Group Signature with Redactable Signature

Abstract. Group signature is a major cryptographic tool allowing

anonymous access to a service. However, in practice, access to a service is usually granted for some periods of time, which implies that the

signing rights must be deactivated the rest of the time. This requirement

thus calls for complex forms of revocation, reminiscent of the concept of

time-bound keys. However, schemes implementing this concept are rare

and only allow revocation with limited granularity. That is, signing keys

are associated with an expiry time and become definitively useless once

the latter has passed.

In this paper, we revisit the notion of group signatures with timebound keys with several contributions. Firstly, we extend this notion to

allow high granularity revocation: a member’s signing key can in particular be deactivated at some moments and then be automatically reinstated. Secondly, we show that this complex property is actually simple to achieve using redactable signature. In particular, we consider in

this context a recent redactable signature scheme from PKC 20 that we

improve by dramatically reducing the size of the public key. The resulting

construction is of independent interest.