# frozen-string-literal: true

require 'openssl'

begin
  OpenSSL::Cipher.new("aes-256-ctr")
rescue OpenSSL::Cipher::CipherError
  # :nocov:
  raise LoadError, "Roda sessions plugin requires the aes-256-ctr cipher"
  # :nocov:
end

require 'base64'
require 'json'
require 'securerandom'
require 'zlib'

class Roda
  module RodaPlugins
    # The sessions plugin adds support for sessions using cookies. It is the recommended
    # way to support sessions in Roda applications.
    #
    # The session cookies are encrypted with AES-256-CTR and then signed with HMAC-SHA-256.
    # By default, session data over a certain size is compressed to reduced space, and
    # is padded to reduce information leaked based on the session size.
    #
    # Sessions are serialized via JSON, so session information should only store data that
    # allows roundtrips via JSON (String, Integer, Float, Array, Hash, true, false, and nil).
    # In particular, note that Symbol does not round trip via JSON, so symbols should not be
    # used in sessions when this plugin is used.  This plugin sets the
    # +:sessions_convert_symbols+ application option to +true+ if it hasn't been set yet,
    # for better integration with plugins that can use either symbol or string session or 
    # flash keys.  Unlike Rack::Session::Cookie, the session is stored as a plain ruby hash,
    # and does not convert all keys to strings.
    #
    # All sessions are timestamped and session expiration is enabled by default, with sessions
    # being valid for 30 days maximum and 7 days since last use.  Session creation time is
    # reset whenever the session is empty when serialized and also whenever +clear_session+
    # is called while processing the request.
    #
    # Session secrets can be rotated, and if so both the cipher and HMAC secrets should be
    # rotated at the same time.  See options below.
    #
    # The sessions plugin can transparently upgrade sessions from Rack::Session::Cookie
    # if the default Rack::Session::Cookie coder and HMAC are used, see options below.
    # It is recommended to only enable transparent upgrades for a brief transition period,
    # and remove support for them once old sessions have converted or timed out.
    #
    # While session data will be compressed by default for sessions over a certain size,
    # if the final cookie is too large (>=4096 bytes), a Roda::RodaPlugins::Sessions::CookieTooLarge
    # exception will be raised.
    #
    # If the flash plugin is used, the sessions plugin should be loaded after the flash
    # plugin, so that the flash plugin rotates the flash in the session before the sessions
    # plugin serializes the session.
    #
    # = Required Options
    #
    # The session cookies this plugin uses are both encrypted and signed, so two separate
    # secrets are used internally.  However, for ease of use, these secrets are combined into
    # a single +:secret+ option.  The +:secret+ option must be a string of at least 64 bytes
    # and should be randomly generated.  The first 32 bytes are used as the secret for the
    # cipher, any remaining bytes are used for the secret for the HMAC.
    #
    # = Other Options
    #
    # :cookie_options :: Any cookie options to set on the session cookie. By default, uses
    #                    <tt>httponly: true, path: '/', same_site: :lax</tt> so that the cookie is not accessible
    #                    to javascript, allowed for all paths, and will not be used for cross-site non-GET requests
    #                    that.  If the +:secure+ option is not present in the hash, then
    #                    <tt>secure: true</tt> is also set if the request is made over HTTPS.  If this option is
    #                    given, it will be merged into the default cookie options.
    # :gzip_over :: For session data over this many bytes, compress it with the deflate algorithm (default: 128).
    # :key :: The cookie name to use (default: <tt>'roda.session'</tt>)
    # :max_seconds :: The maximum number of seconds to allow for total session lifetime, starting with when
    #                 the session was originally created.  Default is <tt>86400*30</tt> (30 days). Can be set to
    #                 +nil+ to disable session lifetime checks.
    # :max_idle_sessions :: The maximum number of seconds to allow since the session was last updated.
    #                       Default is <tt>86400*7</tt> (7 days).  Can be set to nil to disable session idleness
    #                       checks.
