encryption - BBC Basic Cipher Help Needed -

i doing school project in need make "sort of" vigenere cipher in user inputs both keyword , plaintext. vigenere assumes a=0 whereas assume a=1 , have changed accordingly program. required make cipher work both lower , upper case, how make work lower case, may stupid question i'm confused @ point , i'm new programming, thanks.

  rem variables   plaintext$=""   print "enter text encrypt"   input plaintext$   keyword$=""   print "enter keyword wish use"   input keyword$   encrypted$= fnencrypt(plaintext$, keyword$)     rem printing outputs   print "key = " keyword$   print "plaintext  = " plaintext$   print "encrypted = " encrypted$   print "decrypted  = " fndecrypt(encrypted$, keyword$)   end    def fnencrypt(plain$, keyword$)   local i%, offset%, ascii%, output$   i% = 1 len(plain$)     ascii% = ascmid$(plain$, i%)     if ascii% >= 65 if ascii% <= 90     output$ += chr$((66 + (ascii% + ascmid$(keyword$, offset%+1)) mod 26))     endif     offset% = (offset% + 1) mod len(keyword$)   next   = output$    def fndecrypt(encrypted$, keyword$)   local i%, offset%, n%, o$   i% = 1 len(encrypted$)     n% = ascmid$(encrypted$, i%)     o$ += chr$(64 + (n% + 26 - ascmid$(keyword$, offset%+1)) mod 26)     offset% = (offset% + 1) mod len(keyword$)   next   = output$ 

you can convert upper lowercase , stringlib library contains function doing this.

first import stringlib @ top of program:

import @lib$+"stringlib" 

then convert strings using:

plaintext$ = fn_lower(plaintext$)