local logic  = require("bit")
local json   = require("cjson.safe")
local aes    = require("resty.openssl.cipher")
local random = require("resty.random")
local s256   = require("resty.sha256")
local b64    = require("ssso_base64")
local config = require("ssso_config")
local log    = require("ssso_log")
local nginx  = require("ssso_nginx")
local sites  = require("ssso_sites")

local KEY_SIZE = 32 -- 256 bits for AES-256-GCM’s key and SHA-256
local IV_SIZE  = 12 --  96 bits for AES-256-GCM’s IV
local TAG_SIZE = 16 -- 128 bits for AES-256-GCM’s tag

local gcm_aad = random.bytes(8)

-- https://www.rfc-editor.org/rfc/rfc7518.html#section-6.4
local symkey = random.bytes(KEY_SIZE, true) or random.bytes(KEY_SIZE, false)
local keytype = '{"kty":"oct","k":"' .. b64.encode_base64url(symkey) .. '"}'

-- https://en.wikipedia.org/wiki/HMAC
local i_key_pad = ""
local o_key_pad = ""
for c in symkey:gmatch(".") do
  i_key_pad = i_key_pad .. string.char(logic.bxor(54, c:byte()))
  o_key_pad = o_key_pad .. string.char(logic.bxor(92, c:byte()))
end

-- https://www.rfc-editor.org/rfc/rfc7515.html#appendix-A.1
local jose_256_b64 = b64.encode_base64url('{"alg":"HS256"}')

local function encrypt(bytes)
  local iv = random.bytes(IV_SIZE, true) or random.bytes(IV_SIZE, false)
  local gcm = aes.new("aes-256-gcm")
  local crypted = gcm:encrypt(symkey, iv, bytes, false, gcm_aad)
  if not crypted then
    return nil
  end
  local tag = gcm:get_aead_tag()
  return iv .. crypted .. tag
end

local function decrypt(bytes)
  local iv = bytes:sub(1, IV_SIZE)
  local contents = bytes:sub(IV_SIZE + 1, -TAG_SIZE - 1)
  local tag = bytes:sub(-TAG_SIZE)
  local gcm = aes.new("aes-256-gcm")
  local decrypted = gcm:decrypt(symkey, iv, contents, false, gcm_aad, tag)
  return decrypted
end

local function hmac(message)
  local inner = s256:new()
  inner:update(i_key_pad .. message)
  local outer = s256:new()
  outer:update(o_key_pad .. inner:final())
  return outer:final()
end

local function to_jws(jwt)
  local jwt64 = b64.encode_base64url(json.encode(jwt))
  return jose_256_b64 .. "." .. jwt64 .. "." .. b64.encode_base64url(hmac(jose_256_b64 .. jwt64))
end

local function to_jwt(jws)
  local jwslen = #jws
  local dot1, _ = jws:find("%.")
  if not dot1 or dot1 == jwslen then
    return nil
  end
  local dot2, _ = jws:find("%.", dot1 + 1)
  if not dot2 or dot2 == jwslen then
    return nil
  end
  local jose64 = jws:sub(1, dot1 - 1)
  if jose64 ~= jose_256_b64 then
    return nil
  end
  local js64 = jws:sub(dot1 + 1, dot2 - 1)
  local sig = jws:sub(dot2 + 1)
  if sig ~= b64.encode_base64url(hmac(jose64 .. js64)) then
    return nil
  end
  return json.decode(b64.decode_base64url(js64))
end

-- https://www.rfc-editor.org/rfc/rfc7519
-- https://openid.net/specs/openid-connect-core-1_0.html
local function get_jws_and_tslimit(profile)
  local user = profile:user()
  local ser_profile = profile:serialize()
  log.debug("Creating JWS with profile: " .. ser_profile:gsub("([\031\030\029\028\027\026])", function(s) return "[" .. s:byte() .. "]" end))
  local crypted_profile = encrypt(ser_profile)
  if not user or not crypted_profile then
    return nil, nil
  end
  local iat = nginx.get_seconds_since_epoch()
  local exp = iat + config.get_session_seconds()
  local jwt = {
    iss = "https://" .. config.get_sso_host(),
    sub = user,
    aud = user,
    exp = exp,
    iat = iat,
    x_ssso = b64.encode_base64url(crypted_profile),
  }
  return to_jws(jwt), exp
end

local function get_profile_and_new_jws(jws)
  local jwt = to_jwt(jws)
  local iat = nginx.get_seconds_since_epoch()
  if jwt == nil or not jwt["x_ssso"] or not jwt["exp"] or jwt.exp < iat then
    return nil, nil, nil
  end
  local ser_profile = decrypt(b64.decode_base64url(jwt.x_ssso))
  if not ser_profile then
    return nil, nil, nil
  end
  log.debug("Read profile from JWS: " .. ser_profile:gsub("([\031\030\029\028])", function(s) return "[" .. s:byte() .. "]" end))
  local profile = sites.class__profile:deserialize(ser_profile)
  jwt.iat = iat
  jwt.exp = iat + config.get_session_seconds()
  return profile, to_jws(jwt), jwt.exp
end

return {
  get_jws_and_tslimit = get_jws_and_tslimit,
  get_profile_and_new_jws = get_profile_and_new_jws,
}