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.
