Per-origin and cross-origin implications on double spend state

[chris-wood]

Origins currently choose whether their challenges are per-origin or cross-origin. For cross-origin, double-spend prevention is only as good as the coordination between origins and the Issuer. Per-origin allows double-spend prevention to be isolated to a single origin; also prevents the cache of tokens being take up by some other origin.

[chris-wood]

Write this as "any origins that share a redemption context must also share double spend state for that context."