What is the use of secret_key_base in rails 4

The secret_token.rb file's content includes a long randomized string which is used to verify the integrity of signed cookies (such as user sessions when people are signed into your web app).

Documentation says:

Use your existing secret_key_base from the secret_token.rb initializer to set the SECRET_KEY_BASE environment variable for whichever users run the Rails app in production mode. Alternately, you can simply copy the existing secret_key_base from the secret_token.rb initializer to secrets.yml under the production section, replacing <%= ENV["SECRET_KEY_BASE"] %>.

Since it is important file, and you can't put it to .gitignore, it is treated to be a good practice to use env variable to store secret_key_base value:

create .env or .powenv file and store it as:

export SECRET_TOKEN="9489b3eee4eccf317ed77407553e8adc97baca7c74dc7ee33cd93e4c8b69477eea66eaedeb18af0be2679887c7c69c0a28c0fded0a71ea472a8c4laalal19cb"

And then in config/initializers/secret_token.rb

YourAppName::Application.config.secret_key_base = if Rails.env.development? or Rails.env.test? # generate simple key for test and development environments
  ('a' * 30) # should be at least 30 chars long
else
  ENV['SECRET_TOKEN']
end

This article is (a bit old and) long but really full of useful info on the topic.

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UPDATE 04.05.15

Starting from Rails 4.2 there is no longer secret_token.rb file. By new convention there is a config/secrets.yml file aimed to store application's secrets.

Have a read on how to upgrade an existing app to 4.2.x according to innovations.

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Technically the purpose of secrect_key_base is to be the secret input for the application’s key_generator method (check Rails.application.key_generator).

The application’s key_generator, and thus secret_key_base, are used by three core features within the Rails framework:

• Deriving keys for encrypted cookies which are accessible via cookies.encrypted.
• Deriving the key for HMAC signed cookies which are accessible via cookies.signed.
• Deriving keys for all of the application’s named message_verifier instances.

Check out more on each of the three in the article by @michaeljcoyne.

secret_key_base is used to encrypt and sign session

in order to safely send session back and forth in cookies

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In Rails 4,

1. if your app is called Hello, and
2. you set session['a'] = 'b',

your cookie will look something like this:

_Hello_session=BAh7B0kiD3%3D%3D--dc40a55cd52fe32bb3b84ae0608956dfb5824689

which translates into:

_Hello_session=<encrypted a=b>--<digital signature>

Cookies are set by server and kept client side, with browser resending set cookies to the server every time we request a page.

To prevent evil people from understanding a=b string, it's encrypted.
To prevent evil people from tampering cookies, digital signature is used.

In both cases secret_key_base value is used (to encrypt/decrypt a=b and to validate digital signature).