OAuth 2 access_token vs OpenId Connect id_token

Question

Although I have worked with OAuth 2 before, I am a newbie to Open ID Connect.

Reading the tutorials and documentations I have come across both access_token and id_token where access_token is the random unique string generated according to OAuth 2 and id_token is JSON Web Token which contains information like the id of the user, algorithm, issuer and various other info which can be used to validate it. I have also seen API providers who provide both the access_token and id_token and as far as I know it is for backward compatibility.

My question is that is it possible to use both the access_token and the id_token for accessing the protected resources ? Or is the id_token just for verification purposes and access_token is used for getting access to protected resources ?

Answer 1

Originally, OAuth and OpenId are designed for different purpose: OpenId for authentication and OAuth for authorization. OpenId Connect is a unification of the two and serves for both, but does not change their original functionalities. Keeping that in mind, you should be able to find out yourself. ;-)

The id_token is used to identify the authenticated user, e.g. for SSO. The access_token must be used to prove access rights to protected resources, e.g. for the userinfo endpoint in OpenId Connect.

Answer 2

Another angle to provide an answer:

id_token

• An id_token is a JWT - make note of that!
• It contains claims about the identity of the user/resource owner
• Having a valid id_token means that the user is authenticated

access_token

• An access_token is a bearer token
• A bearer token means that the bearer can access the resource without further identification
• An access_token can be a JWT (see Appendix point 1.) or opaque

If you want to read more: Types of tokens in oidc and oauth