Decrypt Rails 6.0 beta session cookies

verify_and_decrypt_session_cookie60beta1.rb

# in this version i'm explicitly specifying the serializer to test upgrading from :marshal to :hybrid or :json

require 'cgi'
require 'active_support'

def verify_and_decrypt_session_cookie(
  cookie,
  serializer = Marshal,
  secret_key_base = Rails.application.secret_key_base
)
  config = Rails.application.config
  cookie = CGI::unescape(cookie)
  salt   = config.action_dispatch.authenticated_encrypted_cookie_salt
  encrypted_cookie_cipher = config.action_dispatch.encrypted_cookie_cipher || 'aes-256-gcm'

  key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000)
  key_len = ActiveSupport::MessageEncryptor.key_len(encrypted_cookie_cipher)
  secret = key_generator.generate_key(salt, key_len)
  encryptor = ActiveSupport::MessageEncryptor.new(secret, cipher: encrypted_cookie_cipher, serializer: serializer)

  encryptor.decrypt_and_verify(cookie)
end