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

Daniel Kahn Gillmor <dkg@fifthhorseman.net> Thu, 19 September 2013 16:18 UTC

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Date: Thu, 19 Sep 2013 12:18:15 -0400
From: 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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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.

	--dkg