Re: [AVT] SRTP Question

["Steffen Fries" <steffen.fries at siemens.com>]

Hi Mat,

agreed, that's also my point of view.

Ciao
	Steffen


Date sent:      	Tue, 20 Jul 2004 11:43:25 +0200
From:           	Mats Näslund <mats.naslund at ericsson.com>
To:             	steffen.fries at siemens.com
Copies to:      	Mark Baugher <mbaugher at cisco.com>, avt at ietf.org
Subject:        	Re: [AVT] SRTP Question

> Hi
>
> Just to make sure we mean the same thing:
>
> Steffen Fries wrote:
>
> > Hi Mark,
> >
> > thanks for your comments.
> >
> > Yes, you are right, it might not hit us soon, it was more a
> > question of completeness. Nevertheless, there might be scenarios
> > were one of the participants chooses just a small intervall <From,
> > To> and uses video and audio, this condition may occur.
>
> Note: setting a very small <From, To> interval but "forgetting"
> to re-key as the interval expires is *not* a problem from security
> point of view. Sure, sender/receiver may loose synch, but security
> isn't compromised.
>
> The only bound relating to security is the absloute 2^48 packets
> upper bound. Note also that this is in terms of number of packets.
> Thus, even if the 48-bit index should wrap modulo 2^48 (e.g. due to
> initially not starting from zero) this is still fine as long as you
> maintain/enforce the 2^48 packer limit.
>
> Best,
>
> /Mats
> >
> > Anyway, your answer suggest exactly what I was thinking.
> >
> > Ciao
> >         Steffen
> >
> >
> > Date sent:              Sat, 17 Jul 2004 12:29:59 -0700
> > To:                     steffen.fries at siemens.com
> > From:                   Mark Baugher <mbaugher at cisco.com>
> > Subject:                Re: [AVT] SRTP Question
> > Copies to:              avt at ietf.org
> >
> >  > At 08:05 AM 7/16/2004, Steffen Fries wrote:
> >  > >Hi,
> >  > >
> >  > >I've got another question to SRTP ;-)
> >  > >
> >  > >Within RFC3711 it is mentioned in section 3.3.1:
> >  > >"...  After that number of SRTP packets have been sent
> >  > >with a given (master or session) key, the sender MUST NOT send
> >  > >any more packets with that key. ..."
> >  > >
> >  > >Further, section 8.1.1 reads:
> >  > >"... The default values for the <From, To> are "from the first
> >  > >observed packet" and "until further notice".  However, the
> >  > >maximum limit of SRTP/SRTCP packets that are sent under each
> >  > >given master/session key (Section 9.2) MUST NOT be exceeded...."
> >  > >
> >  > >Does this mean, that both sender and receiver have to ensure,
> >  > >that the maximum number of packets encrypted with the same key
> >  > >is not exceeded? If yes, is the recommendation, that the
> >  > >receiver discards the packets that are received after the key
> >  > >usage has reached the maximum limit?
> >  >
> >  > This probably should have been spelled out in RFC 3711 because
> >  > there are number of alternative actions such as discard the
> >  > packets, stop receiving the stream in the case of multicast,
> >  > leave the session regardless of how many senders are securely
> >  > sending when one sender violates the key lifetime.
> >  >
> >  > I would say that the session is no longer secure and the receiver
> >  > should leave the session and log the condition.
> >  >
> >  > Thankfully, this is a very unlikely event in the forseeable
> >  > future.
> >  >
> >  > Mark
> >  >
> >  >
> >  > >Ciao
> >  > >         Steffen
> >  > >
>
>



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