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Re: [TLS] Enforcing stronger server side signature/hash combinations in TLS 1.2

mrex@sap.com (Martin Rex) Fri, 24 March 2017 15:40 UTC

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In-Reply-To: <7970606B-0096-475D-8842-2A14E5168413@dukhovni.org>
To: TLS WG <tls@ietf.org>
Date: Fri, 24 Mar 2017 16:40:34 +0100
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From: mrex@sap.com
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Subject: Re: [TLS] Enforcing stronger server side signature/hash combinations in TLS 1.2
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Viktor Dukhovni wrote:
> 
> > On Mar 24, 2017, at 1:08 AM, Martin Thomson <martin.thomson@gmail.com> wrote:
> > 
> >> I've never seen
> >> a TLS server that has multiple chains to choose from for the same
> >> server identity.
>
  [ https://www.cloudflare.com/  ]
> 
> Both chains of course use SHA256.

Actually, looking at the DigiCert issued ECC cert for www.cloudflare.com
I'm a little confused.

This is the cert chain (as visualized by Microsoft CryptoAPI):

  server-cert:  CN=cloudflare.com, ...
                contains ECDSA P-256 public key
                is allegedly signed with sha256ECDSA

  intermediate CA:  CN=DigiCert ECC Extended Validation Server CA
                contains ECDSA P-384 public key
                is allegedly signed with sha384RSA

  root CA:      CN=DigiCert High Assurance EV Root CA
                contains RSA 2048-bit public key
                is self-signed with sha1WithRsaEncryption

For those who insist on reading rfc5246 verbatim, this chain requires

   ECDSA+SHA384:RSA+SHA384:RSA+SHA1


The digital signature on the server certificate looks bogus to me,
that should be a sha384ECDSA signature according to NIST, because
it uses a P-384 signing key.

The signature on the intermediate CA is imbalanced, and
should be sha256RSA rather than sha384RSA. (that is only an interop issue,
not a security issue).


-Martin

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