BBC Basic Cipher Help Needed - encryption

Question

I am doing a school project in which I need to make a "sort of" vigenere cipher in which the user inputs both the keyword and plaintext. However the vigenere assumes a=0 whereas I am to assume a=1 and I have changed this accordingly for my program. However I am required to make my cipher work for both lower and upper case, How could I make this also work for lower case, it may be a stupid question but I'm very confused at this point and I'm new to programming, thanks.
REM Variables
plaintext$=""
PRINT "Enter the text you would like to encrypt"
INPUT plaintext$
keyword$=""
PRINT "Enter the keyword you wish to use"
INPUT keyword$
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$
FOR i% = 1 TO LEN(plain$)
Ascii% = ASCMID$(plain$, i%)
IF Ascii% >= 65 IF Ascii% <= 90 THEN
output$ += 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$
FOR i% = 1 TO 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$

Answer 1

You can always convert from upper to lowercase and the Stringlib library contains a function for doing this.
First import stringlib at the top of your program:
import #lib$+"stringlib"
then convert strings using:
plaintext$ = fn_lower(plaintext$)