openidc.lua

--[[
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or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.

***************************************************************************
Copyright (C) 2017-2023 ZmartZone Holding B.V.
Copyright (C) 2015-2017 Ping Identity Corporation
All rights reserved.

DISCLAIMER OF WARRANTIES:

THE SOFTWARE PROVIDED HEREUNDER IS PROVIDED ON AN "AS IS" BASIS, WITHOUT
ANY WARRANTIES OR REPRESENTATIONS EXPRESS, IMPLIED OR STATUTORY; INCLUDING,
WITHOUT LIMITATION, WARRANTIES OF QUALITY, PERFORMANCE, NONINFRINGEMENT,
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.  NOR ARE THERE ANY
WARRANTIES CREATED BY A COURSE OR DEALING, COURSE OF PERFORMANCE OR TRADE
USAGE.  FURTHERMORE, THERE ARE NO WARRANTIES THAT THE SOFTWARE WILL MEET
YOUR NEEDS OR BE FREE FROM ERRORS, OR THAT THE OPERATION OF THE SOFTWARE
WILL BE UNINTERRUPTED.  IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
EXEMPLARY, OR CONSEQUENTIAL DAMAGES HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

@Author: Hans Zandbelt - hans.zandbelt@zmartzone.eu
--]]

local require = require
local cjson = require("cjson")
local cjson_s = require("cjson.safe")
local http = require("resty.http")
local r_session = require("resty.session")
local string = string
local table = table
local ipairs = ipairs
local pairs = pairs
local type = type
local ngx = ngx
local b64 = ngx.encode_base64
local b64url = require("ngx.base64").encode_base64url
local unb64url = require("ngx.base64").decode_base64url

local log = ngx.log
local DEBUG = ngx.DEBUG
local ERROR = ngx.ERR
local WARN = ngx.WARN

local function token_auth_method_precondition(method, required_field)
  return function(opts)
    if not opts[required_field] then
      log(DEBUG, "Can't use " .. method .. " without opts." .. required_field)
      return false
    end
    return true
  end
end

local supported_token_auth_methods = {
  client_secret_basic = true,
  client_secret_post = true,
  private_key_jwt = token_auth_method_precondition('private_key_jwt', 'client_rsa_private_key'),
  client_secret_jwt = token_auth_method_precondition('client_secret_jwt', 'client_secret')
}

local openidc = {
  _VERSION = "1.8.0"
}
openidc.__index = openidc

local function store_in_session(opts, feature)
  -- We don't have a whitelist of features to enable
  if not opts.session_contents then
    return true
  end

  return opts.session_contents[feature]
end

local function is_session(o)
  return o ~= nil and o.save and type(o.save) == "function"
end

local function is_session_present(session)
  return session ~= nil and next(session:get_data()) ~= nil
end

-- set value in server-wide cache if available
local function openidc_cache_set(type, key, value, exp)
  local dict = ngx.shared[type]
  if dict and (exp > 0) then
    local success, err, forcible = dict:set(key, value, exp)
    log(DEBUG, "cache set: success=", success, " err=", err, " forcible=", forcible)
  end
end

-- retrieve value from server-wide cache if available
local function openidc_cache_get(type, key)
  local dict = ngx.shared[type]
  local value
  if dict then
    value = dict:get(key)
    if value then log(DEBUG, "cache hit: type=", type, " key=", key) end
  end
  return value
end

-- invalidate values of server-wide cache
local function openidc_cache_invalidate(type)
  local dict = ngx.shared[type]
  if dict then
    log(DEBUG, "flushing cache for " .. type)
    dict.flush_all(dict)
    local nbr = dict.flush_expired(dict)
  end
end

-- invalidate all server-wide caches
function openidc.invalidate_caches()
  openidc_cache_invalidate("discovery")
  openidc_cache_invalidate("jwks")
  openidc_cache_invalidate("introspection")
  openidc_cache_invalidate("jwt_verification")
end

-- validate the contents of and id_token
local function openidc_validate_id_token(opts, id_token, nonce)

  -- check issuer
  if opts.discovery.issuer ~= id_token.iss then
    log(ERROR, "issuer \"", id_token.iss, "\" in id_token is not equal to the issuer from the discovery document \"", opts.discovery.issuer, "\"")
    return false
  end

  -- check sub
  if not id_token.sub then
    log(ERROR, "no \"sub\" claim found in id_token")
    return false
  end

  -- check nonce
  if nonce and nonce ~= id_token.nonce then
    log(ERROR, "nonce \"", id_token.nonce, "\" in id_token is not equal to the nonce that was sent in the request \"", nonce, "\"")
    return false
  end

  -- check issued-at timestamp
  if not id_token.iat then
    log(ERROR, "no \"iat\" claim found in id_token")
    return false
  end

  local slack = opts.iat_slack and opts.iat_slack or 120
  if id_token.iat > (ngx.time() + slack) then
    log(ERROR, "id_token not yet valid: id_token.iat=", id_token.iat, ", ngx.time()=", ngx.time(), ", slack=", slack)
    return false
  end

  -- check expiry timestamp
  if not id_token.exp then
    log(ERROR, "no \"exp\" claim found in id_token")
    return false
  end

  if (id_token.exp + slack) < ngx.time() then
    log(ERROR, "token expired: id_token.exp=", id_token.exp, ", ngx.time()=", ngx.time())
    return false
  end

  -- check audience (array or string)
  if not id_token.aud then
    log(ERROR, "no \"aud\" claim found in id_token")
    return false
  end

  if (type(id_token.aud) == "table") then
    for _, value in pairs(id_token.aud) do
      if value == opts.client_id then
        return true
      end
    end
    log(ERROR, "no match found token audience array: client_id=", opts.client_id)
    return false
  elseif (type(id_token.aud) == "string") then
    if id_token.aud ~= opts.client_id then
      log(ERROR, "token audience does not match: id_token.aud=", id_token.aud, ", client_id=", opts.client_id)
      return false
    end
  end
  return true
end

local function get_first(table_or_string)
  local res = table_or_string
  if table_or_string and type(table_or_string) == 'table' then
    res = table_or_string[1]
  end
  return res
end

local function get_first_header(headers, header_name)
  local header = headers[header_name]
  return get_first(header)
end

local function get_first_header_and_strip_whitespace(headers, header_name)
  local header = get_first_header(headers, header_name)
  return header and header:gsub('%s', '')
end

local function trim(s)
  if s then
    return s:match("^%s*(.-)%s*$")
  end
  return s
end

local function get_forwarded_parameter(headers, param_name)
  local forwarded = get_first_header(headers, 'Forwarded')
  local params = {}
  if forwarded then
    local function parse_parameter(pv)
      local name, value = pv:match("^%s*([^=]+)%s*=%s*(.-)%s*$")
      if name and value then
        if value:sub(1, 1) == '"' then
          value = value:sub(2, -2)
        end
        params[name:lower()] = value
      end
    end

    -- this assumes there is no quoted comma inside the header's value
    -- which should be fine as comma is not legal inside a node name,
    -- a URI scheme or a host name. The only thing that might bite us
    -- are extensions.
    local first_part = forwarded
    local first_comma = forwarded:find("%s*,%s*")
    if first_comma then
      first_part = forwarded:sub(1, first_comma - 1)
    end
    first_part:gsub("[^;]+", parse_parameter)
  end
  return params[param_name:gsub("^%s*(.-)%s*$", "%1"):lower()]
end

local function get_scheme(headers)
  return get_forwarded_parameter(headers, 'proto')
      or get_first_header_and_strip_whitespace(headers, 'X-Forwarded-Proto')
      or ngx.var.scheme
end

local function get_host_name_from_x_header(headers)
  local header = get_first_header_and_strip_whitespace(headers, 'X-Forwarded-Host')
  return header and header:gsub('^([^,]+),?.*$', '%1')
end

local function get_host_name(headers)
  return get_forwarded_parameter(headers, 'host')
      or get_host_name_from_x_header(headers)
      or ngx.var.http_host
end

