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

mrex@sap.com (Martin Rex) Thu, 19 September 2013 09:54 UTC

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Date: Thu, 19 Sep 2013 11:54:34 +0200 (CEST)
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From: mrex@sap.com (Martin Rex)
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Subject: [TLS] which alert for TLS client cert w/o keyUsage digitalSignature
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I'm wondering which ssl/tls alert would be the most appropriate
for a TLS server to send if the server requested a client cert
and the client presents a certificate with a keyUsage that
does not have digitalSignature asserted.

OpenSSL seems to send "illegal parameter" alert when processing
CertificateVerify.  I might have chosen "bad certificate" alert.

A related issue would be that a TLS server is using a server
certificate with an RSA key and a keyUsage attribute with just
keyEncipherment.  Such a TLS server cert seems to preclude
its use for DHE_RSA and ECDHE_RSA cipher suites.  What would
be the appropriate alert (for the client to send) if the server
had such a certificate and chose a DHE_RSA or ECDHE_RSA cipher
suite anyway?

It is my impression that there exists CA software that creates
RSA certificates with ExtendedKeyUsage id-kp-serverAuth and
keyUsage limited to keyEncipherment by default.

Do we have a rough idea how common such TLS server certs are
(e.g. from SSL Observatory database)?