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

Eric Rescorla <ekr@rtfm.com> Tue, 07 July 2015 17:46 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 07 Jul 2015 10:45:25 -0700
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To: Dave Garrett <davemgarrett@gmail.com>
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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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On Tue, Jul 7, 2015 at 9:57 AM, Dave Garrett <davemgarrett@gmail.com> wrote:

> On Tuesday, July 07, 2015 12:42:22 pm Dave Garrett wrote:
> > How does it prohibit SHA1 in certs? The commit message might be unclear,
> but it only modifies the signature algorithms extension.
>
> The relevant commit with a fixed message:
>
>
> https://github.com/davegarrett/tls13-spec/commit/56e481f19ed4d2ed374a6fb6e24a66637eb07b3c


http://tlswg.github.io/tls13-spec/#server-certificate:

If the client provided a “signature_algorithms” extension, then all
certificates provided by the server MUST be signed by a hash/signature
algorithm pair that appears in that extension. Note that this implies that
a certificate containing a key for one signature algorithm MAY be signed
using a different signature algorithm (for instance, an RSA key signed with
a DSA key).
So if you can't express SHA1 in signature_algorithms, then the server can't
send you a certificate signed with SHA-1.

-Ekr


> Dave
>

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