diff --git a/lib/qiniu/auth.rb b/lib/qiniu/auth.rb
index 867e4a3..1f5d59a 100755
--- a/lib/qiniu/auth.rb
+++ b/lib/qiniu/auth.rb
@@ -1,5 +1,7 @@
 # -*- encoding: utf-8 -*-
 
+require 'hmac-sha1'
+
 require 'qiniu/exceptions'
 
 module Qiniu
@@ -19,7 +21,54 @@ module Qiniu
           [code, data]
         end
 
-      end
+        EMPTY_ARGS = {}
+
+        ### 生成下载授权URL
+        def authorize_download_url(url, args = EMPTY_ARGS)
+          ### 提取AK/SK信息
+          ak = args[:access_key]
+          if ak.nil? then
+            ak = Config.settings[:access_key]
+          end
+
+          sk = args[:access_key]
+          if sk.nil? then
+            sk = Config.settings[:secret_key]
+          end
+          
+          ### 授权期计算
+          if args[:expires].is_a?(Integer) && args[:expires] > 0 then
+            # 指定相对时间，单位：秒
+            e = Time.now.to_i + args[:expires]
+          elsif args[:deadline].is_a?(Integer) then
+            # 指定绝对时间，常用于调试和单元测试
+            e = args[:deadline]
+          else
+            # 默认授权期1小时
+            e = Time.now.to_i + 3600
+          end
+
+          ### URL变换：追加授权期参数
+          if url.index('?').is_a?(Fixnum) then
+            # 已有参数
+            download_url = "#{url}&e=#{e}"
+          else
+            # 尚无参数
+            download_url = "#{url}?e=#{e}"
+          end
+
+          ### 生成数字签名
+          sign = HMAC::SHA1.new(secret_key).update(download_url)
+          encoded_sign = Utils.urlsafe_base64_encode(sign)
+
+          ### 生成下载授权凭证
+          dntoken = "#{access_key}:#{encoded_sign}"
+
+          ### 返回下载授权URL及相关参数
+          return "#{download_url}&token=#{dntoken}", e, dntoken
+        end # authorize_download_url
+
+      end # class << self
 
     end # module Auth
 end # module Qiniu