11.3. Re-keying and access control

11.3. Re-keying and access control

Re-keying may occur due to access control (e.g., when a member is removed during a multicast RTP session), or for pure cryptographic reasons (e.g., the key is at the end of its lifetime). When using SRTP default transforms, the master key MUST be replaced before any of the index spaces are exhausted for any of the streams protected by one and the same master key.

How key management re-keys SRTP implementations is out of scope, but it is clear that there are straightforward ways to manage keys for a multicast group. In one-sender multicast, for example, it is typically the responsibility of the sender to determine when a new key is needed. The sender is the one entity that can keep track of when the maximum number of packets has been sent, as receivers may join and leave the session at any time, there may be packet loss and delay etc. In scenarios other than one-sender multicast, other methods can be used. Here, one must take into consideration that key exchange can be a costly operation, taking several seconds for a single exchange. Hence, some time before the master key is exhausted/expires, out-of-band key management is initiated, resulting in a new master key that is shared with the receiver(s). In any event, to maintain synchronization when switching to the new key, group policy might choose between using the MKI and the <From, To>, as described in Section 8.1.

For access control purposes, the <From, To> periods are set at the desired granularity, dependent on the packet rate. High rate re-keying can be problematic for SRTCP in some large-group scenarios. As mentioned, there are potential problems in using the SRTP index, rather than the SRTCP index, for determining the master key. In particular, for short periods during switching of master keys, it may be the case that SRTCP packets are not under the current master key of the correspondent SRTP. Therefore, using the MKI for re-keying in such scenarios will produce better results.