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Re: [TLS] Deprecate SHA1 for signatures in TLS 1.3 (was Re: TLS 1.3 draft-07 sneak peek)

Andrei Popov <Andrei.Popov@microsoft.com> Tue, 14 July 2015 20:06 UTC

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From: Andrei Popov <Andrei.Popov@microsoft.com>
To: Martin Thomson <martin.thomson@gmail.com>
Thread-Topic: [TLS] Deprecate SHA1 for signatures in TLS 1.3 (was Re: TLS 1.3 draft-07 sneak peek)
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Subject: Re: [TLS] Deprecate SHA1 for signatures in TLS 1.3 (was Re: TLS 1.3 draft-07 sneak peek)
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If opportunistic TLS handshakes need to be indistinguishable from server-authenticated TLS handshakes, then perhaps opportunistic clients have no choice but to send the signature_algorithms extension (when offering TLS1.2). The absence of signature_algorithms in a TLS 1.2 ClientHello can be used as a signal by the MITM.

-----Original Message-----
From: Martin Thomson [mailto:martin.thomson@gmail.com] 
Sent: Tuesday, July 14, 2015 12:56 PM
To: Andrei Popov
Cc: tls@ietf.org
Subject: Re: [TLS] Deprecate SHA1 for signatures in TLS 1.3 (was Re: TLS 1.3 draft-07 sneak peek)

On 14 July 2015 at 12:30, Andrei Popov <Andrei.Popov@microsoft.com> wrote:
> The downside is of course that the attacker can easily distinguish opportunistic clients from server-authenticating ones. Is this a concern for the opportunistic TLS community?

I raised the concern about this previously.  Opportunistic MitM happens, and providing a strong signal that the connection won't be (or couldn't be) authenticated somehow is a problem for that.  I'd rather have opportunistic security be indistinguishable from "real"
security.  It also means that you don't have separate code paths to support.

The anonymous modes serve a different purpose.  For instance tcpinc could use them.

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