[Perc] Question on diet Design?

[Eric Rescorla <[e-mail]>]

I've read this document and I'm finding it hard to understand the
basic design. Ignoring all the details, we have a node i which
connects to the Key Distributor (KD), and we want to establish a
shared key for each node i = 0, 1, ... n (this is just to establish
E2E keys. I'm ignoring HBH keys for now). So, as I read it, the system
works like this.

- Node i connects to KD and establishes a DTLS connection which establishes
  a pairwise key K_d_i.
- The KD then generates a fresh EKT key K_e_i and sends it to i encrypted
  under K_d_i.
- The KD then generates a shared group key K_g and sends it individually
  to all i in SRTP/EKT encrypted under K_e_i.

Is that correct so far?

If so, my question is why you need K_e_i? You have already established
a shared secret via the DTLS connection and you can make as many fresh
pairwise encryption keys as you want from that, so why do you need
to make a new encryption key K_e_i and send it from KD to i?

-Ekr