-- assemble the redirect_uri
local function openidc_get_redirect_uri(opts, session)
  local path = opts.redirect_uri_path
  if opts.redirect_uri then
    if opts.redirect_uri:sub(1, 1) == '/' then
      path = opts.redirect_uri
    else
      return opts.redirect_uri
    end
  end
  local headers = ngx.req.get_headers()
  local scheme = opts.redirect_uri_scheme or get_scheme(headers)
  local host = get_host_name(headers)
  if not host then
    -- possibly HTTP 1.0 and no Host header
    if session then session:close() end
    ngx.exit(ngx.HTTP_BAD_REQUEST)
  end
  return scheme .. "://" .. host .. path
end

local function openidc_combine_uri(uri, params)
  if params == nil or next(params) == nil then
    return uri
  end
  local sep = "?"
  if string.find(uri, "?", 1, true) then
    sep = "&"
  end
  return uri .. sep .. ngx.encode_args(params)
end

local function decorate_request(http_request_decorator, req)
  return http_request_decorator and http_request_decorator(req) or req
end

local sha256 = (require 'resty.sha256'):new()
local function openidc_s256(verifier)
  sha256:update(verifier)
  local s256 = b64url(sha256:final())
  sha256:reset()
  return s256
end

-- send the browser of to the OP's authorization endpoint
local function openidc_authorize(opts, session, target_url, prompt)
  local resty_random = require("resty.random")
  local resty_string = require("resty.string")
  local err

  -- generate state and nonce
  local state = resty_string.to_hex(resty_random.bytes(16))
  local nonce = (opts.use_nonce == nil or opts.use_nonce)
    and resty_string.to_hex(resty_random.bytes(16))
  local code_verifier = opts.use_pkce and b64url(resty_random.bytes(32))

  -- assemble the parameters to the authentication request
  local params = {
    client_id = opts.client_id,
    response_type = "code",
    scope = opts.scope and opts.scope or "openid email profile",
    redirect_uri = openidc_get_redirect_uri(opts, session),
    state = state,
  }

  if nonce then
    params.nonce = nonce
  end

  if prompt then
    params.prompt = prompt
  end

  if opts.display then
    params.display = opts.display
  end

  if code_verifier then
    params.code_challenge_method = 'S256'
    params.code_challenge = openidc_s256(code_verifier)
  end

  if opts.response_mode then
    params.response_mode = opts.response_mode
  end

  -- merge any provided extra parameters
  if opts.authorization_params then
    for k, v in pairs(opts.authorization_params) do params[k] = v end
  end

  -- store state in the session
  session:set("original_url", target_url)
  session:set("state", state)
  session:set("nonce", nonce)
  session:set("code_verifier", code_verifier)
  session:set("last_authenticated", ngx.time())

  if opts.lifecycle and opts.lifecycle.on_created then
    err = opts.lifecycle.on_created(session, params)
    if err then
      log(WARN, "failed in `on_created` handler: " .. err)
      return err
    end
  end

  local res
  res, err = session:save()
  if err then
    log(WARN, "unable to save session: " .. err)
  end

  -- redirect to the /authorization endpoint
  ngx.header["Cache-Control"] = "no-cache, no-store, max-age=0"
  return ngx.redirect(openidc_combine_uri(opts.discovery.authorization_endpoint, params))
end

-- parse the JSON result from a call to the OP
local function openidc_parse_json_response(response, ignore_body_on_success)
  local ignore_body_on_success = ignore_body_on_success or false

  local err
  local res

  -- check the response from the OP
  if response.status ~= 200 then
    err = "response indicates failure, status=" .. response.status .. ", body=" .. response.body
  else
    if ignore_body_on_success then
      return nil, nil
    end

    -- decode the response and extract the JSON object
    res = cjson_s.decode(response.body)

    if not res then
      err = "JSON decoding failed"
    end
  end

  return res, err
end

local function openidc_configure_timeouts(httpc, timeout)
  if timeout then
    if type(timeout) == "table" then
      local r, e = httpc:set_timeouts(timeout.connect or 0, timeout.send or 0, timeout.read or 0)
    else
      local r, e = httpc:set_timeout(timeout)
    end
  end
end

-- Set outgoing proxy options
local function openidc_configure_proxy(httpc, proxy_opts)
  if httpc and proxy_opts and type(proxy_opts) == "table" then
    log(DEBUG, "openidc_configure_proxy : use http proxy")
    httpc:set_proxy_options(proxy_opts)
  else
    log(DEBUG, "openidc_configure_proxy : don't use http proxy")
  end
end

-- make a call to the token endpoint
function openidc.call_token_endpoint(opts, endpoint, body, auth, endpoint_name, ignore_body_on_success)
  local ignore_body_on_success = ignore_body_on_success or false

  local ep_name = endpoint_name or 'token'
  if not endpoint then
    return nil, 'no endpoint URI for ' .. ep_name
  end

  local headers = {
    ["Content-Type"] = "application/x-www-form-urlencoded"
  }

  if auth then
    if auth == "client_secret_basic" then
      if opts.client_secret then
        headers.Authorization = "Basic " .. b64(ngx.escape_uri(opts.client_id) .. ":" .. ngx.escape_uri(opts.client_secret))
      else
      -- client_secret must not be set if Windows Integrated Authentication (WIA) is used with
      -- Active Directory Federation Services (AD FS) 4.0 (or newer) on Windows Server 2016 (or newer)
        headers.Authorization = "Basic " .. b64(ngx.escape_uri(opts.client_id) .. ":")
      end
      log(DEBUG, "client_secret_basic: authorization header '" .. headers.Authorization .. "'")

    elseif auth == "client_secret_post" then
      body.client_id = opts.client_id
      if opts.client_secret then
        body.client_secret = opts.client_secret
      end
      log(DEBUG, "client_secret_post: client_id and client_secret being sent in POST body")

    elseif auth == "private_key_jwt" or auth == "client_secret_jwt" then
      local key = auth == "private_key_jwt" and opts.client_rsa_private_key or opts.client_secret
      if not key then
        return nil, "Can't use " .. auth .. " without a key."
      end
      body.client_id = opts.client_id
      body.client_assertion_type = "urn:ietf:params:oauth:client-assertion-type:jwt-bearer"
      local now = ngx.time()
      local assertion = {
        header = {
          typ = "JWT",
          alg = auth == "private_key_jwt" and "RS256" or "HS256",
        },
        payload = {
          iss = opts.client_id,
          sub = opts.client_id,
          aud = endpoint,
          jti = ngx.var.request_id,
          exp = now + (opts.client_jwt_assertion_expires_in and opts.client_jwt_assertion_expires_in or 60),
          iat = now
        }
      }
      if auth == "private_key_jwt" then
        assertion.header.kid = opts.client_rsa_private_key_id
      end

      local r_jwt = require("resty.jwt")
      body.client_assertion = r_jwt:sign(key, assertion)
      log(DEBUG, auth .. ": client_id, client_assertion_type and client_assertion being sent in POST body")
    end
  end

  local pass_cookies = opts.pass_cookies
  if pass_cookies then
    if ngx.req.get_headers()["Cookie"] then
      local t = {}
      for cookie_name in string.gmatch(pass_cookies, "%S+") do
        local cookie_value = ngx.var["cookie_" .. cookie_name]
        if cookie_value then
          table.insert(t, cookie_name .. "=" .. cookie_value)
        end
      end
      headers.Cookie = table.concat(t, "; ")
    end
  end

  log(DEBUG, "request body for " .. ep_name .. " endpoint call: ", ngx.encode_args(body))

