# frozen-string-literal: true

require 'base64'
require 'openssl'
require 'securerandom'
require 'uri'
require 'rack/utils'

class Roda
  module RodaPlugins
    # The route_csrf plugin is the recommended plugin to use to support
    # CSRF protection in Roda applications. This plugin allows you set
    # where in the routing tree to enforce CSRF protection.  Additionally,
    # the route_csrf plugin uses modern security practices.
    #
    # By default, the plugin requires tokens be specific to the request
    # method and request path, so a CSRF token generated for one form will
    # not be usable to submit a different form.
    #
    # This plugin also takes care to not expose the underlying CSRF key
    # (except in the session), so that it is not possible for an attacker
    # to generate valid CSRF tokens specific to an arbitrary request method
    # and request path even if they have access to a token that is not
    # specific to request method and request path.  To get this security
    # benefit, you must ensure an attacker does not have access to the
    # session.  Rack::Session::Cookie uses signed sessions, not encrypted
    # sessions, so if the attacker has the ability to read cookie data
    # and you are using Rack::Session::Cookie, it will still be possible
    # for an attacker to generate valid CSRF tokens specific to arbitrary
    # request method and request path.  Roda's session plugin uses
    # encrypted sessions and therefore is safe even if the attacker can
    # read cookie data.
    #
    # == Usage
    #
    # It is recommended to use the plugin defaults, loading the
    # plugin with no options:
    #
    #   plugin :route_csrf
    #
    # This plugin supports the following options:
    #
    # :field :: Form input parameter name for CSRF token (default: '_csrf')
    # :header :: HTTP header name for CSRF token (default: 'X-CSRF-Token')
    # :key :: Session key for CSRF secret (default: '_roda_csrf_secret')
    # :require_request_specific_tokens :: Whether request-specific tokens are required (default: true).
    #                                     A false value will allow tokens that are not request-specific
    #                                     to also work.  You should only set this to false if it is
    #                                     impossible to use request-specific tokens.  If you must
    #                                     use non-request-specific tokens in certain cases, it is best
    #                                     to leave this option true by default, and override it on a
    #                                     per call basis in those specific cases.
    # :csrf_failure :: The action to taken if a request fails the CSRF check (default: :raise).  Options:
    #                  :raise :: raise a Roda::RodaPlugins::RouteCsrf::InvalidToken exception
    #                  :empty_403 :: return a blank 403 page (rack_csrf's default behavior)
    #                  :clear_session :: Clear the current session
    #                  Proc :: Treated as a routing block, called with request object
    # :check_header :: Whether the HTTP header should be checked for the token value (default: false).
    #                  If true, checks the HTTP header after checking for the form input parameter.
    #                  If :only, only checks the HTTP header and doesn't check the form input parameter.
    # :check_request_methods :: Which request methods require CSRF protection
    #                           (default: <tt>['POST', 'DELETE', 'PATCH', 'PUT']</tt>)
    # :upgrade_from_rack_csrf_key :: If provided, the session key that should be checked for the
    #                                rack_csrf raw token.  If the session key is present, the value
    #                                will be checked against the submitted token, and if it matches,
    #                                the CSRF check will be passed.  Should only be set temporarily
    #                                if upgrading from using rack_csrf to the route_csrf plugin, and
    #                                should be removed as soon as you are OK with CSRF forms generated
    #                                before the upgrade not longer being usable. The default rack_csrf
    #                                key is <tt>'csrf.token'</tt>.
    #
    # The plugin also supports a block, in which case the block will be used
    # as the value of the :csrf_failure option.
    #
    # == Methods
    #
    # This adds the following instance methods:
    #
    # check_csrf!(opts={}) :: Used for checking if the submitted CSRF token is valid.
    #                         If a block is provided, it is treated as a routing block if the
    #                         CSRF token is not valid.  Otherwise, by default, raises a
    #                         Roda::RodaPlugins::RouteCsrf::InvalidToken exception if a CSRF
    #                         token is necessary for the request and there is no token provided
    #                         or the provided token is not valid. Options can be provided to
    #                         override any of the plugin options for this specific call.
    #                         The :token option can be used to specify the provided CSRF token
    #                         (instead of looking for the token in the submitted parameters).
    # csrf_field :: The field name to use for the hidden tag containing the CSRF token.
    # csrf_path(action) :: This takes an argument that would be the value of the HTML form's
    #                      action attribute, and returns a path you can pass to csrf_token
    #                      that should be valid for the form submission.  The argument should
    #                      either be nil or a string representing a relative path, absolute
    #                      path, or full URL.
    # csrf_tag(path=nil, method='POST') :: An HTML hidden input tag string containing the CSRF token, suitable
    #                                      for placing in an HTML form.  Takes the same arguments as csrf_token.
    # csrf_token(path=nil, method='POST') :: The value of the csrf token, in case it needs to be accessed
    #                                        directly.  It is recommended to call this method with a
    #                                        path, which will create a request-specific token.  Calling
    #                                        this method without an argument will create a token that is
    #                                        not specific to the request, but such a token will only
    #                                        work if you set the :require_request_specific_tokens option
    #                                        to false, which is a bad idea from a security standpoint.
    # use_request_specific_csrf_tokens? :: Whether the plugin is configured to only support
    #                                      request-specific tokens, true by default.
    # valid_csrf?(opts={}) :: Returns whether the submitted CSRF token is valid (also true if
    #                         the request does not require a CSRF token).  Takes same option hash
    #                         as check_csrf!.
    #
    # This plugin also adds the following instance methods for compatibility with the
    # older csrf plugin, but it is not recommended to use these methods in new code:
    #
    # csrf_header :: The header name to use for submitting the CSRF token via an HTTP header
    #                (useful for javascript). Note that this plugin will not look in
    #                the HTTP header by default, it will only do so if the :check_header
    #                option is used.
    # csrf_metatag :: An HTML meta tag string containing the CSRF token, suitable
    #                 for placing in the page header.  It is not recommended to use
    #                 this method, as the token generated is not request-specific and
    #                 will not work unless you set the :require_request_specific_tokens option to
    #                 false, which is a bad idea from a security standpoint.
    #
    # == Token Cryptography
    #
    # route_csrf uses HMAC-SHA-256 to generate all CSRF tokens.  It generates a random 32-byte secret,
    # which is stored base64 encoded in the session.  For each CSRF token, it generates 31 bytes
    # of random data.
    #
    # For request-specific CSRF tokens, this pseudocode generates the HMAC: 
    #
    #   hmac = HMAC(secret, method + path + random_data)
    #
    # For CSRF tokens not specific to a request, this pseudocode generates the HMAC: 
    #
    #   hmac = HMAC(secret, random_data)
    #
    # This pseudocode generates the final CSRF token in both cases:
    #
    #   token = Base64Encode(random_data + hmac)
    #
    # Using this construction for generating CSRF tokens means that generating any
    # valid CSRF token without knowledge of the secret is equivalent to a successful generic attack
    # on HMAC-SHA-256.
    #
    # By using an HMAC for tokens not specific to a request, it is not possible to use a
    # valid CSRF token that is not specific to a request to generate a valid request-specific
    # CSRF token.  
    #
    # By including random data in the HMAC for all tokens, different tokens are generated
    # each time, mitigating compression ratio attacks such as BREACH.
    module RouteCsrf
      # Default CSRF option values
      DEFAULTS = {
        :field => '_csrf'.freeze,
        :header => 'X-CSRF-Token'.freeze,
        :key => '_roda_csrf_secret'.freeze,
        :require_request_specific_tokens => true,
        :csrf_failure => :raise,
        :check_header => false,
        :check_request_methods => %w'POST DELETE PATCH PUT'.freeze.each(&:freeze)
      }.freeze

