Re: [TLS] WGLC for draft-ietf-tls-md5-sha1-deprecate

Hubert Kario <> Fri, 22 November 2019 19:18 UTC

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From: Hubert Kario <>
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Date: Fri, 22 Nov 2019 20:18:47 +0100
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Subject: Re: [TLS] WGLC for draft-ietf-tls-md5-sha1-deprecate
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On Friday, 22 November 2019 03:25:24 CET, David Benjamin wrote:
> On Fri, Nov 22, 2019 at 8:35 AM Salz, Rich <> wrote:
>>> ...
>> SHA-1 signature hashes in TLS 1.2" draft available
>> Please review the document and send your comments to the list by 2359 UTC
>> on 13 December 2019.
>> I just re-read this.  Looks good. Perhaps a sentence of rationale in ...
> To that end, the combination of client advice in sections 2 and 4 is a bit
> odd. Section 2 uses SHOULD NOT include MD5 and SHA-1, but section 4 says
> the client MUST NOT accept the MD5 SHA-1, even if it included it. Why would
> the client include it in that case? It seems the two should either both be
> MUST NOT or both be SHOULD NOT.

because it also influences certificate selection, and getting a certificate
signed with SHA-1 isn't an automatically disqualifying property?
(it may be an intermediate CA that's not used, it may be an explicitly 
certificate, etc.)

> I think client-sent alerts in section 4 are also wrong. handshake_failure
> means the sender was unable to negotiate parameters, and
> insufficient_security is a variant of handshake_failure. This is the client
> reacting to the server sending something inconsistent with the ClientHello,
> so it should be illegal_parameter. In the context of ServerKeyExchange
> signatures, handshake_failure or insufficient_security would be sent by the
> *server* if the *client* only advertised MD5 and SHA-1.

well, it depends if the SHA-1 was advertised by client or not

if it was advertised (because of certificates, see above), then
handshake_failure is correct; if it wasn't advertised but the
signature_algorithms were included, then yes, client should abort with

Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic