use_nonce (OpenID::Store::Filesystem)

    # File lib/openid/store/filesystem.rb, line 142
          def use_nonce(server_url, timestamp, salt)
            return false if (timestamp - Time.now.to_i).abs > Nonce.skew

            if server_url and !server_url.empty?
              proto, rest = server_url.split('://',2)
            else
              proto, rest = '',''
            end
            raise "Bad server URL" unless proto && rest

            domain = filename_escape(rest.split('/',2)[0])
            url_hash = safe64(server_url)
            salt_hash = safe64(salt)

            nonce_fn = '%08x-%s-%s-%s-%s'%[timestamp, proto, domain, url_hash, salt_hash]

            filename = @nonce_dir.join(nonce_fn)

            begin
              fd = File.new(filename, File::CREAT | File::EXCL | File::WRONLY, 0200)
              fd.close
              return true
            rescue Errno::EEXIST
              return false
            end
          end