We deal with the problem of a center sending a secret message to
a group of users such that some subset of the users is considered
revoked and should not be able to obtain the content of the
message. We concentrate on the stateless receiver case, where
the users do not (necessarily) update their state from session to
session. We present a framework called the Subset-Cover
framework, which abstracts a variety of revocation schemes
including some previously known ones.

We suggest an algorithm in this scheme that works for any number
of revoked users. To revoke $r$ users the resulting message length
is at most $2r$ keys. The storage requirements are
$\frac{1}{2} \log^2 N$ where $N$ is the total number
of users. We also provide a `tracing' mechanism
allowing us to detect the non-compliant devices leaking the keys.