    # :old_secret :: The previous secret to use, allowing for secret rotation.  Must be a string of at least 64
    #                bytes if given.
    # :pad_size :: Pad session data (after possible compression, before encryption), to a multiple of this
    #              many bytes (default: 32).  This can be between 2-4096 bytes, or +nil+ to disable padding.
    # :parser :: The parser for the serialized session data (default: <tt>JSON.method(:parse)</tt>).
    # :serializer :: The serializer for the session data (default +:to_json.to_proc+).
    # :skip_within :: If the last update time for the session cookie is less than this number of seconds from the
    #                 current time, and the session has not been modified, do not set a new session cookie
    #                 (default: 3600).
    # :upgrade_from_rack_session_cookie_key :: The cookie name to use for transparently upgrading from
    #                                          Rack::Session:Cookie (defaults to <tt>'rack.session'</tt>).
    # :upgrade_from_rack_session_cookie_secret :: The secret for the HMAC-SHA1 signature when allowing
    #                                             transparent upgrades from Rack::Session::Cookie. Using this
    #                                             option is only recommended during a short transition period,
    #                                             and is not enabled by default as it lowers security.
    # :upgrade_from_rack_session_cookie_options :: Options to pass when deleting the cookie used by
    #                                              Rack::Session::Cookie after converting it to use the session
    #                                              cookies used by this plugin.
    #
    # = Not a Rack Middleware
    # 
    # Unlike some other approaches to sessions, the sessions plugin does not use
    # a rack middleware, so session information is not available to other rack middleware,
    # only to the application itself, with the session not being loaded from the cookie
    # until the +session+ method is called.
    #
    # If you need rack middleware to access the session information, then
    # <tt>require 'roda/session_middleware'</tt> and <tt>use RodaSessionMiddleware</tt>.
    # <tt>RodaSessionMiddleware</tt> passes the options given to this plugin.
    #
    # = Session Cookie Cryptography/Format
    #
    # Session cookies created by this plugin use the following format:
    #
    #   urlsafe_base64(version + IV + auth tag + encrypted session data + HMAC)
    #
    # where:
    #
    # version :: 1 byte, currently must be 0, other values reserved for future expansion.
    # IV :: 16 bytes, initialization vector for AES-256-CTR cipher.
    # encrypted session data :: >=12 bytes of data encrypted with AES-256-CTR cipher, see below.
    # HMAC :: 32 bytes, HMAC-SHA-256 of all preceding data plus cookie key (so that a cookie value
    #         for a different key cannot be used even if the secret is the same).
    #  
    # The encrypted session data uses the following format:
    #
    #   bitmap + creation time + update time + padding + serialized data
    #
    # where:
    # 
    # bitmap :: 2 bytes in little endian format, lower 12 bits storing number of padding
    #           bytes, 13th bit storing whether serialized data is compressed with deflate.
    #           Bits 14-16 reserved for future expansion.
    # creation time :: 4 byte integer in unsigned little endian format, storing unix timestamp
    #                  since session initially created.
    # update time :: 4 byte integer in unsigned little endian format, storing unix timestamp
    #                since session last updated.
    # padding :: >=0 padding bytes specified in bitmap, filled with random data, can be ignored.
    # serialized data :: >=2 bytes of serialized data in JSON format.  If the bitmap indicates
    #                    deflate compression, this contains the deflate compressed data.
    module Sessions
      DEFAULT_COOKIE_OPTIONS = {:httponly=>true, :path=>'/'.freeze, :same_site=>:lax}.freeze
      DEFAULT_OPTIONS = {:key => 'roda.session'.freeze, :max_seconds=>86400*30, :max_idle_seconds=>86400*7, :pad_size=>32, :gzip_over=>128, :skip_within=>3600}.freeze
      DEFLATE_BIT  = 0x1000
      PADDING_MASK = 0x0fff
      SESSION_CREATED_AT = 'roda.session.created_at'.freeze
      SESSION_UPDATED_AT = 'roda.session.updated_at'.freeze
      SESSION_SERIALIZED = 'roda.session.serialized'.freeze
      SESSION_DELETE_RACK_COOKIE = 'roda.session.delete_rack_session_cookie'.freeze

