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The content borne by a ticket granting ticket in CAS 3 is more than merely a String username. The ticket validation response can include arbitrary attributes. All of this content would need to be signed or encrypted into the ticket granting cookie for this self-validating ticket-granting-cookie scheme to work. Which might be fine – presumably a Cookie could accomodate this.
Alternatives
An alternative approach is to cluster CAS by sharing state across CAS server instances in a cluster. This shared state could be in an RDBMS or in a messaging, in-memory Java state cache.