What is config.active_support.hash_digest_class = OpenSSL::Digest::SHA256?

I occasionally update the config.load_defaults version at work in Rails applications. This involves understanding the purpose of each new configuration in the target version, assessing its impact, and creating individual pull requests for each change. Based on my experience, this process can be quite challenging, and I suspect few projects carry it out with a thorough understanding of the impact. I decided to write this blog post to assist others planning to update config.load_defaults.

Today, I’ll focus on config.active_support.hash_digest_class = OpenSSL::Digest::SHA256, which is included in config.load_defaults 7.0.

• Relevant Commit: Change the default digest for new apps to SHA256 · rails/rails@ba9207f
• Related Documentation: Upgrading Ruby on Rails — Ruby on Rails Guides

This setting changes the default hash function used for cache keys to SHA256. The default was originally MD5, then updated to SHA1 in Rails 5.2, and now to SHA256 in Rails 7.0.

Since this hash function primarily applies to cache keys, the change wasn’t prompted by security issues with SHA1. Instead, the shift to SHA256 aligns Rails with modern security standards, simplifying compliance checks.

Related Areas in Rails

The following are the main areas in Rails where this hash function is used:

• Determining keys for fragment cache templates
  • Used to compute the template tree digest in the cache method.
• Setting cache keys when using cache(User.all) with an ActiveRecord::Relation
  • Specifically, it computes the id portion of the key.
• Generating strong ETags
  • Reference: Rails Caching Guide
• Shortening long cache keys

You can safely apply this setting change if your project doesn’t rely on these caching features.

How to Verify Affected Areas

You can verify whether caching is used by adding a monkey patch like the following, which logs whenever ActiveSupport::Digest.hexdigest is called. This works for all related areas except "shortening long cache keys." Merge logs appropriately if your CI runs tests in parallel.

# config/application.rb or config/initializers/something.rb

module MonkeyPatch
  def monkeypatch_logger
    @monkeypatch_logger ||= Logger.new(Rails.root.join('log/monkeypatch.log'))
  end

  def hexdigest(*)
    monkeypatch_logger.debug("ActiveSupport::Digest.hexdigest is called from: #{caller(2, 1).first}")
    super
  end
end

class ActiveSupport::Digest
  class << self
    prepend MonkeyPatch
  end
end

Before running tests, enable caching and set the cache store to :null_store to avoid unexpected behavior:

# config/environments/test.rb
config.action_controller.perform_caching = true
config.cache_store = :null_store

If you find any areas using caching, note that this change will invalidate all related caches after deployment. This could increase server or database load, so consider temporarily scaling up your application servers and database resources to handle the extra traffic.