$=""
cls
print "(C)odeing or (D)ecodeing?"
1:
if inkey$()="c" then goto code
if inkey$()="d" then goto decode
goto 1

rem !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! C O D E !!!!!!!!!!!!!!!!!!!!!!
code:
isanyfile=FILE EXIST("file.cod")
size=FILE SIZE("file.cod")
if isanyfile=0
print "No file,please rename the file to 'file.cod'"
goto 0
endif
open to read 1,"file.cod"
read string 1,file$
close file 1
fileis$="This is the file: "+file$
print
print
print "The file is opend,ready to code."
print fileis$
print "Please enter the codeing key."
input key


print
print "codeing..."

dim ASCII(size)
for reading=1 to (size)
ASCII(reading)=ASC(mid$(file$,(reading)))
next reading

rem codeing step 1
for code=1 to (size)
ASCII(code)=ASCII(code)+key
4:
if ASCII(code)>255
ASCII(code)=ASCII(code)-255
goto 4
endif
next code

rem codeing step 2

for code=1 to (size)
ASCII(code)=ASCII(code)+code
5:
if ASCII(code)>255
ASCII(code)=ASCII(code)-255
goto 5
endif
next code

rem codeing step 3

rem codeing step 4

rem codeing step 5

rem codeing step 6

rem restringing
dim car$(size)
for writing=1 to (size)
car$(writing)=chr$(ASCII(writing))
next writing


for write=1 to (size)
coded$=coded$+car$(write)
next write
if file exist("coded.cod") then delete file "coded.cod"
rem make file "coded.cod"
 open to write 2,"coded.cod"
 write string 2,coded$
 close file 2
print
print "The codeing is complite."
print "The file:"
print file$
print
print "The coded file:"
print coded$
wait key
goto 0

rem !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! D E C O D E !!!!!!!!!!!!!!!!!!
decode:
isanyfile=FILE EXIST("coded.cod")
size=(FILE SIZE("coded.cod")-2)
if isanyfile=0
print "No file,please rename the file to 'coded.cod'"
goto 0
endif
open to read 3,"coded.cod"
read string 3,file$
close file 3
fileis$="This is the file: "+file$
print
print
print "The file is opend,ready to decode."
print fileis$
print "Please enter the decoding key."
input key


print
print "Decodeing..."

dim ASCII(size)
for reading=1 to (size)
ASCII(reading)=ASC(mid$(file$,(reading)))
next reading

rem decodeing step 1
for code=1 to (size)
ASCII(code)=ASCII(code)-key
3:
if ASCII(code)<1
ASCII(code)=ASCII(code)+255
goto 3
endif
next code

rem decodeing step 2
for code=1 to (size)
ASCII(code)=ASCII(code)-code
2:
if ASCII(code)<1
ASCII(code)=ASCII(code)+255
goto 2
endif
next code

rem decodeing step 3

rem decodeing step 4

rem decodeing step 5

rem decodeing step 6

rem restringing
dim car$(size)
for writing=1 to (size)
car$(writing)=chr$(ASCII(writing))
next writing


for write=1 to (size)
decoded$=decoded$+car$(write)
next write
if file exist("decoded.cod") then delete file "decoded.cod"
rem make file "decoded.cod"
 open to write 4,"decoded.cod"
 write string 4,decoded$
 close file 4
print
print "The decode is compite."
print "The coded text:"
print file$
print
print "The decoded text:"
print decoded$
wait key
goto 0