      # Exception class used when creating a session cookie that would exceed the
      # allowable cookie size limit.
      class CookieTooLarge < RodaError
      end

      # Split given secret into a cipher secret and an hmac secret.
      def self.split_secret(name, secret)
        raise RodaError, "sessions plugin :#{name} option must be a String" unless secret.is_a?(String)
        raise RodaError, "invalid sessions plugin :#{name} option length: #{secret.bytesize}, must be >=64" unless secret.bytesize >= 64
        hmac_secret = secret = secret.dup.force_encoding('BINARY')
        cipher_secret = secret.slice!(0, 32)
        [cipher_secret.freeze, hmac_secret.freeze]
      end

      # Configure the plugin, see Sessions for details on options.
      def self.configure(app, opts=OPTS)
        plugin_opts = opts
        opts = (app.opts[:sessions] || DEFAULT_OPTIONS).merge(opts)
        co = opts[:cookie_options] = DEFAULT_COOKIE_OPTIONS.merge(opts[:cookie_options] || OPTS).freeze
        opts[:parser] ||= app.opts[:json_parser] || JSON.method(:parse)
        opts[:serializer] ||= app.opts[:json_serializer] || :to_json.to_proc

        if opts[:upgrade_from_rack_session_cookie_secret]
          opts[:upgrade_from_rack_session_cookie_key] ||= 'rack.session'
          rsco = opts[:upgrade_from_rack_session_cookie_options] = Hash[opts[:upgrade_from_rack_session_cookie_options] || OPTS]
          rsco[:path] ||= co[:path]
          rsco[:domain] ||= co[:domain]
        end

        opts[:cipher_secret], opts[:hmac_secret] = split_secret(:secret, opts[:secret])
        opts[:old_cipher_secret], opts[:old_hmac_secret] = (split_secret(:old_secret, opts[:old_secret]) if opts[:old_secret])

        case opts[:pad_size]
        when nil
          # no changes
        when Integer
          raise RodaError, "invalid :pad_size: #{opts[:pad_size]}, must be >=2, < 4096" unless opts[:pad_size] >= 2 && opts[:pad_size] < 4096
        else
          raise RodaError, "invalid :pad_size option: #{opts[:pad_size].inspect}, must be Integer or nil"
        end
        
        app.opts[:sessions] = opts.freeze
        app.opts[:sessions_convert_symbols] = true unless app.opts.has_key?(:sessions_convert_symbols)
      end

      module InstanceMethods
        # If session information has been set in the request environment,
        # update the rack response headers to set the session cookie in
        # the response.
        def call
          res = super

          if session = env['rack.session']
            @_request.persist_session(res[1], session)
          end

          res
        end

        # Clear data from the session, and update the request environment
        # so that the session cookie will use a new creation timestamp
        # instead of the previous creation timestamp.
        def clear_session
          session.clear
          env.delete(SESSION_CREATED_AT)
          env.delete(SESSION_UPDATED_AT)
          nil
        end
      end

      module RequestMethods
        # Load the session information from the cookie.  With the sessions
        # plugin, you must call this method to get the session, instead of
        # trying to access the session directly through the request environment.
        # For maximum compatibility with other software that uses rack sessions,
        # this method stores the session in 'rack.session' in the request environment,
        # but that does not happen until this method is called.
        def session
          @env['rack.session'] ||= _load_session
        end

