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

mrex@sap.com (Martin Rex) Tue, 12 January 2016 13:24 UTC

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To: Tony Arcieri <bascule@gmail.com>
Date: Tue, 12 Jan 2016 14:24:31 +0100 (CET)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] Deprecating TLS 1.0, 1.1 and SHA1 signature algorithms
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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



> 
> My understanding is TLS 1.2 specifically was amended to allow MD5
> signatures even though this was not the case in previous TLS versions, or
> at least that was the claim of the miTLS presenters on SLOTH at
> RealWorldCrypto 2016.
> 
> If this is the case, this seems like a big regression in TLS 1.2.

And a fairly well-known & discussed regression, e.g.

http://www.ietf.org/mail-archive/web/tls/current/msg10664.html

that was subsequently removed in OpenSSL 1.0.1f in January 2014,
i.e. 2 years before the SLOTH paper.


I'm also wondering whether it might be misleading to lump the
(in)significance of the currently known collisions for HMAC-SHA1
and HMAC-MD5 together with the (in)significance for 
(general, low-frequent) digital signatures and together with
PKCS#10 & Certificate-issuance design flaw that enables a
mere collision attack to achieve what would normally require
a successful 2nd preimage attack.

Compare the Security Considerations of rfc2104 for the (in)significance
of current collision attacks for HMAC.

https://tools.ietf.org/html/rfc2104#section-6


-Martin