rails session decryptor

decryptor.rb

def decrypt_cookie(cookie)
  cookie = CGI.unescape(cookie)
  data, iv, auth_tag = cookie.split("--").map { |v| Base64.strict_decode64(v) }
  raise InvalidMessage if (auth_tag.nil? || auth_tag.bytes.length != 16)

  cipher = OpenSSL::Cipher.new("aes-256-gcm")
  secret = OpenSSL::PKCS5.pbkdf2_hmac(
    Rails.application.secret_key_base,
    Rails.configuration.action_dispatch.authenticated_encrypted_cookie_salt,
    1000,
    cipher.key_len,
    Rails.configuration.active_support.hash_digest_class.new
  )

  # Setup cipher for decryption and add inputs
  cipher.decrypt
  cipher.key = secret
  cipher.iv  = iv
  cipher.auth_tag = auth_tag
  cipher.auth_data = ""

  # Perform decryption
  cookie_payload = cipher.update(data)
  cookie_payload << cipher.final
  cookie_payload = JSON.parse(cookie_payload)

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

taken from here discussion in here https://gist.github.com/wildjcrt/6359713fa770d277927051fdeb30ebbf?permalink_comment_id=4565360#gistcomment-4565360