        # Persist the session data as a cookie.  If transparently upgrading from
        # Rack::Session::Cookie, mark the related cookie for expiration so it isn't
        # sent in the future.
        def persist_session(headers, session)
          opts = roda_class.opts[:sessions]

          if session.empty?
            if env[SESSION_SERIALIZED]
              # If session was submitted and is now empty, remove the cookie
              Rack::Utils.delete_cookie_header!(headers, opts[:key])
            # else
              # If no session was submitted, and the session is empty
              # then there is no need to do anything
            end
          elsif cookie_value = _serialize_session(session)
            cookie = Hash[opts[:cookie_options]]
            cookie[:value] = cookie_value
            cookie[:secure] = true if !cookie.has_key?(:secure) && ssl?
            Rack::Utils.set_cookie_header!(headers, opts[:key], cookie)
          end
          
          if env[SESSION_DELETE_RACK_COOKIE]
            Rack::Utils.delete_cookie_header!(headers, opts[:upgrade_from_rack_session_cookie_key], opts[:upgrade_from_rack_session_cookie_options])
          end

          nil
        end

        private

        # Load the session by looking for the appropriate cookie, or falling
        # back to the rack session cookie if configured.
        def _load_session
          opts = roda_class.opts[:sessions]
          cs = cookies

          if data = cs[opts[:key]]
            _deserialize_session(data)
          elsif (key = opts[:upgrade_from_rack_session_cookie_key]) && (data = cs[key])
            _deserialize_rack_session(data)
          end || {}
        end

        # If 'rack.errors' is set, write the error message to it.
        # This is used for errors that shouldn't be raised as exceptions,
        # such as improper session cookies.
        def _session_serialization_error(msg)
          return unless error_stream = @env['rack.errors']
          error_stream.puts(msg)
          nil
        end

        # Interpret given cookie data as a Rack::Session::Cookie
        # serialized session using the default Rack::Session::Cookie
        # hmac and coder.
        def _deserialize_rack_session(data)
          opts = roda_class.opts[:sessions]
          key = opts[:upgrade_from_rack_session_cookie_key]
          secret = opts[:upgrade_from_rack_session_cookie_secret]
          data, digest = data.split("--", 2)
          unless digest
            return _session_serialization_error("Not decoding Rack::Session::Cookie session: invalid format")
          end
          unless Rack::Utils.secure_compare(digest, OpenSSL::HMAC.hexdigest(OpenSSL::Digest::SHA1.new, opts[:upgrade_from_rack_session_cookie_secret], data))
            return _session_serialization_error("Not decoding Rack::Session::Cookie session: HMAC invalid")
          end

          begin
            session = Marshal.load(data.unpack('m').first)
          rescue
            return _session_serialization_error("Error decoding Rack::Session::Cookie session: not base64 encoded marshal dump")
          end

          # Mark rack session cookie for deletion on success
          env[SESSION_DELETE_RACK_COOKIE] = true

          # Convert the rack session by roundtripping it through
          # the parser and serializer, so that you would get the
          # same result as you would if the session was handled
          # by this plugin.
          env[SESSION_SERIALIZED] = data = opts[:serializer].call(session)
          env[SESSION_CREATED_AT] = Time.now.to_i
          opts[:parser].call(data)
        end

        # Interpret given cookie data as a Rack::Session::Cookie
        def _deserialize_session(data)
          opts = roda_class.opts[:sessions]

          begin
            data = Base64.urlsafe_decode64(data)
          rescue ArgumentError
            return _session_serialization_error("Unable to decode session: invalid base64")
          end
          length = data.bytesize
          if data.length < 61
            # minimum length (1+16+12+32) (version+cipher_iv+minimum session+hmac)
            # 1 : version
            # 16 : cipher_iv
            # 12 : minimum_session
            #      2 : bitmap for gzip + padding info
            #      4 : creation time
            #      4 : update time
            #      2 : data
            # 32 : HMAC-SHA-256
            return _session_serialization_error("Unable to decode session: data too short")
          end

          unless data.getbyte(0) == 0
            # version marker
            return _session_serialization_error("Unable to decode session: version marker unsupported")
          end

          encrypted_data = data.slice!(0, length-32)
          unless Rack::Utils.secure_compare(data, OpenSSL::HMAC.digest(OpenSSL::Digest::SHA256.new, opts[:hmac_secret], encrypted_data+opts[:key]))
            if opts[:old_hmac_secret] && Rack::Utils.secure_compare(data, OpenSSL::HMAC.digest(OpenSSL::Digest::SHA256.new, opts[:old_hmac_secret], encrypted_data+opts[:key]))
              use_old_cipher_secret = true
            else
              return _session_serialization_error("Not decoding session: HMAC invalid")
            end
          end

          encrypted_data.slice!(0)
          cipher = OpenSSL::Cipher.new("aes-256-ctr")