  local httpc = http.new()
  openidc_configure_timeouts(httpc, opts.timeout)
  openidc_configure_proxy(httpc, opts.proxy_opts)
  local res, err = httpc:request_uri(endpoint, decorate_request(opts.http_request_decorator, {
    method = "POST",
    body = ngx.encode_args(body),
    headers = headers,
    ssl_verify = (opts.ssl_verify ~= "no"),
    keepalive = (opts.keepalive ~= "no")
  }))
  if not res then
    err = "accessing " .. ep_name .. " endpoint (" .. endpoint .. ") failed: " .. err
    log(ERROR, err)
    return nil, err
  end

  log(DEBUG, ep_name .. " endpoint response: ", res.body)

  return openidc_parse_json_response(res, ignore_body_on_success)
end

-- computes access_token expires_in value (in seconds)
local function openidc_access_token_expires_in(opts, expires_in)
  return (expires_in or opts.access_token_expires_in or 3600) - 1 - (opts.access_token_expires_leeway or 0)
end

local function openidc_load_jwt_none_alg(enc_hdr, enc_payload)
  local header = cjson_s.decode(unb64url(enc_hdr))
  local payload = cjson_s.decode(unb64url(enc_payload))
  if header and payload and header.alg == "none" then
    return {
      raw_header = enc_hdr,
      raw_payload = enc_payload,
      header = header,
      payload = payload,
      signature = ''
    }
  end
  return nil
end

-- get the Discovery metadata from the specified URL
local function openidc_discover(url, ssl_verify, keepalive, timeout, exptime, proxy_opts, http_request_decorator)
  log(DEBUG, "openidc_discover: URL is: " .. url)

  local json, err
  local v = openidc_cache_get("discovery", url)
  if not v then

    log(DEBUG, "discovery data not in cache, making call to discovery endpoint")
    -- make the call to the discovery endpoint
    local httpc = http.new()
    openidc_configure_timeouts(httpc, timeout)
    openidc_configure_proxy(httpc, proxy_opts)
    local res, error = httpc:request_uri(url, decorate_request(http_request_decorator, {
      ssl_verify = (ssl_verify ~= "no"),
      keepalive = (keepalive ~= "no")
    }))
    if not res then
      err = "accessing discovery url (" .. url .. ") failed: " .. error
      log(ERROR, err)
    else
      log(DEBUG, "response data: " .. res.body)
      json, err = openidc_parse_json_response(res)
      if json then
        openidc_cache_set("discovery", url, cjson.encode(json), exptime or 24 * 60 * 60)
      else
        err = "could not decode JSON from Discovery data" .. (err and (": " .. err) or '')
        log(ERROR, err)
      end
    end

  else
    json = cjson.decode(v)
  end

  return json, err
end

-- turn a discovery url set in the opts dictionary into the discovered information
local function openidc_ensure_discovered_data(opts)
  local err
  if type(opts.discovery) == "string" then
    local discovery
    discovery, err = openidc_discover(opts.discovery, opts.ssl_verify, opts.keepalive, opts.timeout, opts.discovery_expires_in, opts.proxy_opts,
                                      opts.http_request_decorator)
    if not err then
      opts.discovery = discovery
    end
  end
  return err
end

-- make a call to the userinfo endpoint
function openidc.call_userinfo_endpoint(opts, access_token)
  local err = openidc_ensure_discovered_data(opts)
  if err then
    return nil, err
  end
  if not (opts and opts.discovery and opts.discovery.userinfo_endpoint) then
    log(DEBUG, "no userinfo endpoint supplied")
    return nil, nil
  end

  local headers = {
    ["Authorization"] = "Bearer " .. access_token,
  }

  log(DEBUG, "authorization header '" .. headers.Authorization .. "'")

  local httpc = http.new()
  openidc_configure_timeouts(httpc, opts.timeout)
  openidc_configure_proxy(httpc, opts.proxy_opts)
  local res, err = httpc:request_uri(opts.discovery.userinfo_endpoint,
                                     decorate_request(opts.http_request_decorator, {
    headers = headers,
    ssl_verify = (opts.ssl_verify ~= "no"),
    keepalive = (opts.keepalive ~= "no")
  }))
  if not res then
    err = "accessing (" .. opts.discovery.userinfo_endpoint .. ") failed: " .. err
    return nil, err
  end

  log(DEBUG, "userinfo response: ", res.body)

  -- handle if the response type is a jwt/signed payload
  local responseType = string.lower(res.headers["Content-Type"])
  if string.find(responseType, "application/jwt") then
    local json, err = openidc.jwt_verify(res.body, opts)
    if err then
      err = "userinfo jwt could not be verified: " .. err
      return nil, err
    end
    return json
  end

  -- parse the response from the user info endpoint
  return openidc_parse_json_response(res)
end

local function can_use_token_auth_method(method, opts)
  local supported = supported_token_auth_methods[method]
  return supported and (type(supported) ~= 'function' or supported(opts))
end

-- get the token endpoint authentication method
local function openidc_get_token_auth_method(opts)

  if opts.token_endpoint_auth_method ~= nil and not can_use_token_auth_method(opts.token_endpoint_auth_method, opts) then
    log(ERROR, "configured value for token_endpoint_auth_method (" .. opts.token_endpoint_auth_method .. ") is not supported, ignoring it")
    opts.token_endpoint_auth_method = nil
  end

  local result
  if opts.discovery.token_endpoint_auth_methods_supported ~= nil then
    -- if set check to make sure the discovery data includes the selected client auth method
    if opts.token_endpoint_auth_method ~= nil then
      for index, value in ipairs(opts.discovery.token_endpoint_auth_methods_supported) do
        log(DEBUG, index .. " => " .. value)
        if value == opts.token_endpoint_auth_method then
          log(DEBUG, "configured value for token_endpoint_auth_method (" .. opts.token_endpoint_auth_method .. ") found in token_endpoint_auth_methods_supported in metadata")
          result = opts.token_endpoint_auth_method
          break
        end
      end
      if result == nil then
        log(ERROR, "configured value for token_endpoint_auth_method (" .. opts.token_endpoint_auth_method .. ") NOT found in token_endpoint_auth_methods_supported in metadata")
        return nil
      end
    else
      for index, value in ipairs(opts.discovery.token_endpoint_auth_methods_supported) do
        log(DEBUG, index .. " => " .. value)
        if can_use_token_auth_method(value, opts) then
          result = value
          log(DEBUG, "no configuration setting for option so select the first supported method specified by the OP: " .. result)
          break
        end
      end
    end
  else
    result = opts.token_endpoint_auth_method
  end

  -- set a sane default if auto-configuration failed
  if result == nil then
    result = "client_secret_basic"
  end

  log(DEBUG, "token_endpoint_auth_method result set to " .. result)

  return result
end

-- ensure that discovery and token auth configuration is available in opts
local function ensure_config(opts)
  local err
  err = openidc_ensure_discovered_data(opts)
  if err then
    return err
  end

  -- set the authentication method for the token endpoint
  opts.token_endpoint_auth_method = openidc_get_token_auth_method(opts)
end

