Improved Privacy of the Tree-Based Hash Protocols Using Physically Unclonable Function

In 2004, Molnar and Wagner introduced a very appealing scheme dedicated to the identification of RFID tags. Their protocol relies on a binary tree of secrets which are shared – for all nodes except the leaves – amongst the tags. Hence the compromise of one tag also has implications on the other tags with whom it shares keys. We describe a new man-in-the-middle attack against this protocol which allows to break privacy even without opening tags. Moreover, it can be applied to some other RFID protocols which use correlated keys as the one described recently by Damg̊ard and Pedersen at CT-RSA 2008. We introduce a modification of the initial scheme to allow us to thwart this and to strengthen RFID tags by implementing secrets with Physical Obfuscated Keys (POKs). This doing, we augment tags and scheme privacy, particularly general resistance against physical threats.