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Re: [TLS] TLS and KCI vulnerable handshakes

Karthikeyan Bhargavan <karthik.bhargavan@gmail.com> Tue, 11 August 2015 18:25 UTC

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From: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
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Date: Tue, 11 Aug 2015 14:25:42 -0400
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References: <55C8CD7A.7030309@rise-world.com> <9A043F3CF02CD34C8E74AC1594475C73F4AD80F3@uxcn10-5.UoA.auckland.ac.nz>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
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Cc: Clemens Hlauschek <clemens.hlauschek@rise-world.com>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] TLS and KCI vulnerable handshakes
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>> https://www.usenix.org/conference/woot15/workshop-program/presentation/hlauschek
> 
> Some comments on this, it looks like it requires a "cert with static (EC)DH
> key" in order to work, which would mean an X9.42 cert.  Since no (public) CA
> that I know of can handle or issue such certs, this probably provides a
> reasonable amount of defence against this attack…

No, a regular ECDSA certificate would do.
That is, the attack would work as long as 
- a client has an ECDSA certificate, and
- it enables any static TLS_ECDH_* cipher suite, and
- its ECDSA private key has been stolen (or chosen) by an attacker.

Best,
Karthik


> 
> In terms of the suggested countermeasures:
> 
>> Set appropriate X509 Key Usage extension for ECDSA and DSS certificates, and
>> disable specifically the KeyAgreement flag
> 
> Since the keyUsage flags are widely ignored by implementations, this won't
> provide the protection that the text implies.
> 
> Peter.
> _______________________________________________
> TLS mailing list
> TLS@ietf.org
> https://www.ietf.org/mailman/listinfo/tls

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