Rails Cookie Decrypt Test

decryptor.rb

# Run this code in Rails(7 or above) console 
class Decryptor
  attr_reader :key_generator, :cookie

  def initialize(cookie:)
    @cookie = CGI::unescape(cookie)
    set_key_generator
  end

  def set_sha1
    @key_generator.instance_variable_set(:@hash_digest_class, OpenSSL::Digest::SHA1)
  end

  def set_sha256
    @key_generator.instance_variable_set(:@hash_digest_class, OpenSSL::Digest::SHA256)
  end

  def decrypt
    encrypted_cookie_cipher = config.action_dispatch.encrypted_cookie_cipher || 'aes-256-gcm'
    key_len = ActiveSupport::MessageEncryptor.key_len(encrypted_cookie_cipher)
    salt   = config.action_dispatch.authenticated_encrypted_cookie_salt
    secret = @key_generator.generate_key(salt, key_len)
    serializer = ActiveSupport::MessageEncryptor::NullSerializer
    encryptor = ActiveSupport::MessageEncryptor.new(secret, cipher: encrypted_cookie_cipher, serializer: serializer)
    session_key = config.session_options[:key].freeze
    encryptor.decrypt_and_verify(@cookie, purpose: "cookie.#{session_key}")
  end

  private 

  def set_key_generator
    secret_key_base = Rails.application.secret_key_base    
    @key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000)
  end

  def config
    Rails.application.config
  end
end

cookie = "..."
decryptor = Decryptor.new(cookie: cookie)
decryptor.set_sha1 # or decryptor.set_sha256
decryptor.decrypt