-- query for discovery endpoint data
function openidc.get_discovery_doc(opts)
  local err = openidc_ensure_discovered_data(opts)
  if err then
    log(ERROR, "error getting endpoints definition using discovery endpoint")
  end

  return opts.discovery, err
end

local function openidc_jwks(url, force, ssl_verify, keepalive, timeout, exptime, proxy_opts, http_request_decorator)
  log(DEBUG, "openidc_jwks: URL is: " .. url .. " (force=" .. force .. ") (decorator=" .. (http_request_decorator and type(http_request_decorator) or "nil"))

  local json, err, v

  if force == 0 then
    v = openidc_cache_get("jwks", url)
  end

  if not v then

    log(DEBUG, "cannot use cached JWKS data; making call to jwks endpoint")
    -- make the call to the jwks endpoint
    local httpc = http.new()
    openidc_configure_timeouts(httpc, timeout)
    openidc_configure_proxy(httpc, proxy_opts)
    local res, error = httpc:request_uri(url, decorate_request(http_request_decorator, {
      ssl_verify = (ssl_verify ~= "no"),
      keepalive = (keepalive ~= "no")
    }))
    if not res then
      err = "accessing jwks url (" .. url .. ") failed: " .. error
      log(ERROR, err)
    else
      log(DEBUG, "response data: " .. res.body)
      json, err = openidc_parse_json_response(res)
      if json then
        openidc_cache_set("jwks", url, cjson.encode(json), exptime or 24 * 60 * 60)
      end
    end

  else
    json = cjson.decode(v)
  end

  return json, err
end

local function split_by_chunk(text, chunkSize)
  local s = {}
  for i = 1, #text, chunkSize do
    s[#s + 1] = text:sub(i, i + chunkSize - 1)
  end
  return s
end

local function get_jwk(keys, kid)

  local rsa_keys = {}
  for _, value in pairs(keys) do
    if value.kty == "RSA" and (not value.use or value.use == "sig") then
      table.insert(rsa_keys, value)
    end
  end

  if kid == nil then
    if #rsa_keys == 1 then
      log(DEBUG, "returning only RSA key of JWKS for keyid-less JWT")
      return rsa_keys[1], nil
    else
      return nil, "JWT doesn't specify kid but the keystore contains multiple RSA keys"
    end
  end
  for _, value in pairs(rsa_keys) do
    if value.kid == kid then
      return value, nil
    end
  end

  return nil, "RSA key with id " .. kid .. " not found"
end

local wrap = ('.'):rep(64)

local envelope = "-----BEGIN %s-----\n%s\n-----END %s-----\n"

local function der2pem(data, typ)
  typ = typ:upper() or "CERTIFICATE"
  data = b64(data)
  return string.format(envelope, typ, data:gsub(wrap, '%0\n', (#data - 1) / 64), typ)
end


local function encode_length(length)
  if length < 0x80 then
    return string.char(length)
  elseif length < 0x100 then
    return string.char(0x81, length)
  elseif length < 0x10000 then
    return string.char(0x82, math.floor(length / 0x100), length % 0x100)
  end
  error("Can't encode lengths over 65535")
end


local function encode_sequence(array, of)
  local encoded_array = array
  if of then
    encoded_array = {}
    for i = 1, #array do
      encoded_array[i] = of(array[i])
    end
  end
  encoded_array = table.concat(encoded_array)

  return string.char(0x30) .. encode_length(#encoded_array) .. encoded_array
end

local function encode_binary_integer(bytes)
  if bytes:byte(1) > 127 then
    -- We currenly only use this for unsigned integers,
    -- however since the high bit is set here, it would look
    -- like a negative signed int, so prefix with zeroes
    bytes = "\0" .. bytes
  end
  return "\2" .. encode_length(#bytes) .. bytes
end

local function encode_sequence_of_integer(array)
  return encode_sequence(array, encode_binary_integer)
end

local function encode_bit_string(array)
  local s = "\0" .. array -- first octet holds the number of unused bits
  return "\3" .. encode_length(#s) .. s
end

local function openidc_pem_from_x5c(x5c)
  log(DEBUG, "Found x5c, getting PEM public key from x5c entry of json public key")
  local chunks = split_by_chunk(x5c[1], 64)
  local pem = "-----BEGIN CERTIFICATE-----\n" ..
      table.concat(chunks, "\n") ..
      "\n-----END CERTIFICATE-----"
  log(DEBUG, "Generated PEM key from x5c:", pem)
  return pem
end

local function openidc_pem_from_rsa_n_and_e(n, e)
  log(DEBUG, "getting PEM public key from n and e parameters of json public key")

  local der_key = {
    unb64url(n), unb64url(e)
  }
  local encoded_key = encode_sequence_of_integer(der_key)
  local pem = der2pem(encode_sequence({
    encode_sequence({
      "\6\9\42\134\72\134\247\13\1\1\1" -- OID :rsaEncryption
          .. "\5\0" -- ASN.1 NULL of length 0
    }),
    encode_bit_string(encoded_key)
  }), "PUBLIC KEY")
  log(DEBUG, "Generated pem key from n and e: ", pem)
  return pem
end

local function openidc_pem_from_jwk(opts, kid)
  local err = openidc_ensure_discovered_data(opts)
  if err then
    return nil, err
  end

  if not opts.discovery.jwks_uri or not (type(opts.discovery.jwks_uri) == "string") or (opts.discovery.jwks_uri == "") then
    return nil, "opts.discovery.jwks_uri is not present or not a string"
  end

  local cache_id = opts.discovery.jwks_uri .. '#' .. (kid or '')
  local v = openidc_cache_get("jwks", cache_id)

  if v then
    return v
  end

  local jwk, jwks

  for force = 0, 1 do
    jwks, err = openidc_jwks(opts.discovery.jwks_uri, force, opts.ssl_verify, opts.keepalive, opts.timeout, opts.jwk_expires_in, opts.proxy_opts,
                             opts.http_request_decorator)
    if err then
      return nil, err
    end

    jwk, err = get_jwk(jwks.keys, kid)

    if jwk and not err then
      break
    end
  end

  if err then
    return nil, err
  end

  local x5c = jwk.x5c
  if x5c and type(x5c) ~= 'table' then
    log(WARN, "Found invalid JWK with x5c claim not being an array but a " .. type(x5c))
    x5c = nil
  end
  if x5c and #(jwk.x5c) == 0 then
    log(WARN, "Found invalid JWK with empty x5c array, ignoring x5c claim")
    x5c = nil
  end

  local pem
  if x5c then
    pem = openidc_pem_from_x5c(x5c)
  elseif jwk.kty == "RSA" and jwk.n and jwk.e then
    pem = openidc_pem_from_rsa_n_and_e(jwk.n, jwk.e)
  else
    return nil, "don't know how to create RSA key/cert for " .. cjson.encode(jwk)
  end

  openidc_cache_set("jwks", cache_id, pem, opts.jwk_expires_in or 24 * 60 * 60)
  return pem
end

-- does lua-resty-jwt and/or we know how to handle the algorithm of the JWT?
local function is_algorithm_supported(jwt_header)
  return jwt_header and jwt_header.alg and (jwt_header.alg == "none"
      or string.sub(jwt_header.alg, 1, 2) == "RS"
      or string.sub(jwt_header.alg, 1, 2) == "HS")
end

-- is the JWT signing algorithm an asymmetric one whose key might be
-- obtained from the discovery endpoint?
local function uses_asymmetric_algorithm(jwt_header)
  return string.sub(jwt_header.alg, 1, 2) == "RS"
end

-- is the JWT signing algorithm one that has been expected?
local function is_algorithm_expected(jwt_header, expected_algs)
  if expected_algs == nil or not jwt_header or not jwt_header.alg then
    return true
  end
  if type(expected_algs) == 'string' then
    return expected_algs == jwt_header.alg
  end

  for _, alg in ipairs(expected_algs) do
    if alg == jwt_header.alg then
      return true
    end
  end
  return false
end

