Re: [TLS] which alert for TLS client cert w/o keyUsage digitalSignature

Michael D'Errico <mike-list@pobox.com> Thu, 19 September 2013 17:01 UTC

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Date: Thu, 19 Sep 2013 10:01:29 -0700
From: Michael D'Errico <mike-list@pobox.com>
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To: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
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Subject: Re: [TLS] which alert for TLS client cert w/o keyUsage digitalSignature
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Daniel Kahn Gillmor wrote:
> On 09/19/2013 12:10 PM, Michael D'Errico wrote:
>> Wan-Teh Chang wrote:
>>> Note: I don't understand why RFC 4492 says "It MUST be signed with RSA."
>> A certificate authority could, in theory, sign a certificate using
>> any algorithm it chooses.  Since the client is known to have code
>> for dealing with RSA (it asked for ECDHE_RSA) but not whether it
>> has code to validate any other type of signature algorithm, it makes
>> (practical) sense to limit the CA signature to RSA.
> 
> But do we limit the certificate signature in any other context?  if not,
> why limit it here?  for example, if the EE certificate is signed by an
> intermediate CA using RSA, but that CA's own cert is signed using DSA or
> ECDSA, is that OK?
> 
> Why haven't we applied these same-algorithm constraints to EE certs used
> in DHE_RSA or RSA key exchanges for that matter?
> 
> I can see why the spec might recommend the use of RSA signatures on
> certs that supply RSA keys (for the implementation-simplifying reasons
> you propose), but making it an RFC 2119 MUST is pretty strong stuff, and
> weirdly inconsistent with the rest of the specs, as i understand them.

Page 47 of RFC 5246 hints at these rules:

    If the client does not send the signature_algorithms extension, the
    server MUST do the following:

    -  If the negotiated key exchange algorithm is one of (RSA, DHE_RSA,
       DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had
       sent the value {sha1,rsa}.

    -  If the negotiated key exchange algorithm is one of (DHE_DSS,
       DH_DSS), behave as if the client had sent the value {sha1,dsa}.

    -  If the negotiated key exchange algorithm is one of (ECDH_ECDSA,
       ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}.

Prior to TLS 1.2 it could be assumed that all clients supported RSA, and
if any supported DSA, they would be asking for DSS cipher suites.  Once
people started wanting to use new algorithms (ECC), it became necessary
to have a way for a client to indicate which algorithms it could process.

Mike