Dump encrypted rails 6 session cookie

session_cookie_debug.rb

class SessionCookieDebug
  def initialize(key:)
    raise ArgumentError, "Get key from production Rails.application.secret_key_base" unless key

    @key = key
  end

  def dump_raw(cookie:)
    # Most of the leg work cribbed from 
    # https://binarysolo.chapter24.blog/demystifying-cookies-in-rails-6/
    data, iv, auth_tag = CGI.unescape(cookie).split("--").map { |p| Base64.decode64(p) }

    cipher = OpenSSL::Cipher.new("aes-256-gcm")
    secret = OpenSSL::PKCS5.pbkdf2_hmac_sha1(@key, "authenticated encrypted cookie", 1000, cipher.key_len)

    cipher.decrypt
    cipher.key = secret
    cipher.iv  = iv
    cipher.auth_tag = auth_tag
    cipher.auth_data = ""

    cookie_payload = cipher.update(data)
    cookie_payload << cipher.final
    JSON.parse(cookie_payload)
  end

  def dump(cookie:)
    raw = dump_raw(cookie: cookie)

    JSON.parse(Base64.decode64(raw["_rails"]["message"]))
  end
end