local board  = "ET AON RISBCDFGHJKLMPQ/UVWXYZ."
local digits = "0123456789"
local rows   = " 26"
local escape = "62"
local key    = "0452"

local function encrypt(message)
    local msg = message:upper():split("")
                               :filtered(|c| -> (c in board or c in digits) and !(c in " /"))
                               :reorder()
    local sb = ""
    for msg as c do
        local idx = board:find(c, 1, true)
        if idx then
            idx -= 1
            local row = idx // 10
            local col = idx % 10
            sb ..= (row == 0) ? $"{col}" : $"{rows[row + 1]}{col}"
        else
            sb ..= $"{escape}{c}"
        end
    end
    local enc = {sb:byte(1, #sb)}
    local i = 0
    for enc as c do
        local k = key[i % 4 + 1]:byte() - 48
        if k != 0 then
            local j = c - 48
            enc[i + 1] = 48 + ((j + k) % 10)
        end
        i += 1
    end

    return enc:map(|b| -> string.char(b)):concat("")
end

local function decrypt(encoded)
    local enc = {encoded:byte(1, #encoded)}
    local i = 0
    for enc as c do
        local k = key[i % 4 + 1]:byte() - 48
        if k != 0 then
            local j = c - 48
            enc[i + 1] = 48 + ((j >= k) ? (j - k) % 10 : (10 + j - k) % 10)
        end
        i += 1
    end
    local len = #enc
    local sb = ""
    i = 0
    while i < len do
        local c = enc[i + 1]
        local idx = rows:find(tostring(c - 48), 1, true)
        if !idx then
            local idx2 = c - 48
            sb ..= board[idx2 + 1]
            i += 1
        elseif $"{c - 48}{enc[i + 2] - 48}" == escape then
            sb ..= tostring(enc[i + 3] - 48)
            i += 3
        else
            local idx2 = (idx - 1) * 10 + enc[i + 2] - 48
            sb ..= board[idx2 + 1]
            i += 2
        end
    end
    return sb
end

local messages = {
    "Attack at dawn",
    "One night-it was on the twentieth of March, 1888-I was returning",
    "In the winter 1965/we were hungry/just barely alive",
    "you have put on 7.5 pounds since I saw you.",
    "The checkerboard cake recipe specifies 3 large eggs and 2.25 cups of flour."
}
for messages as message do
    local encrypted = encrypt(message)
    local decrypted = decrypt(encrypted)
    print($"\nMessage   : {message}")
    print($"Encrypted : {encrypted}")
    print($"Decrypted : {decrypted}")
end