require "bignum"
require "table2"

local pt = "Rosetta Code"
print($"Plain text:            : {pt}")
local n = bigint.new("9516311845790656153499716760847001433441357")
local e = bigint.new("65537")
local d = bigint.new("5617843187844953170308463622230283376298685")
local ptn = bigint.zero
-- Convert plain text to a number.
for {pt:byte(1, #pt)} as b do
    ptn = bigint.bor(bigint.shl(ptn, 8), bigint.new(b))
end
if ptn >= n then
    print("Plain text message too long")
    os.exit(1)
end
print($"Plain text as a number : {ptn}")

-- Encode a single number.
local etn = bigint.modpow(ptn, e, n)
print($"Encoded                : {etn}")

-- Decode a single number.
local dtn = bigint.modpow(etn, d, n)
print($"Decoded                : {dtn}")

-- Convert number to text.
local db = table.rep(16, 0)
local dx = 16
local bff = bigint.new(255)
while dtn:bitlength() > 0 do
    db[dx] = bigint.toplain(bigint.band(dtn, bff))
    dx -= 1
    dtn = bigint.shr(dtn, 8)
end
local s = ""
for i = dx + 1, 16 do s ..= string.char(db[i]) end
print($"Decoded number as text : {s}")