5 files changed, 67 insertions, 1 deletions
diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 41d77ab6c1..4e397ea110 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -48,6 +48,7 @@ module ActiveSupport
     autoload :Gzip
     autoload :Inflector
     autoload :JSON
+    autoload :KeyGenerator
     autoload :MessageEncryptor
     autoload :MessageVerifier
     autoload :Multibyte
diff --git a/activesupport/lib/active_support/key_generator.rb b/activesupport/lib/active_support/key_generator.rb
new file mode 100644
index 0000000000..04d170f801
--- /dev/null
+++ b/activesupport/lib/active_support/key_generator.rb
@@ -0,0 +1,23 @@
+require 'openssl'
+
+module ActiveSupport
+  # KeyGenerator is a simple wrapper around OpenSSL's implementation of PBKDF2
+  # It can be used to derive a number of keys for various purposes from a given secret.
+  # This lets rails applications have a single secure secret, but avoid reusing that
+  # key in multiple incompatible contexts.
+  class KeyGenerator
+    def initialize(secret, options = {})
+      @secret = secret
+      # The default iterations are higher than required for our key derivation uses
+      # on the off chance someone uses this for password storage
+      @iterations = options[:iterations] || 2**16
+    end
+
+    # Returns a derived key suitable for use.  The default key_size is chosen
+    # to be compatible with the default settings of ActiveSupport::MessageVerifier.
+    # i.e. OpenSSL::Digest::SHA1#block_length
+    def generate_key(salt, key_size=64)
+      OpenSSL::PKCS5.pbkdf2_hmac_sha1(@secret, salt, @iterations, key_size)
+    end
+  end
+end
diff --git a/activesupport/test/key_generator_test.rb b/activesupport/test/key_generator_test.rb
new file mode 100644
index 0000000000..525082d313
--- /dev/null
+++ b/activesupport/test/key_generator_test.rb
@@ -0,0 +1,32 @@
+require 'abstract_unit'
+
+begin
+  require 'openssl'
+  OpenSSL::PKCS5
+rescue LoadError, NameError
+  $stderr.puts "Skipping KeyGenerator test: broken OpenSSL install"
+else
+
+require 'active_support/time'
+require 'active_support/json'
+
+class KeyGeneratorTest < ActiveSupport::TestCase
+  def setup
+    @secret    = SecureRandom.hex(64)
+    @generator = ActiveSupport::KeyGenerator.new(@secret, :iterations=>2)
+  end
+
+  test "Generating a key of the default length" do
+    derived_key = @generator.generate_key("some_salt")
+    assert_kind_of String, derived_key
+    assert_equal OpenSSL::Digest::SHA1.new.block_length, derived_key.length, "Should have generated a key of the default size"
+  end
+
+  test "Generating a key of an alternative length" do
+    derived_key = @generator.generate_key("some_salt", 32)
+    assert_kind_of String, derived_key
+    assert_equal 32, derived_key.length, "Should have generated a key of the right size"
+  end
+end
+
+end
diff --git a/railties/lib/rails/application.rb b/railties/lib/rails/application.rb
index 0b9ed025db..c8a6600da9 100644
--- a/railties/lib/rails/application.rb
+++ b/railties/lib/rails/application.rb
@@ -101,6 +101,14 @@ module Rails
       routes_reloader.reload!
     end
 
+
+    # Return the application's KeyGenerator
+    def key_generator
+      # number of iterations selected based on consultation with the google security
+      # team. Details at https://github.com/rails/rails/pull/6952#issuecomment-7661220
+      @key_generator ||= ActiveSupport::KeyGenerator.new(config.secret_token, :iterations=>1000)
+    end
+
     # Stores some of the Rails initial environment parameters which
     # will be used by middlewares and engines to configure themselves.
     # Currently stores:
@@ -121,7 +129,8 @@ module Rails
         "action_dispatch.show_exceptions" => config.action_dispatch.show_exceptions,
         "action_dispatch.show_detailed_exceptions" => config.consider_all_requests_local,
         "action_dispatch.logger" => Rails.logger,
-        "action_dispatch.backtrace_cleaner" => Rails.backtrace_cleaner
+        "action_dispatch.backtrace_cleaner" => Rails.backtrace_cleaner,
+        "action_dispatch.key_generator" => key_generator
       })
     end
 
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index d014e5e362..d8d022c265 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -634,6 +634,7 @@ module ApplicationTests
       assert_equal      app.env_config['action_dispatch.show_exceptions'],   app.config.action_dispatch.show_exceptions
       assert_equal      app.env_config['action_dispatch.logger'],            Rails.logger
       assert_equal      app.env_config['action_dispatch.backtrace_cleaner'], Rails.backtrace_cleaner
+      assert_equal      app.env_config['action_dispatch.key_generator'],     Rails.application.key_generator
     end
 
     test "config.colorize_logging default is true" do