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Re: [TLS] Certificate keyUsage enforcement question (new in RFC8446 Appendix E.8)

Geoffrey Keating <geoffk@geoffk.org> Tue, 06 November 2018 04:11 UTC

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From: Geoffrey Keating <geoffk@geoffk.org>
Date: Mon, 05 Nov 2018 20:11:12 -0800
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Subject: Re: [TLS] Certificate keyUsage enforcement question (new in RFC8446 Appendix E.8)
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Viktor Dukhovni <ietf-dane@dukhovni.org> writes:

> TL;DR:  Should TLS client abort DHE-RSA handshakes with a peer
> certificate that *only* lists:
> 
>             X509v3 Key Usage: 
>                 Key Encipherment, Data Encipherment

Yes, because in DHE-RSA, the RSA key is used for signing, and this is
an encryption-only key.

It's much more important in the DHE-ECDSA case, because using an
encryption-only EC key for signing can lead to key compromise (IIRC).
As far as I know there's no similar attack on RSA, but I think this is
not a well-examined area.

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