      # Exception class raised when :csrf_failure option is :raise and
      # a valid CSRF token was not provided.
      class InvalidToken < RodaError; end

      def self.configure(app, opts=OPTS, &block)
        options = app.opts[:route_csrf] = (app.opts[:route_csrf] || DEFAULTS).merge(opts)
        if block || opts[:csrf_failure].is_a?(Proc)
          if block && opts[:csrf_failure]
            raise RodaError, "Cannot specify both route_csrf plugin block and :csrf_failure option"
          end
          block ||= opts[:csrf_failure]
          options[:csrf_failure] = :csrf_failure_method
          app.define_roda_method(:_roda_route_csrf_failure, 1, &app.send(:convert_route_block, block))
        end
        options[:env_header] = "HTTP_#{options[:header].to_s.gsub('-', '_').upcase}".freeze
        options.freeze
      end

      module InstanceMethods
        # Check that the submitted CSRF token is valid, if the request requires a CSRF token.
        # If the CSRF token is valid or the request does not require a CSRF token, return nil.
        # Otherwise, if a block is given, treat it as a routing block and yield to it, and
        # if a block is not given, use the :csrf_failure option to determine how to handle it.
        def check_csrf!(opts=OPTS, &block)
          if msg = csrf_invalid_message(opts)
            if block
              @_request.on(&block)
            end
            
            case failure_action = opts.fetch(:csrf_failure, csrf_options[:csrf_failure])
            when :raise
              raise InvalidToken, msg
            when :empty_403
              @_response.status = 403
              @_response.headers.replace('Content-Type'=>'text/html', 'Content-Length'=>'0')
              throw :halt, @_response.finish_with_body([])
            when :clear_session
              session.clear
            when :csrf_failure_method
              @_request.on{_roda_route_csrf_failure(@_request)}
            when Proc
              RodaPlugins.warn "Passing a Proc as the :csrf_failure option value to check_csrf! is deprecated"
              @_request.on{instance_exec(@_request, &failure_action)} # Deprecated
            else
              raise RodaError, "Unsupported :csrf_failure option: #{failure_action.inspect}"
            end
          end
        end

