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Re: [Acme] S/MIME and ACME: concerns about dual certificates (maybe pull the S/MIME draft back to AC

Benjamin Kaduk <kaduk@mit.edu> Thu, 26 November 2020 03:14 UTC

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Date: Wed, 25 Nov 2020 19:14:12 -0800
From: Benjamin Kaduk <kaduk@mit.edu>
To: Sebastian Nielsen <sebastian=40sebbe.eu@dmarc.ietf.org>
Cc: 'Mailing List' <acme@ietf.org>
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Subject: Re: [Acme] S/MIME and ACME: concerns about dual certificates (maybe pull the S/MIME draft back to AC
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Hi Sebastian,

On Fri, Nov 20, 2020 at 07:05:08PM +0100, Sebastian Nielsen wrote:
> The certificates can have different extended key usages, either digital
> signature or encryption - and thus an email client will automatically pick
> the right certificate.
> 
> However, the reason the same key shouldn't be used for both signing and
> encryption, is that in textbook RSA, it exist sheninigans, where a attacker
> could send an snooped encrypted message, to the receiver, but blinded with a
> random number. (multiplied).

While this may be *a* reason, I don't think it's the only reason.  Another
reason that comes up with some regularity is that isolating a given key to
a single usage greatly simplifies formal analysis of the containing
protocol's security properties, as cross-protocol and cross-algorithm
attacks are definitionally out of scope.

Thanks,

Ben

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