[ruby-dev:31928] securerandom.rb for 1.8

securerandom.rb を 1.8 に追加し、cgi/session.rb に使わせたい
のですが、どうでしょうか。

cgi/session.rb に限りませんが、session cookie の生成には暗号
学的にそれなりに安全な乱数が必要です。

しかし、現状の Ruby ではそのような安全な乱数を得る簡単な API
が用意されていないため、安全なんかどうなのかよくわからない怪
しげなコードで session cookie を生成することがあります。

やはり、そういう難しい暗号の話は、餅は餅屋ということで外部
(OpenSSL や /dev/urandom) にまかせるのがいいのではないでしょ
うか。

Index: lib/securerandom.rb
===================================================================
--- lib/securerandom.rb	(リビジョン 0)
+++ lib/securerandom.rb	(リビジョン 0)
@@ -0,0 +1,137 @@
+# = Secure random number generator interface.
+#
+# This library is an interface for secure random number generator which is
+# suitable for generating session key in HTTP cookies, etc.
+#
+# It supports following secure random number generators.
+#
+# * openssl
+# * /dev/urandom
+#
+# == Example
+#
+# # random hexadecimal string.
+# p SecureRandom.hex(10) #=> "52750b30ffbc7de3b362"
+# p SecureRandom.hex(10) #=> "92b15d6c8dc4beb5f559"
+# p SecureRandom.hex(11) #=> "6aca1b5c58e4863e6b81b8"
+# p SecureRandom.hex(12) #=> "94b2fff3e7fd9b9c391a2306"
+# p SecureRandom.hex(13) #=> "39b290146bea6ce975c37cfc23"
+# ...
+#
+# # random base64 string.
+# p SecureRandom.base64(10) #=> "EcmTPZwWRAozdA=="
+# p SecureRandom.base64(10) #=> "9b0nsevdwNuM/w=="
+# p SecureRandom.base64(10) #=> "KO1nIU+p9DKxGg=="
+# p SecureRandom.base64(11) #=> "l7XEiFja+8EKEtY="
+# p SecureRandom.base64(12) #=> "7kJSM/MzBJI+75j8"
+# p SecureRandom.base64(13) #=> "vKLJ0tXBHqQOuIcSIg=="
+# ...
+#
+# # random binary string.
+# p SecureRandom.random_bytes(10) #=> "\016\t{\370g\310pbr\301"
+# p SecureRandom.random_bytes(10) #=> "\323U\030TO\234\357\020\a\337"
+# ...
+
+begin
+  require 'openssl'
+rescue LoadError
+end
+
+module SecureRandom
+  # SecureRandom.random_bytes generates a random binary string.
+  #
+  # The argument n specifies the length of the result string.
+  #
+  # If n is not specified, 16 is assumed.
+  # It may be larger in future.
+  #
+  # If secure random number generator is not available,
+  # NotImplementedError is raised.
+  def self.random_bytes(n=nil)
+    n ||= 16
+    if defined? OpenSSL::Random
+      return OpenSSL::Random.random_bytes(n)
+    end
+    if !defined?(@has_urandom) || @has_urandom
+      @has_urandom = false
+      flags = File::RDONLY
+      flags |= File::NONBLOCK if defined? File::NONBLOCK
+      flags |= File::NOCTTY if defined? File::NOCTTY
+      flags |= File::NOFOLLOW if defined? File::NOFOLLOW
+      begin
+        File.open("/dev/urandom", flags) {|f|
+          unless f.stat.chardev?
+            raise Errno::ENOENT
+          end
+          @has_urandom = true
+          ret = f.readpartial(n)
+          if ret.length != n
+            raise NotImplementedError, "Unexpected partial read from random device"
+          end
+          return ret
+        }
+      rescue Errno::ENOENT
+        raise NotImplementedError, "No random device"
+      end
+    end
+    raise NotImplementedError, "No random device"
+  end
+
+  # SecureRandom.hex generates a random hex string.
+  #
+  # The argument n specifies the length of the random length.
+  # The length of the result string is twice of n.
+  #
+  # If n is not specified, 16 is assumed.
+  # It may be larger in future.
+  #
+  # If secure random number generator is not available,
+  # NotImplementedError is raised.
+  def self.hex(n=nil)
+    random_bytes(n).unpack("H*")[0]
+  end
+
+  # SecureRandom.base64 generates a random base64 string.
+  #
+  # The argument n specifies the length of the random length.
+  # The length of the result string is about 4/3 of n.
+  #
+  # If n is not specified, 16 is assumed.
+  # It may be larger in future.
+  #
+  # If secure random number generator is not available,
+  # NotImplementedError is raised.
+  def self.base64(n=nil)
+    [random_bytes(n)].pack("m*").delete("\n")
+  end
+
+  # SecureRandom.random_number generates a random number.
+  #
+  # If an positive integer is given as n,
+  # SecureRandom.random_number returns an integer:
+  # 0 <= SecureRandom.random_number(n) < n.
+  #
+  # If 0 is given or an argument is not given,
+  # SecureRandom.random_number returns an float:
+  # 0.0 <= SecureRandom.random_number() < 1.0.
+  def self.random_number(n=0)
+    if 0 < n
+      hex = n.to_s(16)
+      hex = '0' + hex if (hex.length & 1) == 1
+      bin = [hex].pack("H*")
+      mask = bin[0]
+      mask |= mask >> 1
+      mask |= mask >> 2
+      mask |= mask >> 4
+      begin
+        rnd = SecureRandom.random_bytes(bin.length)
+        rnd[0] = (rnd[0] & mask).chr
+      end until rnd < bin
+      rnd.unpack("H*")[0].hex
+    else
+      # assumption: Float::MANT_DIG <= 64
+      i64 = SecureRandom.random_bytes(8).unpack("Q")[0]
+      Math.ldexp(i64 >> (64-Float::MANT_DIG), -Float::MANT_DIG)
+    end
+  end
+end
Index: lib/cgi/session.rb
===================================================================
--- lib/cgi/session.rb	(リビジョン 13603)
+++ lib/cgi/session.rb	(作業コピー)
@@ -174,16 +174,22 @@
     # is used internally for automatically generated
     # session ids. 
     def create_new_id
-      require 'digest/md5'
-      md5 = Digest::MD5::new
-      now = Time::now
-      md5.update(now.to_s)
-      md5.update(String(now.usec))
-      md5.update(String(rand(0)))
-      md5.update(String($$))
-      md5.update('foobar')
+      require 'securerandom'
+      begin
+        session_id = SecureRandom.hex(16)
+      rescue NotImplementedError
+        require 'digest/md5'
+        md5 = Digest::MD5::new
+        now = Time::now
+        md5.update(now.to_s)
+        md5.update(String(now.usec))
+        md5.update(String(rand(0)))
+        md5.update(String($$))
+        md5.update('foobar')
+        session_id = md5.hexdigest
+      end
       @new_session = true
-      md5.hexdigest
+      session_id
     end
     private :create_new_id
 
-- 
[田中 哲][たなか あきら][Tanaka Akira]