RE: MD5 is slow

[Vasilenko Eduard via NANOG <nanog () lists nanog org>]

Hi Ytti,
Hi Dan,
Your comments on the performance are very important.
I still believe that any Hash must be slow enough, because if it were fast, then the attacker could take a big GPU and 
brute force it
(The routing message is very predictable; only the password is not known, but could be tested from the dictionary).
But what is slow? us or ms?
In support of the latter, look to https://www.ijcna.org/Manuscripts/IJCNA-2020-O-01.pdf.
It is hundreds of cycles per byte.
Acceleration helps, but not much (around 3x) https://github.com/minio/sha256-simd/blob/master/README.md.
A few milliseconds per every hop is expensive.
Eduard
-----Original Message-----
From: Saku Ytti <saku () ytti fi> 
Sent: Friday, September 5, 2025 18:55
To: North American Network Operators Group <nanog () lists nanog org>
Cc: Vasilenko Eduard <vasilenko.eduard () huawei com>
Subject: Re: MD5 is slow

On Fri, 5 Sept 2025 at 10:22, Vasilenko Eduard via NANOG <nanog () lists nanog org> wrote:

> Any hash MUST be slow (by design) to withstand brute force. In the network device case, it is about 5ms for SHA-2 (of 
> course, dependent on the control plane processor).

Out of curiosity, how did you arrive at 5ms?  I don't think it is important, but it is interesting to me.

I'm more arriving at around 1us on core from <10years ago (w/ SHA instruction set) or 10us on older core per ISIS LSA.

But we can't still include even this 1us or 10us to the convergence budget, because NOS almost always has most of the 
cores doing nothing, due to poor design and no commercial pressure to improve. So if this would actually matter, you 
could at the first point when receiving LSA call sha_validate on another core with access to a shared pointer to 
boolean sha_valid=false, which this other core sets to true, upon validating SHA. Then the original core which is 
guaranteed to do work exceeding 1us or 10us for that LSA will continue its work, and finally check that sha_valid is 
true, if not reject the work it did, making the integrity validation free provided it takes less time to validate the 
integrity than it takes to calculate the topology.

--
  ++ytti
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