-- parse a JWT and verify its signature (if present)
local function openidc_load_jwt_and_verify_crypto(opts, jwt_string, asymmetric_secret,
symmetric_secret, expected_algs, ...)
  local r_jwt = require("resty.jwt")
  local enc_hdr, enc_payload, enc_sign = string.match(jwt_string, '^(.+)%.(.+)%.(.*)$')
  if enc_payload and (not enc_sign or enc_sign == "") then
    local jwt = openidc_load_jwt_none_alg(enc_hdr, enc_payload)
    if jwt then
      if opts.accept_none_alg then
        log(DEBUG, "accept JWT with alg \"none\" and no signature")
        return jwt
      else
        return jwt, "token uses \"none\" alg but accept_none_alg is not enabled"
      end
    end -- otherwise the JWT is invalid and load_jwt produces an error
  end

  local jwt_obj = r_jwt:load_jwt(jwt_string, nil)
  if not jwt_obj.valid then
    local reason = "invalid jwt"
    if jwt_obj.reason then
      reason = reason .. ": " .. jwt_obj.reason
    end
    return nil, reason
  end

  if not is_algorithm_expected(jwt_obj.header, expected_algs) then
    local alg = jwt_obj.header and jwt_obj.header.alg or "no algorithm at all"
    return nil, "token is signed by unexpected algorithm \"" .. alg .. "\""
  end

  local secret
  if is_algorithm_supported(jwt_obj.header) then
    if uses_asymmetric_algorithm(jwt_obj.header) then
      if opts.secret then
        log(WARN, "using deprecated option `opts.secret` for asymmetric key; switch to `opts.public_key` instead")
      end
      secret = asymmetric_secret or opts.secret
      if not secret and opts.discovery then
        log(DEBUG, "using discovery to find key")
        local err
        secret, err = openidc_pem_from_jwk(opts, jwt_obj.header.kid)

        if secret == nil then
          log(ERROR, err)
          return nil, err
        end
      end
    else
      if opts.secret then
        log(WARN, "using deprecated option `opts.secret` for symmetric key; switch to `opts.symmetric_key` instead")
      end
      secret = symmetric_secret or opts.secret
    end
  end

  if #{ ... } == 0 then
    -- an empty list of claim specs makes lua-resty-jwt add default
    -- validators for the exp and nbf claims if they are
    -- present. These validators need to know the configured slack
    -- value
    local jwt_validators = require("resty.jwt-validators")
    jwt_validators.set_system_leeway(opts.iat_slack and opts.iat_slack or 120)
  end

  jwt_obj = r_jwt:verify_jwt_obj(secret, jwt_obj, ...)
  if jwt_obj then
    log(DEBUG, "jwt: ", cjson.encode(jwt_obj), " ,valid: ", jwt_obj.valid, ", verified: ", jwt_obj.verified)
  end
  if not jwt_obj.verified then
    local reason = "jwt signature verification failed"
    if jwt_obj.reason then
      reason = reason .. ": " .. jwt_obj.reason
    end
    return jwt_obj, reason
  end
  return jwt_obj
end

--
-- Load and validate id token from the id_token properties of the token endpoint response
-- Parameters :
--     - opts the openidc module options
--     - jwt_id_token the id_token from the id_token properties of the token endpoint response
--     - session the current session
-- Return the id_token, nil if valid
-- Return nil, the error if invalid
--
local function openidc_load_and_validate_jwt_id_token(opts, jwt_id_token, session)

  local jwt_obj, err = openidc_load_jwt_and_verify_crypto(opts, jwt_id_token, opts.public_key, opts.client_secret,
    opts.discovery.id_token_signing_alg_values_supported)
  if err then
    local alg = (jwt_obj and jwt_obj.header and jwt_obj.header.alg) or ''
    local is_unsupported_signature_error = jwt_obj and not jwt_obj.verified and not is_algorithm_supported(jwt_obj.header)
    if is_unsupported_signature_error then
      if opts.accept_unsupported_alg == nil or opts.accept_unsupported_alg then
        log(WARN, "ignored id_token signature as algorithm '" .. alg .. "' is not supported")
      else
        err = "token is signed using algorithm \"" .. alg .. "\" which is not supported by lua-resty-jwt"
        log(ERROR, err)
        return nil, err
      end
    else
      log(ERROR, "id_token '" .. alg .. "' signature verification failed")
      return nil, err
    end
  end
  local id_token = jwt_obj.payload

  log(DEBUG, "id_token header: ", cjson.encode(jwt_obj.header))
  log(DEBUG, "id_token payload: ", cjson.encode(jwt_obj.payload))

  -- validate the id_token contents
  if openidc_validate_id_token(opts, id_token, session:get("nonce")) == false then
    err = "id_token validation failed"
    log(ERROR, err)
    return nil, err
  end

  return id_token
end

-- handle a "code" authorization response from the OP
local function openidc_authorization_response(opts, session)
  local args, err, log_err, client_err

  if opts.response_mode and opts.response_mode == "form_post" then
    ngx.req.read_body()
    args = ngx.req.get_post_args()
  else
    args = ngx.req.get_uri_args()
  end

  if not args.code or not args.state then
    err = "unhandled request to the redirect_uri: " .. ngx.var.request_uri
    log(ERROR, err)
    return nil, err, session:get("original_url"), session
  end

  -- check that the state returned in the response against the session; prevents CSRF
  local state = session:get("state")
  if args.state ~= state then
    log_err = "state from argument: " .. (args.state and args.state or "nil") .. " does not match state restored from session: " .. (state and state or "nil")
    client_err = "state from argument does not match state restored from session"
    log(ERROR, log_err)
    return nil, client_err, session:get("original_url"), session
  end

  err = ensure_config(opts)
  if err then
    return nil, err, session:get("original_url"), session
  end

  -- check the iss if returned from the OP
  if args.iss and args.iss ~= opts.discovery.issuer then
    log_err = "iss from argument: " .. args.iss .. " does not match expected issuer: " .. opts.discovery.issuer
    client_err = "iss from argument does not match expected issuer"
    log(ERROR, log_err)
    return nil, client_err, session:get("original_url"), session
  end

  -- check the client_id if returned from the OP
  if args.client_id and args.client_id ~= opts.client_id then
    log_err = "client_id from argument: " .. args.client_id .. " does not match expected client_id: " .. opts.client_id
    client_err = "client_id from argument does not match expected client_id"
    log(ERROR, log_err)
    return nil, client_err, session:get("original_url"), session
  end

  -- assemble the parameters to the token endpoint
  local body = {
    grant_type = "authorization_code",
    code = args.code,
    redirect_uri = openidc_get_redirect_uri(opts, session),
    state = state,
    code_verifier = session:get("code_verifier")
  }

  log(DEBUG, "Authentication with OP done -> Calling OP Token Endpoint to obtain tokens")

  local current_time = ngx.time()
  -- make the call to the token endpoint
  local json
  json, err = openidc.call_token_endpoint(opts, opts.discovery.token_endpoint, body, opts.token_endpoint_auth_method)
  if err then
    return nil, err, session:get("original_url"), session
  end

  local id_token, err = openidc_load_and_validate_jwt_id_token(opts, json.id_token, session);
  if err then
    return nil, err, session:get("original_url"), session
  end

  -- mark this sessions as authenticated
  session:set("authenticated", true)
  session:set("nonce", nil)
  session:set("state", nil)
  session:set("code_verifier", nil)

  if store_in_session(opts, 'id_token') then
    session:set("id_token", id_token)
  end

  if store_in_session(opts, 'user') then
    -- call the user info endpoint
    -- TODO: should this error be checked?
    local user
    user, err = openidc.call_userinfo_endpoint(opts, json.access_token)

    if err then
      log(ERROR, "error calling userinfo endpoint: " .. err)
    elseif user then
      if id_token.sub ~= user.sub then
        err = "\"sub\" claim in id_token (\"" .. (id_token.sub or "null") .. "\") is not equal to the \"sub\" claim returned from the userinfo endpoint (\"" .. (user.sub or "null") .. "\")"
        log(ERROR, err)
      else
        session:set("user", user)
      end
    end
  end

  if store_in_session(opts, 'enc_id_token') then
    session:set("enc_id_token", json.id_token)
  end

  if store_in_session(opts, 'access_token') then
    session:set("access_token", json.access_token)
    session:set("access_token_expiration", current_time + openidc_access_token_expires_in(opts, json.expires_in))

    if json.refresh_token ~= nil then
      session:set("refresh_token", json.refresh_token)
    end
  end

  if opts.lifecycle and opts.lifecycle.on_authenticated then
    err = opts.lifecycle.on_authenticated(session, id_token, json)
    if err then
      log(WARN, "failed in `on_authenticated` handler: " .. err)
      return nil, err, session:get("original_url"), session
    end
  end

