What are Bearer Tokens and token_type in OAuth 2?

token_type is a parameter in Access Token generate call to Authorization server, which essentially represents how an access_token will be generated and presented for resource access calls. You provide token_type in the access token generation call to an authorization server.

If you choose Bearer (default on most implementation), an access_token is generated and sent back to you. Bearer can be simply understood as "give access to the bearer of this token." One valid token and no question asked. On the other hand, if you choose Mac and sign_type (default hmac-sha-1 on most implementation), the access token is generated and kept as secret in Key Manager as an attribute, and an encrypted secret is sent back as access_token.

Yes, you can use your own implementation of token_type, but that might not make much sense as developers will need to follow your process rather than standard implementations of OAuth.

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Anyone can define "token_type" as an OAuth 2.0 extension, but currently "bearer" token type is the most common one.

https://www.rfc-editor.org/rfc/rfc6750

Basically that's what Facebook is using. Their implementation is a bit behind from the latest spec though.

If you want to be more secure than Facebook (or as secure as OAuth 1.0 which has "signature"), you can use "mac" token type.

However, it will be hard way since the mac spec is still changing rapidly.

https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-http-mac-05

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From RFC 6750, Section 1.2:

Bearer Token A security token with the property that any party in possession of the token (a "bearer") can use the token in any way that any other party in possession of it can. Using a bearer token does not require a bearer to prove possession of cryptographic key material (proof-of-possession).

The Bearer Token or Refresh token is created for you by the Authentication server. When a user authenticates your application (client) the authentication server then goes and generates for your a Bearer Token (refresh token) which you can then use to get an access token.

The Bearer Token is normally some kind of cryptic value created by the authentication server, it isn't random it is created based upon the user giving you access and the client your application getting access.

See also: Mozilla MDN Header Information.

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