Re: [IPsec] Avoiding Authentication Header (AH)

[RJ Atkinson <[e-mail]>]

On 02  Jan 2012, at 20:06 , Nico Williams wrote:
> On Mon, Jan 2, 2012 at 6:45 PM, RJ Atkinson <[e-mail]> wrote:
> With tunnel mode you effectively repeat the options inside and outside
> the tunnel.  

One could repeat the headers; some implementations don't.  
It doesn't help transit devices to repeat the headers
if they can't validate them before acting upon them.

> Routers can't validate the integrity protection
> regardless of whether AH or ESP-NULL in tunnel mode is used,

Disagree.  Intermediate authentication can be performed
by routers/firewalls, at least when AH is used.  The
router/firewall could then act on the options in the
packet having reasonable assurance that the option itself,
and its contents, were valid for that packet.

> but assuming that an attacker can only modify options at one place in the
> path then the recipient can see that options were modified.

Unfortunately, it is not really a generally valid assumption
that there is only 1 potential attack point along a path.

>>>>        - ESP null cannot reliably be parsed past.
>>> 
>>> WESP is supposed to provide this.
>> 
>> Sadly, at present there is still no 100% reliable
>> method for parsing past an ESP header with NULL encryption.
>> There are various documents describing methods which
>> have various success probabilities, but none that is
>> 100% reliable.
> 
> Sure, this is necessarily true until any replacement for AH is
> universally deployed and that indicates that only integrity
> protection is provided.

If some such mechanism exists, and if it provides 100% reliable
ability to parse-past/parse-into, and if it is plausibly efficient 
to parse-into/parse-past in real-time for high-performance 
switches/routers/firewalls, then this discussion might be 
more useful than it is at present.  That is rather a lot
of IFs, however, none of which seem true today.

Yours,

Ran