        # The name of the hidden input tag containing the CSRF token.  Also used as the name
        # for the meta tag.
        def csrf_field
          csrf_options[:field]
        end

        # The HTTP header name to use when submitting CSRF tokens in an HTTP header, if
        # such support is enabled (it is not by default).
        def csrf_header
          csrf_options[:header]
        end

        # An HTML meta tag string containing a CSRF token that is not request-specific.
        # It is not recommended to use this, as it doesn't support request-specific tokens.
        def csrf_metatag
          "<meta name=\"#{csrf_field}\" content=\"#{csrf_token}\" \/>"
        end

        # Given a form action, return the appropriate path to use for the CSRF token.
        # This makes it easier to generate request-specific tokens without having to
        # worry about the different types of form actions (relative paths, absolute
        # paths, URLs, empty paths).
        def csrf_path(action)
          case action
          when nil, '', /\A[#?]/
            # use current path
            request.path
          when /\A(?:https?:\/)?\//
            # Either full URI or absolute path, extract just the path
            URI.parse(action).path
          else
            # relative path, join to current path
            URI.join(request.url, action).path
          end
        end

        # An HTML hidden input tag string containing the CSRF token.  See csrf_token for
        # arguments.
        def csrf_tag(*args)
          "<input type=\"hidden\" name=\"#{csrf_field}\" value=\"#{csrf_token(*args)}\" \/>"
        end

        # The value of the csrf token.  For a path specific token, provide a path
        # argument.  By default, it a path is provided, the POST request method will
        # be assumed.  To generate a token for a non-POST request method, pass the
        # method as the second argument.
        def csrf_token(path=nil, method=('POST' if path))
          token = SecureRandom.random_bytes(31)
          token << csrf_hmac(token, method, path)
          Base64.strict_encode64(token)
        end

        # Whether request-specific CSRF tokens should be used by default.
        def use_request_specific_csrf_tokens?
          csrf_options[:require_request_specific_tokens]
        end

        # Whether the submitted CSRF token is valid for the request.  True if the
        # request does not require a CSRF token.
        def valid_csrf?(opts=OPTS)
          csrf_invalid_message(opts).nil?
        end

        private

        # Returns error message string if the CSRF token is not valid.
        # Returns nil if the CSRF token is valid.
        def csrf_invalid_message(opts)
          opts = opts.empty? ? csrf_options : csrf_options.merge(opts)
          method = request.request_method

          unless opts[:check_request_methods].include?(method)
            return
          end

          unless encoded_token = opts[:token]
            encoded_token = case opts[:check_header]
            when :only
              env[opts[:env_header]]
            when true
              return (csrf_invalid_message(opts.merge(:check_header=>false)) && csrf_invalid_message(opts.merge(:check_header=>:only)))
            else
              @_request.params[opts[:field]]
            end
          end

          unless encoded_token.is_a?(String)
            return "encoded token is not a string"
          end

          if (rack_csrf_key = opts[:upgrade_from_rack_csrf_key]) && (rack_csrf_value = session[rack_csrf_key]) && csrf_compare(rack_csrf_value, encoded_token)
            return
          end

          # 31 byte random initialization vector
          # 32 byte HMAC
          # 63 bytes total
          # 84 bytes when base64 encoded
          unless encoded_token.bytesize == 84
            return "encoded token length is not 84"
          end

          begin
            submitted_hmac = Base64.strict_decode64(encoded_token)
          rescue ArgumentError
            return "encoded token is not valid base64"
          end

          random_data = submitted_hmac.slice!(0...31)

          if csrf_compare(csrf_hmac(random_data, method, @_request.path), submitted_hmac)
            return
          end

          if opts[:require_request_specific_tokens]
            "decoded token is not valid for request method and path"
          else
            unless csrf_compare(csrf_hmac(random_data, '', ''), submitted_hmac)
              "decoded token is not valid for either request method and path or for blank method and path"
            end
          end
        end
        
        # Helper for getting the plugin options.
        def csrf_options
          opts[:route_csrf]
        end

        # Perform a constant-time comparison of the two strings, returning true if they match and false otherwise.
        def csrf_compare(s1, s2)
          Rack::Utils.secure_compare(s1, s2)
        end

        # Return the HMAC-SHA-256 for the secret and the given arguments.
        def csrf_hmac(random_data, method, path)
          OpenSSL::HMAC.digest(OpenSSL::Digest::SHA256.new, csrf_secret, "#{method.to_s.upcase}#{path}#{random_data}")
        end

        # If a secret has not already been specified, generate a random 32-byte
        # secret, stored base64 encoded in the session (to handle cases where
        # JSON is used for session serialization).
        def csrf_secret
          key = session[csrf_options[:key]] ||= SecureRandom.base64(32)
          Base64.strict_decode64(key)
        end
      end
    end

    register_plugin(:route_csrf, RouteCsrf)
  end
end