a little demo i made..
sync on : sync rate 0
input "Secret word? ",magicword$
open to read 1,"c:\windows\desktop\mine\darkedit\darkedit.exe"
open to write 2,"c:\windows\desktop\mine\darkedit\darkedit_exe.enc"
wordpointer=1
time=timer()
while file end(1)=0
read byte 1,a
a=a-asc(mid$(magicword$,wordpointer))
if a<0 then a=a+256
write byte 2,a
inc wordpointer
if wordpointer>len(magicword$) then wordpointer=1
endwhile
time=timer()-time
print "time taken: ",time
wait key
close file 1
close file 2
it's not too hard to understand, and to reverse it, change the lines:
a=a-asc(mid$(magicword$,wordpointer))
if a<0 then a=a+256
to
a=a+asc(mid$(magicword$,wordpointer))
if a>255 then a=a-256

it's not the fastest routine but it can encrypt a 3MB file in, oh, 20 seconds or so.
i'm looking at yooooou!