Subject: Re: proposal for RTP header authentication method / anti-spamming

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 29 Feb 2000 18:06:02 -0500

Michael Thomas wrote:
> 
> Henning Schulzrinne writes:
>  > However, one consideration is that this is needed not just for audio,
>  > but also for video, in which case running MAC over the whole packet is
>  > probably a good idea (2 Mb/s may not do the trick there, particularly
>  > since you don't want to dedicate your whole CPU to this).
> 
>    So, I'm confused: you're saying that the MAC
>    *should* be run over the RTP header + payload?
>    If so, I agree, and that is in fact what the
>    packetcable spec mandates.

I think the problem is that Henning left out a "not" in the sentence
above, so it should rather read:

However, one consideration is that this is needed not just for audio,
but also for video, in which case running MAC over the whole packet is
probably *NOT* a good idea (2 Mb/s may not do the trick there,
particularly
since you don't want to dedicate your whole CPU to this).

>    It wasn't clear given my cursory reading of their
>    spec whether the MAC was run over the payload or
>    not. Assuming that it is not, the obvious
>    complaint is that it is susceptible to man in
>    the middle attacks which tamper with the
>    payload.

Actually, you don't need man in the middle to tamper. You can simply
snoop packets, make a copy, remove the payload, insert your own payload,
and then send those. The receiver will get one or the other first; if
the faked ones are received first you will be able to insert your forged
audio stream. 

 While I guess this comes down to how
>    rigorous you want to be, it seems there should
>    be a huge amount of motivation to *not* do the
>    more secure thing. I'm not convinced at all for
>    audio that there is any such motivation, and
>    I'm pretty doubtful about video as well.

Just to make sure we're not missing a not here also - you're arguing for
the less secure mechanism? 

-Jonathan R.

-- 
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