On preventing type flaw attacks on security protocols with a simplified tagging scheme

A type flaw attack on a security protocol is an attack in which a field in a message that was originally intended to have one type is subsequently interpreted as having another type. Heather et al. proved that type flaw attacks can be prevented by tagging each field with the information that indicates its intended type. We simplify Heather et al.'s tagging scheme by combining all the tags inside each encrypted component into a single tag and by omitting the tags on the outermost level. The simplification process reduces the sizes of messages in the security protocol. We also formally prove that our simplified tagging scheme is as secure as Heather et al.' with the strand space method.