The inter-operation among heterogeneous wireless networks is crucial to support ubiquitous mobility and seamless roaming. Handoffs across wireless networksin separate administrative domains should ensure uninterrupted service and authenticity of the entities involved. However, the re-authentication of a mobile node (MN) during a handoff across administrative domains typically involves several round trips to the home domain which produce long latencies [9, 12]. A new authentication protocol based on the use of proof tokens is presented in this paper which allows a MN to authenticate in a foreign domain without requiring communication with its home domain. Moreover, the foreign domain is only required to have a trust relationship with any one of MN's previously visited domains, and not necessarily with the home domain. Our objective is to provide an improved authentication approach for ubiquitous mobility and seamless roaming that does not compromise the security of the handoff process while providing low delays.