local function caesar(s, k, e=true)
    -- Parameters: s (string), k (key)
    -- e (true=encode, false=decode)
    if !e do k=-k end
    local out = ""
    for i = 1, #s do
        local c = s[i]
        if c:islower() then
            out ..= string.char((c:byte() - 97 + k) % 26 + 97)
        elseif c:isupper() then
            out ..= string.char((c:byte() - 65 + k) % 26 + 65)
        else
            out ..= c
        end
    end
    return out
end

print(caesar("The quick brown fox jumps over the lazy dog.", 3))
print(caesar("Wkh txlfn eurzq ira mxpsv ryhu wkh odcb grj.", 3, false))