[standards-jig] A late security comment on JEP-0020

[Paul Lloyd]

Hi,

I'd like to make on comment on the negotiation feature presented in JEP-0020.

The examples are interesting because they present the negotiation of
various security related algorithms/properties. Unfortunately, the
protocol is at odds with current accepted practices for the
negotiation of such items because the values are not protected
in any way, cryptographically or otherwise; one result of this lack
of protection is that an active attacker can launch a downgrade attack.

For example, client A just wants some crypto:

   <iq type="get" id="1" from="A at tld" to="B at tld">
      <query xmlns="jabber:iq:negotiate">
         <feature type="jabber:crypto:cipher">
            <option>beefy algorithm</option>
            <option>wimpy algorithm</option>
         </feature>
      </query>
   </iq>

Client B wants strong crypto and sends:

   <iq type="result" id="1" from="B at tld" to="A at tld">
      <query xmlns="jabber:iq:negotiate">
         <feature type="jabber:crypto:cipher">
            <option>beefy algorithm</option>
         </feature>
      </query>
   </iq>

But an active attacker modifies the response:

   <iq type="result" id="1" from="B at tld" to="A at tld">
      <query xmlns="jabber:iq:negotiate">
         <feature type="jabber:crypto:cipher">
            <option>wimpy algorithm</option>
         </feature>
      </query>
   </iq>

An even more striking example would be for the attacker to simply
forge an error response indicating the crypto feature isn't supported.
Client A may still chose to proceed and expose data that would otherwise
have been properly protected.

Obviously, we can imagine further checks downstream that can be used
to detect such attacks; the point is rather that the negotiation of such
important items should be inherently protected from the start.

Comments encouraged,


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