Re: [Srtp-users] RE: [AVT] <From, To> and MKI, FRC 3711

[Mats Näslund <mats.naslund at ericsson.com>]

Hi

What I think Mark refers to is a situation in which

- several SRTP streams share a master key, and,
- re-keying is based on From-To values.

If you use SRTP in this way (which RFC3711 recommends against)
the receiver may in some cases have problem determining the correct
key. However, there is really no "extra" security problems involved.
What *could* happen is a so-called two-time pad, but this would be
caused by a misstake by the sender, not by the receiver. Furthermore,
this key sharing is not allowed in RFC3711, unless you can guarantee
distinct SSRCs for the RTP sessions sharing the key. Whence, if you
(somehow) could guarantee such SSRC-distincness, two-time pads would
anyway not occur.

Cheers

/Mats

Mark Baugher wrote:

> hi
>   Using both mechanisms is a lot of complexity and I'm hard pressed to 
> find examples of when we would need two mechanisms, which do the same 
> thing.  I don't like that final sentence in 8.1.1 in RFC 3711 because 
> complexity undermines security and using both mechanisms is complex.
>
>   From/To has the advantage of not adding a field to the packet, but it 
> has the disadvantage of insecure interactions with the AES counter mode 
> transform.  MKI adds a field but does not suffer from this problem.  I 
> would choose one or the other but not both.
>
>   As it is described below, use of both mechanisms is superfluous AFAICT.
>
> Mark
>
> At 05:56 AM 5/11/2004, Sylvain Latulippe wrote:
>
>  
> > Hi,
> >
> > My understanding is the following:
> >
> > Both mechanisms can be used whitin the same session. When using MKI, 
> > <From, To> mechanism can be used by the sender side for synchronizing key 
> > transitions (re-keying).
> >
> > For example:
> >
> > A session is started and 2 keys (KEY_1 and KEY_2) are given to the session.
> >
> > Session parameters:
> > KEY      FROM    TO        MKI
> > KEY_1    0       10        1
> > KEY_2    11      x         2
> >
> > The sender uses KEY_1 to encrypt packets with index 0 to 10. MKI = 1 is 
> > also appended to packets 0 to 10. When building packet with index 11, the 
> > sender switches to KEY_2 and appends MKI = 2 to the packet.
> >
> > On the receiver side, <From, To> is not required in this scenario. MKI is 
> > extracted from the received packet and used to retrieve the right key.
> >
> > Sylvain
> >
> >
> >
> >
> > -----Original Message-----
> > From: avt-admin@ietf.org [mailto:avt-admin@ietf.org]On Behalf Of Ofer Goren
> > Sent: 11 mai, 2004 07:35
> > To: avt@ietf.org; srtp-users@lists.sourceforge.net
> > Subject: [AVT] <From, To> and MKI, FRC 3711
> >
> >
> > Hi.
> >
> > In RFC 3711, section 8.1.1, it says that "using the MKI does not exclude 
> > using <From, To> key
> >   lifetime simultaneously".
> >
> > As I understand it, both mechanisms can be used for the same session 
> > simultaneously. However, does the <From, To> mechanism is MANDATORY to be 
> > used every time (whether <From, To> is used or not), or can I omit it if 
> > I'm using MKI?
> >
> > Thanks,
> >
> > Ofer.
> >
> >
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> >    
>  


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