Ephemeral Secure Token with JWT

ephemeral_token.rb

class EphemeralToken
  attr_reader :origin, :targets, :expires_at, :payload
  
  class TokenInvalid < StandardError; end
  class TokenExpired < TokenInvalid; end
  
  Algorithm = 'HS512'
  Secret = ENV['SECRET_EPHEMERAL_TOKEN_KEY']
  ObjectToNotation = -> (target) { [target.class.model_name.name, target.id] }
  NotationToObject = -> ((model_name, model_id)) { model_name.constantize.find_by_id(model_id) }
  
  class << self
    def parse (token)
      payload, _ = decode_jwt(token)
      data, exp = payload.values_at('data', 'exp')
      
      new({
        origin: data['origin'].try(&NotationToObject),
        targets: data['targets'].map(&NotationToObject).compact,
        expires_at: exp.try { |x| Time.at(x) }
      })
    end
  
  private
    def decode_jwt (token)
      JWT.decode(token, Secret)
    rescue JWT::ExpiredSignature
      raise TokenExpired.new 'the token has expired'
    rescue JWT::DecodeError
      raise TokenInvalid.new 'the token is not valid'
    end
  end
  
  def initialize (origin: nil, targets: [], expires_at: nil)
    @origin = origin
    @targets = Array(targets)
    @expires_at = expires_at
  end
  
  def payload
    @payload ||= build_payload
  end
  
  def to_jwt
    JWT.encode payload, Secret, Algorithm
  end
  
  def has_target? (target)
    targets.any? { |x|
      (x.id == target.id) && (x.class.model_name.name == target.class.model_name.name)
    }
  end
  
private
  def build_payload
    {
      data: {
        origin: origin.try(&ObjectToNotation),
        targets: targets.map(&ObjectToNotation)
      }
    }.tap { |x|
      if expires_at.present?
        x[:exp] = expires_at.to_time.to_i
      end
    }
  end
end

has_preview_token.rb

module Concerns::HasPreviewToken
  extend ActiveSupport::Concern

  def authorize_preview! (target)
    unless preview_token.has_target?(target)
      deny_preview
    end
  rescue EphemeralToken::TokenInvalid
    deny_preview
  end
  
  def deny_preview
    raise CanCan::AccessDenied.new('you are not authorized to view this page.')
  end
  
  def preview_token
    @preview_token ||= EphemeralToken.parse(params[:preview_token])
  end
end