Optimizing Identity Tokens for size

Generally speaking, you want to keep your (identity) tokens small. They often need to be transferred via length constrained transport mechanisms – especially the browser URL which might have limitations (e.g. 2 KB in IE). You also need to somehow store the identity token for the length of a session if you want to use the post logout redirect feature at logout time.

Therefore the OpenID Connect specification suggests the following (in section 5.4):

The Claims requested by the profile, email, address, and phone scope values are returned from the UserInfo Endpoint, as described in Section 5.3.2, when a response_type value is used that results in an Access Token being issued. However, when no Access Token is issued (which is the case for the response_type value id_token), the resulting Claims are returned in the ID Token.

IOW – if only an identity token is requested, put all claims into the token. If however an access token is requested as well (e.g. via id_token token or code id_token), it is OK to remove the claims from the identity token and rather let the client use the userinfo endpoint to retrieve them.

That’s how we always handled identity token generation in IdentityServer by default. You could then override our default behaviour by setting the AlwaysIncludeInIdToken flag on the ScopeClaim class.

When we did the configuration re-design in IdentityServer4, we asked ourselves if this override feature is still required. Times have changed a bit and the popular client libraries out there (e.g. the ASP.NET Core OpenID Connect middleware or Brock’s JS client) automatically use the userinfo endpoint anyways as part of the authentication process.

So we removed it.

Shortly after that, several people brought to our attention that they were actually relying on that feature and are now missing their claims in the identity token without a way to change configuration. Sorry about that.

Post RC5, we brought this feature back – it is now a client setting, and not a claims setting anymore. It will be included in RTM next week and documented in our docs.

I hope this post explains our motivation, and some background, why this behaviour existed in the first place.