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

Michael D'Errico <mike-list@pobox.com> Thu, 19 September 2013 16:11 UTC

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Date: Thu, 19 Sep 2013 09:10:45 -0700
From: Michael D'Errico <mike-list@pobox.com>
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To: Wan-Teh Chang <wtc@google.com>
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Subject: Re: [TLS] which alert for TLS client cert w/o keyUsage digitalSignature
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Wan-Teh Chang wrote:
> 
> RFC 4492 says:
> 
>           Key Exchange Algorithm  Server Certificate Type
>           ----------------------  -----------------------
> 
>           ...
> 
>           ECDHE_RSA               Certificate MUST contain an
>                                   RSA public key authorized for
>                                   use in digital signatures.  It
>                                   MUST be signed with RSA.
> 
> 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.

Mike

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