Subject: Re: doc/draft/draft-ietf-dnsext-tsig-sha-01.txt

From: Donald Eastlake III <[email-masked]>
Date: Tue, 1 Mar 2005 09:24:45 -0500 (EST)

TSIG is only used by prior agreement of the parties involved. Policies 
as to what algorithms to use and what truncation to allow are all 
deliberately left open. If they feel like it, the parties could not only 
use asymmetric amounts of truncation but different hash algorithms in 
each direction. But one would expect that in most real cases they would 
use a fixed algorithm and truncation. But, since amounts of truncation 
are clearly ordered and, arguably, current hash algorithms also are, I 
suppose that more flexible code could be configured with a minimum hash 
algorithm and a minimum number of octets of hash. If the signature is 
too weak for the receiver's policy, I don't see this as different form 
the signature being missing. This should be probably all be explicitly 
stated in the document.

Truncation is trivial and most IETF security protocols provide 
explicitly for things like SHA-1 truncated to 96 bits. There are people 
who believe that a little truncation improves security by revealing less 
information to crackers. I don't see any reason to fail to provide 
truncation cababilities in TSIG.

Thanks,
Donald
  ====================================================================
  Donald E. Eastlake 3rd                       [email-masked]
  155 Beaver Street              +1-508-634-2066(h) +1-508-786-7554(w)
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On Tue, 1 Mar 2005, Mark Andrews wrote:

> Date: Tue, 01 Mar 2005 16:08:36 +1100
> From: Mark Andrews <[email-masked]>
> To: [email-masked]
> Subject: Re: doc/draft/draft-ietf-dnsext-tsig-sha-01.txt
> 
>
> 	HMAC Truncation.
>
> 	Looking at this there is nothing to say whether a reply
> 	has to use the same number of bits as the query?
>
> 	i.e.
> 		Can a server respond to a truncated HMAC-SHA1 w/
> 	a full HMAC-SHA1?
>
> 	If you are rejecting a TSIG because the number of bits is
> 	too small what is the RCODE in the TSIG?  BADAUTH+????
>
> 	Is the server supposed to auto adjust to the client within
> 	limits (max(80 bits, half digest bits)) or is the server
> 	supposed to specify a absolute number of expected bits or
> 	a minimum number of expected bits.
>
> 	How does a server, if it rejecting because of too few bits,
> 	signal the minimum number of bits it will accept for this
> 	key.
>
> 	Is truncation just too much of a complication?
> -- 
> Mark Andrews, ISC
> 1 Seymour St., Dundas Valley, NSW 2117, Australia
> PHONE:	+61 2 9871 4742		         INTERNET: [email-masked]
>
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