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

Hubert Kario <hkario@redhat.com> Tue, 12 January 2016 14:13 UTC

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From: Hubert Kario <hkario@redhat.com>
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Date: Tue, 12 Jan 2016 15:13:26 +0100
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Subject: Re: [TLS] Deprecating TLS 1.0, 1.1 and SHA1 signature algorithms
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On Tuesday 12 January 2016 14:24:31 Martin Rex wrote:
> Tony Arcieri wrote:
> [ Charset UTF-8 unsupported, converting... ]
> 
> > Peter Gutmann <pgut001@cs.auckland.ac.nz>; wrote:
> >> The vulnerabilities shown in the SLOTH paper were based on the fact
> >> that implementations still allow MD5 for authentication/integrity
> >> protection, even if (for example) it's explicitly disabled in the
> >> config. So the problem wasn't a fault in the protocol, it's buggy
> >> implementations (as it was for ones that allowed 512-bit keys,
> >> non-prime primes,>> 
> >>  and so on).  Throwing out TLS 1.1 based on this seems rather
> >>  premature.
> Actually no, the TLSv1.2 made a few terribly braindead design choices
>   - newly introduce raw md5RSA digital signatures into TLSv1.2 in 2008
> where all prior TLS protocol versions, including SSLv3 had been using
> the concatenation SHA-1||MD5
>   - making the sha1RSA rather than sha256RSA digital signature
> algorithm the default and mandatory-to-implement algorithm for use
> with TLSv1.2(!!) although it was well-known weaker than the algorithm
> (SHA-1||MD5) in all earlier TLS protocol versions, including SSLv3,
>     and in spite of SHA-1 already being officially scheduled for
> end-of-life 2 years later (NIST, SP800-57 pt.1 rev2)
>     This is ridiculous considering that SHA-256 is mandatory-to-use
>     in the TLSv1.2 PRF.
>   - failing to adjust the truncation of the HMAC output in the
>     TLSv1.2 Finished handshake message to be at least half the size of
> the underlying hash function (SHA-256), see RFC 2104 Section 5:
> 
> https://tools.ietf.org/html/rfc2104#section-5

the problem stems from the fact that the same field is used for 
announcing support for signatures in ServerKeyExchange *and* for 
certificates provided by server.

while SKE signatures could have easily been made mandatory to SHA-256 at 
least, the depreciation of SHA-1 signatures for certificates certainly 
wasn't possible at the time - only now we are closing on migration from 
them

so, it was a _bad_ decision, but calling it a "braindead" one is a bit 
over the top, sorry
-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purky┼łova 99/71, 612 45, Brno, Czech Republic