Ruby on Rails implementation of purging AMP cache (https://developers.google.com/amp/cache/update-cache)

amp_purge.rb

# app/models/amp_purge.rb
#
# frozen_string_literal: true
#
# Ruby on Rails implementation of purging AMP cache
# https://developers.google.com/amp/cache/update-cache
#
# Replace "example" with your domain
#
# Run like:
# $ rails c
# > AmpPurge.new("/foo/bar.amp").purge
#
# Open the "Signed URL" in your browser
# You should see an OK response

require "base64"
require "openssl"

class AmpPurge
  attr_accessor :article_path

  def initialize(article_path)
    @article_path = article_path
  end

  def purge
    puts "Unsigned Path ----------------"
    puts unsigned_path

    puts "Valid Signature? -------------"
    puts public_key.verify(digest, signature, unsigned_path)

    puts "Signed URL -------------------"
    puts signed_url
  end

  private

  def base64_signature
    Base64.urlsafe_encode64(signature)
  end

  def base_url
    "https://www-example-com.cdn.ampproject.org"
  end

  def digest
    OpenSSL::Digest::SHA256.new
  end

  def params
    {
      amp_action: "flush",
      amp_ts: Time.now.utc.to_i
    }
  end

  def private_key
    OpenSSL::PKey::RSA.new(
      File.read(Rails.root.join("amp-private-key.pem"))
    )
  end

  def public_key
    OpenSSL::PKey::RSA.new(
      File.read(Rails.root.join("amp-public-key.pem"))
    )
  end

  def signature
    private_key.sign(digest, unsigned_path)
  end

  def signed_url
    "#{base_url}#{unsigned_path}&amp_url_signature=#{base64_signature}"
  end

  def unsigned_path
    "/update-cache/c/s/www.example.com#{article_path}?#{params.to_param}"
  end
end