Overview#Identity Toolkit-enabled] site, Identity Toolkit’s widgets set a cookie named “gtoken”. It is a JSON Web Token (JWT), a cryptographically-signed JSON object encoded in base 64. The Identity Toolkit JWT is very similar to an OpenID Connect ID token and we will refer to this as the Identity Toolkit ID Token.
Identity Toolkit ID Token does NOT conform to the OpenID Connect specification in one important way. The user_id field in an OIDC ID token is the identifier of the user at the IDP. The user_id field in the Identity Toolkit ID Token is a global identifier, unique across all IDPs, for this user, in the context of your site or app. It is not shared with other sites or apps which use Identity Toolkit. In other words, Identity Toolkit does not provide a global identifier across different developers (relying parties).