  -- save the session with the obtained id_token
  local res
  res, err = session:save()
  if err then
    log(WARN, "unable to save session: " .. err)
  end

  -- redirect to the URL that was accessed originally
  local original_url = session:get("original_url")
  log(DEBUG, "OIDC Authorization Code Flow completed -> Redirecting to original URL (" .. original_url .. ")")
  ngx.redirect(original_url)
  return nil, nil, original_url, session
end

-- token revocation (RFC 7009)
local function openidc_revoke_token(opts, token_type_hint, token)
  if not opts.discovery.revocation_endpoint then
    log(DEBUG, "no revocation endpoint supplied. unable to revoke " .. token_type_hint .. ".")
    return nil
  end

  local token_type_hint = token_type_hint or nil
  local body = {
    token = token
  }
  if token_type_hint then
    body['token_type_hint'] = token_type_hint
  end
  local token_type_log = token_type_hint or 'token'

  -- ensure revocation endpoint auth method is properly discovered
  local err = ensure_config(opts)
  if err then
    log(ERROR, "revocation of " .. token_type_log .. " unsuccessful: " .. err)
    return false
  end

  -- call the revocation endpoint
  local _
  _, err = openidc.call_token_endpoint(opts, opts.discovery.revocation_endpoint, body, opts.token_endpoint_auth_method, "revocation", true)
  if err then
    log(ERROR, "revocation of " .. token_type_log .. " unsuccessful: " .. err)
    return false
  else
    log(DEBUG, "revocation of " .. token_type_log .. " successful")
    return true
  end
end

function openidc.revoke_token(opts, token_type_hint, token)
  local err = openidc_ensure_discovered_data(opts)
  if err then
    log(ERROR, "revocation of " .. (token_type_hint or "token (no type specified)") .. " unsuccessful: " .. err)
    return false
  end

  return openidc_revoke_token(opts, token_type_hint, token)
end

function openidc.revoke_tokens(opts, session)
  local err = openidc_ensure_discovered_data(opts)
  if err then
    log(ERROR, "revocation of tokens unsuccessful: " .. err)
    return false
  end

  local access_token = session:get("access_token")
  local refresh_token = session:get("refresh_token")

  local access_token_revoke, refresh_token_revoke
  if refresh_token then
    access_token_revoke = openidc_revoke_token(opts, "refresh_token", refresh_token)
  end
  if access_token then
    refresh_token_revoke = openidc_revoke_token(opts, "access_token", access_token)
  end
  return access_token_revoke and refresh_token_revoke
end

local openidc_transparent_pixel = "\137\080\078\071\013\010\026\010\000\000\000\013\073\072\068\082" ..
    "\000\000\000\001\000\000\000\001\008\004\000\000\000\181\028\012" ..
    "\002\000\000\000\011\073\068\065\084\120\156\099\250\207\000\000" ..
    "\002\007\001\002\154\028\049\113\000\000\000\000\073\069\078\068" ..
    "\174\066\096\130"

local function request_prefers_png_over_html()
  local headers = ngx.req.get_headers()
  local header = get_first(headers['Accept'])
  if not header then return false end

  -- https://httpwg.org/specs/rfc9110.html#field.accept
  local accepted = {}
  local function append_accepted_type(media_range_and_quality)
    local media_range, quality = media_range_and_quality:match("(.+)%s*;%s*q=%s*([^%s]+)")
    if media_range and quality then
      accepted[#accepted + 1] = {media_range=media_range, quality=tonumber(quality)}
    else
      accepted[#accepted + 1] = {media_range=media_range_and_quality, quality=1}
    end
  end
  header:gsub("[^,]+", append_accepted_type)

  table.sort(accepted, function(a1, a2)
      return a1.quality > a2.quality
  end)

  for _, a in ipairs(accepted) do
    if a.media_range:find("text/html") or a.media_range:find("application/xhtml%+xml") then
      return false
    end
    if a.media_range:find("image/png") then
      return true
    end
  end
  return false
end

-- handle logout
local function openidc_logout(opts, session)
  local session_token = session:get("enc_id_token")
  local access_token = session:get("access_token")
  local refresh_token = session:get("refresh_token")
  local err

  if opts.lifecycle and opts.lifecycle.on_logout then
    err = opts.lifecycle.on_logout(session)
    if err then
      log(WARN, "failed in `on_logout` handler: " .. err)
      return err
    end
  end

  if is_session_present(session) then
    session:destroy()
  end

  if opts.revoke_tokens_on_logout then
    log(DEBUG, "revoke_tokens_on_logout is enabled. " ..
      "trying to revoke access and refresh tokens...")
    if refresh_token then
      openidc_revoke_token(opts, "refresh_token", refresh_token)
    end
    if access_token then
      openidc_revoke_token(opts, "access_token", access_token)
    end
  end

  if request_prefers_png_over_html() then
    -- support for Ping Federate's proprietary logout protocol
    ngx.header["Cache-Control"] = "no-cache, no-store"
    ngx.header["Pragma"] = "no-cache"
    ngx.header["P3P"] = "CAO PSA OUR"
    ngx.header["Expires"] = "0"
    ngx.header["X-Frame-Options"] = "DENY"
    ngx.header.content_type = "image/png"
    ngx.print(openidc_transparent_pixel)
    ngx.exit(ngx.OK)
    return
  elseif opts.redirect_after_logout_uri or opts.discovery.end_session_endpoint then
    local uri
    if opts.redirect_after_logout_uri then
      uri = opts.redirect_after_logout_uri
    else
      uri = opts.discovery.end_session_endpoint
    end
    local params = {}
    if (opts.redirect_after_logout_with_id_token_hint or not opts.redirect_after_logout_uri) and session_token then
      params["id_token_hint"] = session_token
    end
    if opts.post_logout_redirect_uri then
      params["post_logout_redirect_uri"] = opts.post_logout_redirect_uri
    end
    return ngx.redirect(openidc_combine_uri(uri, params))
  elseif opts.discovery.ping_end_session_endpoint then
    local params = {}
    if opts.post_logout_redirect_uri then
      params["TargetResource"] = opts.post_logout_redirect_uri
    end
    return ngx.redirect(openidc_combine_uri(opts.discovery.ping_end_session_endpoint, params))
  end

  ngx.header.content_type = "text/html"
  ngx.say("<html><body>Logged Out</body></html>")
  ngx.exit(ngx.OK)
end

-- returns a valid access_token (eventually refreshing the token)
local function openidc_access_token(opts, session, try_to_renew)

  local err

  local access_token = session:get("access_token")
  if access_token == nil then
    return nil, err
  end

  local current_time = ngx.time()
  if current_time < session:get("access_token_expiration") then
    return access_token, err
  end

  if not try_to_renew then
    return nil, "token expired"
  end

  local refresh_token = session:get("refresh_token")
  if refresh_token == nil then
    return nil, "token expired and no refresh token available"
  end

  log(DEBUG, "refreshing expired access_token: ", access_token, " with: ", refresh_token)

  -- retrieve token endpoint URL from discovery endpoint if necessary
  err = ensure_config(opts)
  if err then
    return nil, err
  end

