Hi i have the following questions about H.235 procedures:
The Diffie Hellman exchange as depicted in Fig 1/H.235 is as follows:
EP GK
ClrTkn(Dh_a, Time_a) CryptoTkn[(genId_a, time_a,Dh_a)Sign_a] ---------------------------------------------------------------------->
ClrTkn(Dh_b, Random_b, Time_a) CryptoTkn[{genId_a,Time_b,Dh_b}Sign_b] ------- <-----------------------------------------------------------------------
_____________________________________________________________________________
ClrTkn[{(genId_b XOR Random_b XOR (x)}EHD-secret)] ------- ---------------------------------------------------------------------->
ClrTkn[ (genId_a, Random_b) ] <-----------------------------------------------------------------------
1.In the phase II procedure above, how does the EP know about genId_b ? I feel that the genId in cryptoToken in phase I message from GK to EP (second line in the diagram) should have genId_b and not genId_a. Is my understanding correct ?
2.As applied to RAS protocol in H.323 context, for a non subscription based authentication case:
Dh_a and Dh_b have public keys for EP and Gk respectively.
(x) is requestSeqNum. genId_b has gkId in GCF.
What does genId_a have in GRQ ?
The above exchange is NOT immune to man-in-the-middle attacks. A third party can easily snoop in and find out Dh_a, Dh_b, GkId and IntegrityMechanism algorithms as these are passed un-ecrypted in GRQ-GCF exchange. How is this authentication procedure any different from just passing a GK assigned dynamic identifier (e.g. EndPointId) in all messages to the GK ??
How is this procedure affected if the EP knows Gk's id apriori (through provisioning or out of band methods as in manual discovery)?
3. H.235 also mentions that this procedures may be used on the call signalling channel as well. The scope of the Key generated as a result of this procedure is not clearly specified. Is this key used for encryption on the call control channel or is it valid only for call signalling channel ?
Any pointers would be appreciated.
thanks, aseem@trillium.com