PayPal offers a secure commerce Identity API that lets your customers sign in to your web site using their PayPal credentials. Signing in with PayPal is quick (just a User Id and Password) and most often, transactions are secured with verified PayPal accounts.
PayPal Identity frees you from the need to implement a customer-management system. The Identity API manages and verifies your customer account data for you. This lets you focus on the important details of your business website, your business transactions.
Using the Identity API
Customers can Log In with PayPal to your website and checkout seamlessly.
Use Log In with PayPal, a secure sign-on system that utilizes cutting-edge security standards, to manage the customer log in process; you don't have to store user data on your system.
You can see a demo of Log In with PayPal on the PayPal Demo.
More about Identity
Find developer information for the PayPal Identity API: