Use SHA-1 for non-sensitive digests by default

Instead of providing a configuration option to set the hash function,
switch to SHA-1 for new apps and allow upgrading apps to opt in later
via `new_framework_defaults_5_2.rb`.

2 files changed, 8 insertions, 7 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 3f77b191f9..481b25a545 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -3,12 +3,12 @@
 
     *Daniel Ma*
 
-*   Allow the hash function used to generate non-sensitive digests, such as the
-    ETag header, to be specified with `config.active_support.hash_digest_class`.
+*   Use SHA-1 to generate non-sensitive digests, such as the ETag header.
 
-    The object provided must respond to `#hexdigest`, e.g. `Digest::SHA1`.
+    Enabled by default for new apps; upgrading apps can opt in by setting
+    `config.active_support.use_sha1_digests = true`.
 
-    *Dmitri Dolguikh*
+    *Dmitri Dolguikh*, *Eugene Kenny*
 
 
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
diff --git a/activesupport/lib/active_support/railtie.rb b/activesupport/lib/active_support/railtie.rb
index 6b8fa676a9..605b50d346 100644
--- a/activesupport/lib/active_support/railtie.rb
+++ b/activesupport/lib/active_support/railtie.rb
@@ -70,9 +70,10 @@ module ActiveSupport
     end
 
     initializer "active_support.set_hash_digest_class" do |app|
-      if app.config.active_support.hash_digest_class
-        ActiveSupport::Digest.hash_digest_class =
-          app.config.active_support.hash_digest_class
+      config.after_initialize do
+        if app.config.active_support.use_sha1_digests
+          ActiveSupport::Digest.hash_digest_class = ::Digest::SHA1
+        end
       end
     end
   end