  -- assemble the parameters to the token endpoint
  local body = {
    grant_type = "refresh_token",
    refresh_token = refresh_token,
    scope = opts.scope and opts.scope or "openid email profile"
  }

  local json
  json, err = openidc.call_token_endpoint(opts, opts.discovery.token_endpoint, body, opts.token_endpoint_auth_method)
  if err then
    return nil, err
  end
  local id_token
  if json.id_token then
    id_token, err = openidc_load_and_validate_jwt_id_token(opts, json.id_token, session)
    if err then
      log(ERROR, "invalid id token, discarding tokens returned while refreshing")
      return nil, err
    end
  end
  log(DEBUG, "access_token refreshed: ", json.access_token, " updated refresh_token: ", json.refresh_token)

  session:set("access_token", json.access_token)
  session:set("access_token_expiration", current_time + openidc_access_token_expires_in(opts, json.expires_in))
  if json.refresh_token then
    session:set("refresh_token", json.refresh_token)
  end

  if json.id_token and
      (store_in_session(opts, 'enc_id_token') or store_in_session(opts, 'id_token')) then
    log(DEBUG, "id_token refreshed: ", json.id_token)
    if store_in_session(opts, 'enc_id_token') then
      session:set("enc_id_token", json.id_token)
    end
    if store_in_session(opts, 'id_token') then
      session:set("id_token", id_token)
    end
  end

  if opts.lifecycle and opts.lifecycle.on_regenerated then
    err = opts.lifecycle.on_regenerated(session)
    if err then
      log(WARN, "failed in `on_regenerated` handler: " .. err)
      return nil, err
    end
  end

  -- save the session with the new access_token and optionally the new refresh_token and id_token
  local res
  res, err = session:save()
  if err then
    log(ERROR, "failed to save session: " .. err)
    return nil, err
  end

  return session:get("access_token"), err
end

local function openidc_get_path(uri)
  local without_query = uri:match("(.-)%?") or uri
  return without_query:match(".-//[^/]+(/.*)") or without_query
end

local function openidc_get_redirect_uri_path(opts)
  if opts.local_redirect_uri_path then
    return opts.local_redirect_uri_path
  end
  return opts.redirect_uri and openidc_get_path(opts.redirect_uri) or opts.redirect_uri_path
end

-- main routine for OpenID Connect user authentication
function openidc.authenticate(opts, target_url, unauth_action, session_or_opts)

  if opts.redirect_uri_path then
    log(WARN, "using deprecated option `opts.redirect_uri_path`; switch to using an absolute URI and `opts.redirect_uri` instead")
  end

  local err

  local session
  if is_session(session_or_opts) then
    session = session_or_opts
  else
    local session_error
    session, session_error = r_session.start(session_or_opts)
    if session == nil then
      log(ERROR, "Error starting session: " .. session_error)
      return nil, session_error, target_url, session
    end
  end

  local session_present = is_session_present(session)

  target_url = target_url or ngx.var.request_uri

  local access_token

  -- see if this is a request to the redirect_uri i.e. an authorization response
  local path = openidc_get_path(target_url)
  if path == openidc_get_redirect_uri_path(opts) then
    log(DEBUG, "Redirect URI path (" .. path .. ") is currently navigated -> Processing authorization response coming from OP")

    if not session_present then
      err = "request to the redirect_uri path but there's no session state found"
      log(ERROR, err)
      return nil, err, target_url, session
    end

    return openidc_authorization_response(opts, session)
  end

  -- see if this is a request to logout
  if path == (opts.logout_path or "/logout") then
    log(DEBUG, "Logout path (" .. path .. ") is currently navigated -> Processing local session removal before redirecting to next step of logout process")

    err = ensure_config(opts)
    if err then
      return nil, err, session:get("original_url"), session
    end

    openidc_logout(opts, session)
    return nil, nil, target_url, session
  end

  local token_expired = false
  local try_to_renew = opts.renew_access_token_on_expiry == nil or opts.renew_access_token_on_expiry
  local authenticated = session:get("authenticated")

  if session_present and authenticated
      and store_in_session(opts, 'access_token') then

    -- refresh access_token if necessary
    access_token, err = openidc_access_token(opts, session, try_to_renew)
    if err then
      log(ERROR, "lost access token:" .. err)
      err = nil
    end
    if not access_token then
      token_expired = true
    end
  end

  local id_token = session:get("id_token")

  log(DEBUG,
    "session_present=", session_present,
    ", session.data.id_token=", id_token ~= nil,
    ", session.data.authenticated=", authenticated,
    ", opts.force_reauthorize=", opts.force_reauthorize,
    ", opts.renew_access_token_on_expiry=", opts.renew_access_token_on_expiry,
    ", try_to_renew=", try_to_renew,
    ", token_expired=", token_expired)

  -- if we are not authenticated then redirect to the OP for authentication
  -- the presence of the id_token is check for backwards compatibility
  if not session_present
      or not (id_token or authenticated)
      or opts.force_reauthorize
      or (try_to_renew and token_expired) then
    if unauth_action == "pass" then
      if token_expired then
        session:set("authenticated", false)
        return nil, 'token refresh failed', target_url, session
      end
      return nil, err, target_url, session
    end
    if unauth_action == 'deny' then
      return nil, 'unauthorized request', target_url, session
    end

    err = ensure_config(opts)
    if err then
      return nil, err, session:get("original_url"), session
    end

    log(DEBUG, "Authentication is required - Redirecting to OP Authorization endpoint")
    openidc_authorize(opts, session, target_url, opts.prompt)
    return nil, nil, target_url, session
  end

  -- silently reauthenticate if necessary (mainly used for session refresh/getting updated id_token data)
  if opts.refresh_session_interval ~= nil then
    local last_authenticated = session:get("last_authenticated")
    if last_authenticated == nil or (last_authenticated + opts.refresh_session_interval) < ngx.time() then
      err = ensure_config(opts)
      if err then
        return nil, err, session:get("original_url"), session
      end

      log(DEBUG, "Silent authentication is required - Redirecting to OP Authorization endpoint")
      openidc_authorize(opts, session, target_url, "none")
      return nil, nil, target_url, session
    end
  end

  if store_in_session(opts, 'id_token') then
    -- log id_token contents
    log(DEBUG, "id_token=", cjson.encode(id_token))
  end

  -- return the id_token to the caller Lua script for access control purposes
  return
  {
    id_token = id_token,
    access_token = access_token,
    user = session:get("user")
  },
  err,
  target_url,
  session
end

-- get a valid access_token (eventually refreshing the token), or nil if there's no valid access_token
function openidc.access_token(opts, session_opts)

  local session = r_session.start(session_opts)
  local token, err = openidc_access_token(opts, session, true)
  session:close()
  return token, err
end


-- get an OAuth 2.0 bearer access token from the HTTP request cookies
local function openidc_get_bearer_access_token_from_cookie(opts)

  local err

  log(DEBUG, "getting bearer access token from Cookie")

  local accept_token_as = opts.auth_accept_token_as or "header"
  if accept_token_as:find("cookie") ~= 1 then
    return nil, "openidc_get_bearer_access_token_from_cookie called but auth_accept_token_as wants "
        .. opts.auth_accept_token_as
  end
  local divider = accept_token_as:find(':')
  local cookie_name = divider and accept_token_as:sub(divider + 1) or "PA.global"

  log(DEBUG, "bearer access token from cookie named: " .. cookie_name)

  local cookies = ngx.req.get_headers()["Cookie"]
  if not cookies then
    err = "no Cookie header found"
    log(ERROR, err)
    return nil, err
  end

  local cookie_value = ngx.var["cookie_" .. cookie_name]
  if not cookie_value then
    err = "no Cookie " .. cookie_name .. " found"
    log(ERROR, err)
  end

  return cookie_value, err
end


-- get an OAuth 2.0 bearer access token from the HTTP request
local function openidc_get_bearer_access_token(opts)

  local err

  local accept_token_as = opts.auth_accept_token_as or "header"

  if accept_token_as:find("cookie") == 1 then
    return openidc_get_bearer_access_token_from_cookie(opts)
  end

