# Copyright 2015, Google Inc.
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#
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# modification, are permitted provided that the following conditions are
# met:
#
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# notice, this list of conditions and the following disclaimer.
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# in the documentation and/or other materials provided with the
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# this software without specific prior written permission.
#
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require 'googleauth/signet'
require 'googleauth/credentials_loader'
require 'jwt'
require 'multi_json'

module Google
  # Module Auth provides classes that provide Google-specific authorization
  # used to access Google APIs.
  module Auth
    # Authenticates requests using Google's Service Account credentials.
    #
    # This class allows authorizing requests for service accounts directly
    # from credentials from a json key file downloaded from the developer
    # console (via 'Generate new Json Key').
    #
    # cf [Application Default Credentials](http://goo.gl/mkAHpZ)
    class ServiceAccountCredentials < Signet::OAuth2::Client
      JWT_AUD_URI_KEY = :jwt_aud_uri
      AUTH_METADATA_KEY = Signet::OAuth2::AUTH_METADATA_KEY
      TOKEN_CRED_URI = 'https://www.googleapis.com/oauth2/v3/token'
      extend CredentialsLoader

      # Reads the private key and client email fields from the service account
      # JSON key.
      def self.read_json_key(json_key_io)
        json_key = MultiJson.load(json_key_io.read)
        fail 'missing client_email' unless json_key.key?('client_email')
        fail 'missing private_key' unless json_key.key?('private_key')
        [json_key['private_key'], json_key['client_email']]
      end

      # Initializes a ServiceAccountCredentials.
      #
      # @param json_key_io [IO] an IO from which the JSON key can be read
      # @param scope [string|array|nil] the scope(s) to access
      def initialize(json_key_io, scope = nil)
        private_key, client_email = self.class.read_json_key(json_key_io)
        super(token_credential_uri: TOKEN_CRED_URI,
              audience: TOKEN_CRED_URI,
              scope: scope,
              issuer: client_email,
              signing_key: OpenSSL::PKey::RSA.new(private_key))
      end

      # Extends the superclass behaviour to construct a jwt token if the
      # google jwt uri key is present in the input hash.
      #
      # The jwt is used as the authentication token.
      def apply!(a_hash, opts = {})
        jwt_aud_uri = a_hash.delete(JWT_AUD_URI_KEY)
        unless jwt_aud_uri.nil?
          jwt_token = new_jwt_token(jwt_aud_uri, opts)
          a_hash[AUTH_METADATA_KEY] = "Bearer #{jwt_token}"
          return a_hash
        end
        super
      end

      # Creates a jwt uri token.
      def new_jwt_token(jwt_aud_uri, options = {})
        now = Time.new
        skew = options[:skew] || 60
        assertion = {
          'iss' => issuer,
          'sub' => issuer,
          'aud' => jwt_aud_uri,
          'exp' => (now + expiry).to_i,
          'iat' => (now - skew).to_i
        }
        JWT.encode(assertion, signing_key, signing_algorithm)
      end
    end
  end
end