Prior to reading this guide it is assumed that you are familiar with the terms and concepts described in the Overview and How user authorization works guide.
This guide only covers authorization and data sharing topics. It does not review user authentication, instead see Sign In With Google and the Migrating from Google Sign-In guide for user sign-up and sign-in.
Deciding if the GIS library is right for you
You must choose if using Google’s library, or creating your own best fits your needs. An overview of features and functionality:
- Popup based consent flows to minimize redirects, thus enabling users to remain on your site throughout the authorization process.
- Security features such as Cross-site Request Forgery (CRSF).
- Helper methods to request individual scopes and confirm user consent.
- Human friendly error handling and documentation links for use by engineers during development and later for visitors to your site.
- When implementing without the Identity Services library you are responsible
- Managing requests and responses with Google’s OAuth 2.0 endpoints, including redirects.
- Optimizing the user experience.
- Implementation of security features to validate requests, responses, and to prevent CSRF.
- Methods to confirm a user has granted consent for any requested scopes.
- Managing OAuth 2.0 error codes, creating human readable messages, and links to user help.
In summary, Google offers the GIS library to help you to quickly, and securely implement an OAuth 2.0 client and optimize the user’s authorization experience.
Choosing an authorization flow
Both flows result in an access token which can be used to call Google APIs.
The primary differences between the two flows are:
- the number of user actions,
- whether your app will call Google APIs without the user present,
- if a backend platform is needed to host an endpoint and to store per-user refresh tokens for individual user accounts, and
- higher or lower levels of user security.
When comparing flows and evaluating your security requirements, a factor to consider is that the level of user security varies depending on the scopes you choose. For example, viewing calendar invites as read-only might be considered less risky than using a read/write scope to edit files in Drive.
OAuth 2.0 flow comparison
|Implicit flow||Authorization code flow|
|User consent required||For every token request, including replacing expired tokens.||Only for the first token request.|
|User must be present||Yes||No, supports offline use.|
|User security||Least||Most, has client authentication and avoids in-browser token handling risks.|
|Access token issued||Yes||Yes|
|Refresh token issued||No||Yes|
|Supported browser required||Yes||Yes|
|Access token used to call Google APIs||only from a web app running in the user's browser.||either from a server running on backend platform, or from a web app running in the user's browser.|
|Requires backend platform||No||Yes, for endpoint hosting and storage.|
|Secure storage needed||No||Yes, for refresh token storage.|
|Requires hosting of an authorization code endpoint||No||Yes, to receive authorization codes from Google.|
|Access token expiration behavior||A user gesture such as button press or clicking on a link is required to request and obtain a new, valid access token.||After an initial user request, your platform exchanges the stored refresh token to obtain a new, valid access token necessary to call Google APIs.|