Re: The TCP and UDP checksum algorithm may soon need updating

Nico Williams <nico@cryptonector.com> Tue, 09 June 2020 19:39 UTC

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Date: Tue, 9 Jun 2020 14:39:34 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Craig Partridge <craig@tereschau.net>
Cc: IETF discussion list <ietf@ietf.org>
Subject: Re: The TCP and UDP checksum algorithm may soon need updating
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On Tue, Jun 09, 2020 at 01:31:58PM -0600, Craig Partridge wrote:
> Following up on notes from various folks, most notably John K. and
> Christian H., about checksums and encryption-based integrity protection, I
> wanted to make a couple of clarifying points.
> 
> * If you are not worried about an adversary who seeks to alter your data in
> transit, then integrity protection is a lousy method for ensuring the
> received data is correct.  The reality is that a checksum of width X bits
> can potentially catch 10x as many errors as an integrity check of X bits
> and will likely require 1/X computational power to compute.  The
> requirement to make an integrity check proof against an adversary makes it
> less good. [Happy to explain more -- just trying to keep this note short].

While this is true, application developers who are affected by weak TCP
checksums don't really have a better option, since TLS is basically all
they really have off the shelf, they will get a) [many] more bits of
MAC, b) therefore more protection than a 16-bit checksum could provide,
c) more compute cost.

> * Some folks have assumed the TCP checksum, at 16 bits, misses an error
> about 1 in 2^16 times.  That's not correct.  There are classes of errors
> that TCP catches 100% of the time (e.g. single-bit errors).  There are also
> classes of errors that the TCP checksum misses about 1 in 2^10 times.  This
> latter is because the TCP checksum is not terribly good -- a better
> designed checksum would miss only about 1 in 2^16.

2-bit errors do happen.

Nico
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