How to fix Kamal Rails Master Key Message Encryption error

When deploying a Rails application using Kamal, we set RAILS_MASTER_KEY in our .env file. However, this RAILS_MASTER_KEY is for our production environment. When we try to run the same application on our development environment, we get ActiveSupport::MessageEncryptor::InvalidMessage error because Rails is not able to decrypt the development credentials file. This article will show you how to fix this Master Key Encryption error when using foreman to start the development environment.

This is a typical stack trace we get when this happens:

18:12:06 web.1    | Exiting
18:12:06 web.1    | /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/messages/codec.rb:57:in `catch_and_raise': ActiveSupport::MessageEncryptor::InvalidMessage
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/message_encryptor.rb:242:in `decrypt_and_verify'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/encrypted_file.rb:109:in `decrypt'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/encrypted_file.rb:72:in `read'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/encrypted_configuration.rb:57:in `read'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/encrypted_configuration.rb:76:in `config'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/encrypted_configuration.rb:95:in `options'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/activesupport-7.1.1/lib/active_support/core_ext/module/delegation.rb:332:in `method_missing'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/devise-4.9.2/lib/devise/secret_key_finder.rb:24:in `key_exists?'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/devise-4.9.2/lib/devise/secret_key_finder.rb:10:in `find'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/devise-4.9.2/lib/devise/rails.rb:37:in `block in <class:Engine>'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/railties-7.1.1/lib/rails/initializable.rb:32:in `instance_exec'
18:12:06 web.1    |     from /Users/omkarjoshi/Projects/rubypodcatcher/.bundle/ruby/3.2.0/gems/railties-7.1.1/lib/rails/initializable.rb:32:in `run'

In this case, we have typically run the Rails application in our development environment using the bin/dev command and we use <a href="https://github.com/ddollar/foreman">foreman</a> in our Procfile.dev.

The Fix:

• Create a separate environment variable file for our local environment:

# .env.local

RAILS_MASTER_KEY=<development_master_key>

• Explicitly specify this environment variable when foreman starts in Procfile.dev

# Procfile.dev

exec foreman start -e .env.local -f Procfile.dev "$@"

Why does this work?

• Earlier our Procfile.dev, simply started foreman using this command: exec foreman start -f Procfile.dev "*$@*". This command automatically picks up the .env file which is present in the root directory.
• In our case, this .env file has configuration for the production environment which are required for Kamal.
• In our fix, we explicitly point foreman to a different “local” environment variable file which gets rid of this error.

Check out my Rails Kamal Series for step by step guide to deploy Rails application using Kamal.