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Re: [TLS] Deprecating TLS 1.0, 1.1 and SHA1 signature algorithms

Karthikeyan Bhargavan <karthik.bhargavan@gmail.com> Tue, 12 January 2016 08:58 UTC

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From: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
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Date: Tue, 12 Jan 2016 09:58:15 +0100
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To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
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Subject: Re: [TLS] Deprecating TLS 1.0, 1.1 and SHA1 signature algorithms
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> I'm aware of that (and related) work, but this is about finding
> multicollisions in MD5 || SHA1.

To be clear, there is no published collision on MD5 || SHA1 right now.

In our paper, we only say that *if SHA-1 collisions were to appear* with complexity 2^x, 
then MD5||SHA1 collisions would cost 2^(6+x). Hence, if the current estimate of 2^61 for SHA1
were true, then the cost of MD5||SHA1 is 2^67. 

It is up to protocol designers and implementers to decide whether this is an acceptable security margin.
If we decide to wait for a “real” SHA-1 collision to appear, then we must be prepared for “real” attacks to appear soon after.

Best,
Karthik

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