          # Not rescuing cipher errors.  If there is an error in the decryption, that's
          # either a bug in the plugin that needs to be fixed, or an attacker is already
          # able to forge a valid HMAC, in which case the error should be raised to
          # alert the application owner about the problem.
          cipher.decrypt
          cipher.key = opts[use_old_cipher_secret ? :old_cipher_secret : :cipher_secret]
          cipher_iv = cipher.iv = encrypted_data.slice!(0, 16)
          data = cipher.update(encrypted_data) << cipher.final

          bitmap, created_at, updated_at = data.unpack('vVV')
          padding_bytes = bitmap & PADDING_MASK
          if (max = opts[:max_seconds]) && Time.now.to_i > created_at + max
            return _session_serialization_error("Not returning session: maximum session time expired")
          end
          if (max = opts[:max_idle_seconds]) && Time.now.to_i > updated_at + max
            return _session_serialization_error("Not returning session: maximum session idle time expired")
          end

          data = data.slice(10+padding_bytes, data.bytesize)

          if bitmap & DEFLATE_BIT > 0
            data = Zlib::Inflate.inflate(data)
          end

          env = @env
          env[SESSION_CREATED_AT] = created_at
          env[SESSION_UPDATED_AT] = updated_at
          env[SESSION_SERIALIZED] = data

          opts[:parser].call(data)
        end

        def _serialize_session(session)
          opts = roda_class.opts[:sessions]
          env = @env
          now = Time.now.to_i
          json_data = opts[:serializer].call(session).force_encoding('BINARY')

          if (serialized_session = env[SESSION_SERIALIZED]) &&
             (updated_at = env[SESSION_UPDATED_AT]) &&
             (now - updated_at < opts[:skip_within]) &&
             (serialized_session == json_data)
            return
          end

          bitmap = 0
          json_length = json_data.bytesize

          if json_length > opts[:gzip_over]
            json_data = Zlib.deflate(json_data)
            json_length = json_data.bytesize
            bitmap |= DEFLATE_BIT
          end

          # When calculating padding bytes to use, include 10 bytes for bitmap and
          # session create/update times, so total size of encrypted data is a
          # multiple of pad_size.
          if (pad_size = opts[:pad_size]) && (padding_bytes = (json_length+10) % pad_size) != 0
            padding_bytes = pad_size - padding_bytes
            bitmap |= padding_bytes
            padding_data = SecureRandom.random_bytes(padding_bytes)
          end

          session_create_time = env[SESSION_CREATED_AT]
          serialized_data = [bitmap, session_create_time||now, now].pack('vVV')

          serialized_data << padding_data if padding_data
          serialized_data << json_data

          cipher = OpenSSL::Cipher.new("aes-256-ctr")
          cipher.encrypt
          cipher.key = opts[:cipher_secret]
          cipher_iv = cipher.random_iv
          encrypted_data = cipher.update(serialized_data) << cipher.final

          data = String.new
          data << "\0" # version marker
          data << cipher_iv
          data << encrypted_data
          data << OpenSSL::HMAC.digest(OpenSSL::Digest::SHA256.new, opts[:hmac_secret], data+opts[:key])

          data = Base64.urlsafe_encode64(data)

          if data.bytesize >= 4096
            raise CookieTooLarge, "attempted to create cookie larger than 4096 bytes"
          end

          data
        end
      end
    end

    register_plugin(:sessions, Sessions)
  end
end