key change for TCP-MD5

Thu Jun 22 21:43:41 UTC 2006

On 22-jun-2006, at 23:17, Steven M. Bellovin wrote:

>> Why not correct the protocol deficiency by introducing a new  
>> option that
>> includes a KeyID? Wouldn't that approach provide a more comprehensive
>> solution to the problem?

> That's a much better long-term strategy, though the exact mechanism  
> still
> has to be defined.  But it's literally years before that will be  
> usable,
> especially because both ends of a connection need to be upgraded  
> before it
> delivers any benefits.

If you want benefits when only one end is upgraded, your mechanism  
for concurrent keys could be used like this:

- the upgraded side installs the new key
- the upgraded side keeps using the old key
- the non-upgraded side installs the new key
- the upgraded side detects that the other side uses the new key and  
switches over itself
- the old key is removed from the upgraded side

This way, it all goes down when the non-upgraded side installs the  
key: they can immediately see the problem if there is some kind of  
issue with the key (for instance someone entered it incorrectly).

It still makes sense to add stuff that allows both ends to manage the  
key rollover when they're both upgraded, since in that case something  
like the above won't work. I think something like this would work well:

- announce key rollover capability at session connect
- when a new key is configured, send a hash of it to the other side
- other side doesn't have the key yet so says "reject"
- other side is also configured with the new key, sends a hash
- first side sees hashes match, starts sending with the new key and  
says "accept"

Bonus points: when no key is configured, one of the routers generates  
one at session start and sends it over in the clear. This protects  
equally well against session reset attacks as a preconfigured key,  
but obviously it can be sniffed by someone with access to the  
infrastructure.

> We both agree that key change is (a) necessary, and (b) very difficult
> with 2385.

How often do you think keys should change? I've never had anyone ask  
to change keys for about 50 session-years.