CONST lc$ = "abcdefghijklmnopqrstuvwxyz"
CONST uc$ = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

CONST txt$ = "The quick brown fox jumps over the lazy dog."

FUNCTION Ceasar$(t$, k)

    lk$ = MID$(lc$ & lc$, k+1, 26)
    uk$ = MID$(uc$ & uc$, k+1, 26)

    RETURN REPLACE$(t$, lc$ & uc$, lk$ & uk$, 2)

END FUNCTION

tokey = RANDOM(25)+1
PRINT "Encrypting text with key: ", tokey

en$ = Ceasar$(txt$, tokey)

PRINT "Encrypted: ", en$
PRINT "Decrypted: ", Ceasar$(en$, 26-tokey)