input "Type some text to be encoded, then ENTER. ";tx$

tex$ = Rot13$(tx$)
print tex$
'check
print Rot13$(tex$)

wait

Function Rot13$(t$)
    if t$="" then
        Rot13$=""
        exit function
    end if
    for i = 1 to len(t$)
        c$=mid$(t$,i,1)
        ch$=c$
        if (asc(c$)>=asc("A")) and (asc(c$)<=asc("Z")) then
            ch$=chr$(asc(c$)+13)
            if (asc(ch$)>asc("Z")) then ch$=chr$(asc(ch$)-26)
        end if
        if (asc(c$)>=asc("a")) and (asc(c$)<=asc("z")) then
            ch$=chr$(asc(c$)+13)
            if (asc(ch$)>asc("z")) then ch$=chr$(asc(ch$)-26)
        end if
        rot$=rot$+ch$
    next
    Rot13$=rot$
    end function