Validating the request

google_validation.rb

require 'json/jwt'
require 'jwt'

module GoogleValidation

  GOOGLE_CERTS_URI = 'https://www.googleapis.com/oauth2/v3/certs'
  REQUIRED_AUDIENCE = ''
  REQUIRED_CLIENT_ID = ''

  def self.token_validation(token)
    google_token = token.split('.')
    # From header find the segnature key used
    header = JSON.parse(Base64.decode64(google_token[0]))
    payload = Base64.decode64(google_token[1])
    signature = Base64.decode64(google_token[2])
    # Find google key from the list of the certs
    public_key = google_certs(header)

    # Decode the google token with public key
    user_info = decode_google_token(token, public_key)
    user_info
  end

  def self.generate_public_key(key)
    jwk = JSON::JWK.new(
      kty: :RSA,
      e: key["e"],
      n: key["n"]
    )
    jwk.to_key
  end

  def self.decode_google_token(token, public_key)
    begin
      decoded_token = JWT.decode(token, public_key, true, { :algorithm => 'RS256' })
      if decoded_token[0]["aud"] == REQUIRED_AUDIENCE && decoded_token[0]["azp"] == REQUIRED_CLIENT_ID
        return true
      else
        return false
      end
    rescue Exception => e
      return e.message
    end

  end

  def self.google_certs(header)
    uri = URI(GOOGLE_CERTS_URI)
    get = Net::HTTP::Get.new uri.request_uri
    http = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true
    res = http.request(get)
    if res.kind_of?(Net::HTTPSuccess)
      new_certs = MultiJson.load(res.body).map{|key,value| value}.first.find{|x|  x["n"] if x["kid"] == header["kid"]}
      # using JWT convert it to public key
      public_key = generate_public_key(new_certs)
    else
      true
    end
  end

end