Re: [IPsec] Avoiding Authentication Header (AH)

[Nico Williams <[e-mail]>]

On Mon, Jan 2, 2012 at 6:45 PM, RJ Atkinson <[e-mail]> wrote:
> On 02  Jan 2012, at 19:28 , Nico Williams wrote:
>> On Mon, Jan 2, 2012 at 3:11 PM, RJ Atkinson <[e-mail]> wrote:
>>> I gave a list earlier of a number of different scenarios where
>>> and reasons why AH is used.  A subset of that list:
>>>        - ESP null does not protect options/optional headers.
>>
>> ESP in tunnel mode is supposed to be the replacement for AH,
>> and gets you this.
>
> Sadly, it cannot do so.
>
> Tunnel-mode isn't especially helpful here -- particularly
> for options or optional headers that are intended to be
> read/seen and their contents considered when forwarding
> transit IP packets.

With tunnel mode you effectively repeat the options inside and outside
the tunnel.  Routers can't validate the integrity protection
regardless of whether AH or ESP-NULL in tunnel mode is used, but
assuming that an attacker can only modify options at one place in the
path then the recipient can see that options were modified.

This is applies to both, IPv4 and v6.

>>>        - 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.

Nico
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