Proxy re-signatures in the standard model

This paper studies proxy re-signature schemes. We first classify the expected security notions for proxy re-signature schemes with different properties. We then show how to attack on a recently proposed bidirectional scheme that is purported to be secure without random oracles, and discuss the flaw in their proof. Next, we show how to design a generic unidirectional proxy re-signature scheme using a new primitive called homomorphic compartment signature as the building block. We give a concrete instantiation which yields the first known unidirectional proxy re-signature scheme which is proven secure under standard assumption in the standard model. We also discuss how to incorporate the concept of forward-security into the proxy re-signature paradigm, such that the signing and the transformation are both time-limited.