[Cryptography] Langsec & authentication

[Stephan Neuhaus]

On 05/27/2014 09:59 PM, Judson Lester wrote:
> On Tue, May 27, 2014 at 12:53 PM, Stephan Neuhaus
> <stephan.neuhaus at tik.ee.ethz.ch> wrote:
>> Once you have ambiguous blobs of data, one of which means "re-order
>> coffee" and the other meaning "nuke North Korea", you've lost the game.
>>   But surely that's not the fault of the authentication subsystem.
>
> So, in that case you need your protocol language that describes
> messages unambiguously - so that there's only one blob that
> legitimately means "nuke North Korea."

My point was rather that if I send you a message which you can 
legitimately decode as either a request for more caffeine or delivery of 
apocalyptic weaponry, authenticating the message is the least of your 
worries and you should redesign your protocol.

One protocol which does it right is IPFIX (see RFC 5101). This is a 
protocol that's designed for the transport of network measurement data. 
(It's a descendent of NetFlow V9, if that helps.) (Conflict-of-Interest 
declaration: I am writing a fast IPFIX/V9 collection/transcoding/export 
library.) The various data items have types, but these aren't the types 
that a CS major would come up with (boolean, integer, float), but rather 
have very specific semantics.  Thus you have, for example, the types "IP 
V4 source address" and "IP V4 destination address".  There is really no 
way to mistake one for the other.  (Well of course there's always a way, 
but the protocol really goes out of the way to make that difficult.)

In this protocol, there's no way to say "bring me more coffee" and have 
it interpreted as "launch nukes", even though there are many ways to say 
"there were 100 bytes exchanged between 1.2.3.4 and 5.6.7.8 last Monday 
between 10.00 am and 10.01 am", through different orderings of the data 
items.  In this case, even reordering the source and destination IP 
addresses would not change the meaning of the data, since the types of 
the data items are not just "IP V4 address" but "IP V4 source address".

Fun,

Stephan