  -- get the access token from the Authorization header
  local headers = ngx.req.get_headers()
  local header_name = opts.auth_accept_token_as_header_name or "Authorization"
  local header = trim(get_first(headers[header_name]))

  if header == nil then
    err = "no Authorization header found"
    log(ERROR, err)
    return nil, err
  end

  local divider = header:find(' ')
  if divider == nil or divider == 0 or string.lower(header:sub(0, divider - 1)) ~= string.lower("Bearer") then
    err = "no Bearer authorization header value found"
    log(ERROR, err)
    return nil, err
  end

  local access_token = header:sub(divider + 1)
  if access_token == "" then
    err = "no Bearer access token value found"
    log(ERROR, err)
    return nil, err
  end

  return access_token, err
end

local function get_introspection_endpoint(opts)
  local introspection_endpoint = opts.introspection_endpoint
  if not introspection_endpoint then
    local err = openidc_ensure_discovered_data(opts)
    if err then
      return nil, "opts.introspection_endpoint not said and " .. err
    end
    local endpoint = opts.discovery and opts.discovery.introspection_endpoint
    if endpoint then
      return endpoint
    end
  end
  return introspection_endpoint
end

local function get_introspection_cache_prefix(opts)
  return (opts.cache_segment and opts.cache_segment:gsub(',', '_') or 'DEFAULT') .. ','
    .. (get_introspection_endpoint(opts) or 'nil-endpoint') .. ','
    .. (opts.client_id or 'no-client_id') .. ','
    .. (opts.client_secret and 'secret' or 'no-client_secret') .. ':'
end

local function get_cached_introspection(opts, access_token)
  local introspection_cache_ignore = opts.introspection_cache_ignore or false
  if not introspection_cache_ignore then
    return openidc_cache_get("introspection",
                             get_introspection_cache_prefix(opts) .. access_token)
  end
end

local function set_cached_introspection(opts, access_token, encoded_json, ttl)
  local introspection_cache_ignore = opts.introspection_cache_ignore or false
  if not introspection_cache_ignore then
    openidc_cache_set("introspection",
                      get_introspection_cache_prefix(opts) .. access_token,
                      encoded_json, ttl)
  end
end

-- main routine for OAuth 2.0 token introspection
function openidc.introspect(opts)

  -- get the access token from the request
  local access_token, err = openidc_get_bearer_access_token(opts)
  if access_token == nil then
    return nil, err
  end

  -- see if we've previously cached the introspection result for this access token
  local json
  local v = get_cached_introspection(opts, access_token)

  if v then
    json = cjson.decode(v)

    if not json or not json.active then
        err = "invalid cached token"
    end

    return json, err
  end

  -- assemble the parameters to the introspection (token) endpoint
  local token_param_name = opts.introspection_token_param_name and opts.introspection_token_param_name or "token"

  local body = {}

  body[token_param_name] = access_token

  if opts.client_id then
    body.client_id = opts.client_id
  end
  if opts.client_secret then
    body.client_secret = opts.client_secret
  end

  -- merge any provided extra parameters
  if opts.introspection_params then
    for key, val in pairs(opts.introspection_params) do body[key] = val end
  end

  -- call the introspection endpoint
  local introspection_endpoint
  introspection_endpoint, err = get_introspection_endpoint(opts)
  if err then
    return nil, err
  end
  json, err = openidc.call_token_endpoint(opts, introspection_endpoint, body, opts.introspection_endpoint_auth_method, "introspection")

  if not json then
    return json, err
  end

  -- check if negative cache should be in use
  local introspection_enable_negative_cache = opts.introspection_enable_negative_cache or false
  if not json.active and not introspection_enable_negative_cache then
    err = "invalid token"
    return json, err
  end

  -- cache the results
  local introspection_cache_ignore = opts.introspection_cache_ignore or false
  local expiry_claim = opts.introspection_expiry_claim or "exp"


  if not introspection_cache_ignore and json[expiry_claim] then
    local introspection_interval = opts.introspection_interval or 0
    local ttl = json[expiry_claim]
    if expiry_claim == "exp" then --https://tools.ietf.org/html/rfc7662#section-2.2
      ttl = ttl - ngx.time()
    end
    if introspection_interval > 0 then
      if ttl > introspection_interval then
        ttl = introspection_interval
      end
    end
    log(DEBUG, "cache token ttl: " .. ttl)
    set_cached_introspection(opts, access_token, cjson.encode(json), ttl)
  end

  if not json.active then
    err = "invalid token"
  end

  return json, err

end

local function get_jwt_verification_cache_prefix(opts)
  local signing_alg_values_expected = (opts.accept_none_alg and 'none' or 'no-none')
  local expected_algs = opts.token_signing_alg_values_expected or {}
  if type(expected_algs) == 'string' then
    expected_algs = { expected_algs }
  end
  for _, alg in ipairs(expected_algs) do
    signing_alg_values_expected = signing_alg_values_expected .. ',' .. alg
  end
  return (opts.cache_segment and opts.cache_segment:gsub(',', '_') or 'DEFAULT') .. ','
    .. (opts.public_key or 'no-pubkey') .. ','
    .. (opts.symmetric_key or 'no-symkey') .. ','
    .. signing_alg_values_expected .. ':'
end

local function get_cached_jwt_verification(opts, access_token)
  local jwt_verification_cache_ignore = opts.jwt_verification_cache_ignore or false
  if not jwt_verification_cache_ignore then
    return openidc_cache_get("jwt_verification",
                             get_jwt_verification_cache_prefix(opts) .. access_token)
  end
end

local function set_cached_jwt_verification(opts, access_token, encoded_json, ttl)
  local jwt_verification_cache_ignore = opts.jwt_verification_cache_ignore or false
  if not jwt_verification_cache_ignore then
    openidc_cache_set("jwt_verification",
                      get_jwt_verification_cache_prefix(opts) .. access_token,
                      encoded_json, ttl)
  end
end

-- main routine for OAuth 2.0 JWT token validation
-- optional args are claim specs, see jwt-validators in resty.jwt
function openidc.jwt_verify(access_token, opts, ...)
  local err
  local json
  local v = get_cached_jwt_verification(opts, access_token)

  local slack = opts.iat_slack and opts.iat_slack or 120
  if not v then
    local jwt_obj
    jwt_obj, err = openidc_load_jwt_and_verify_crypto(opts, access_token, opts.public_key, opts.symmetric_key,
      opts.token_signing_alg_values_expected, ...)
    if not err then
      json = jwt_obj.payload
      local encoded_json = cjson.encode(json)
      log(DEBUG, "jwt: ", encoded_json)

      set_cached_jwt_verification(opts, access_token, encoded_json,
                                  json.exp and json.exp - ngx.time() or 120)
    end

  else
    -- decode from the cache
    json = cjson.decode(v)
  end

  -- check the token expiry
  if json then
    if json.exp and json.exp + slack < ngx.time() then
      log(ERROR, "token expired: json.exp=", json.exp, ", ngx.time()=", ngx.time())
      err = "JWT expired"
    end
  end

  return json, err
end

function openidc.bearer_jwt_verify(opts, ...)
  local json

  -- get the access token from the request
  local access_token, err = openidc_get_bearer_access_token(opts)
  if access_token == nil then
    return nil, err
  end

  log(DEBUG, "access_token: ", access_token)

  json, err = openidc.jwt_verify(access_token, opts, ...)
  return json, err, access_token
end

-- Passing nil to any of the arguments resets the configuration to default
function openidc.set_logging(new_log, new_levels)
  log = new_log and new_log or ngx.log
  DEBUG = new_levels.DEBUG and new_levels.DEBUG or ngx.DEBUG
  ERROR = new_levels.ERROR and new_levels.ERROR or ngx.ERR
  WARN = new_levels.WARN and new_levels.WARN or ngx.WARN
end

return openidc