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Re: key change for TCP-MD5

  • From: Steven M. Bellovin
  • Date: Mon Jun 26 20:44:53 2006

On Tue, 20 Jun 2006 17:06:27 -0400, Ross Callon <[email protected]>

> At 12:12 PM 6/20/2006 -0700, Bora Akyol wrote:
> >The draft allows you to have a set of keys in your keychain and
> >the implementation tries all of them before declaring the segment
> >as invalid.
> DoS against routers is of course a major concern. Using
> encryption has the potential of making DoS worse, in the
> sense that the amount of processing that a bogus packet
> can cause is increased by the amount of processing
> needed to check the authentication. If the router needs to
> check multiple keys in the keychain before declaring the
> segment as invalid, then this multiplies the effect of the
> DOS by the number of keys that need to be checked.
You're quite right, and the next version of the draft contains the
following additional text in the Security Considerations:

     Having multiple keys makes CPU denial of service  
     attacks easier.  This suggests that keeping the 
     overlap period reasonably
     short is a good idea.  In 
     addition, the Generalized TTL Security Mechanism
     <xref target="RFC3682" />, if applicable to
     the local topology, can help.
     Note that there would almost never be more than
     two keys in existence at any one time in any event.

		--Steven M. Bellovin,