Scheme III: Achieving Fast Transfer Rates

 

As we mentioned earlier, the receiver needs to be assured that it receives the packet $P$_i before the corresponding key disclosure packet $P$_i+1 is sent by the sender. This condition severely limits the transmission rate of the previous two schemes since $P$_i+1 can only be sent after every receiver has received $P$_i.

We solve this problem by disclosing the key $K$_i of the data packet $P$_i in a later packet $P$_i+d, instead of in the following packet, where $d$ is a delay parameter that is set by the sender and announced as the session set-up.

The sender determines the delay $d$ based on the packet rate $r$, the maximum tolerable synchronization uncertainty $\delta$_tMax, and the maximum tolerable network delay $d$_NMax. Setting $d\; =\; (\delta$_tMax+d_NMax)r allows the receiver to successfully verify the security condition even in the case of maximum allowable network delay and maximal synchronization error. The choice of $\delta$_tMax and $d$_NMax presents the following tradeoff: Large delay values will cause a large $d$ which results in long delays until the packet authentication. On the other hand, short maximum delays cause the the security condition to drop packets at receivers with a slow network connection. However, multimedia data packets become obsolete if they are received after their segment of the stream was already played or presented to the user. In that case, dropping unsafe packets might not interfere with the multimedia stream since the packets are likely to be obsolete. We stress that the choice of $d$ does not affect the security of the scheme, only its usability.

For the case of a constant packet rate, the security condition is easy to state. We assume that the sending time of the first packet is $T$₀ and the sending time of packet $P$_i is $T$_i = T₀ + i / r. To verify the security condition for an incoming packet, the receiver checks that $ArrT$_i + δ_t < T_i+d, where $ArrT$_i is the arrival time of packet $P$_i at the receiver.