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From Dr Stephen Henson <>
Subject Re: DH params and multiple certificates
Date Thu, 20 Feb 2014 03:06:09 GMT
On 19/02/2014 17:30, Falco Schwarz wrote:
> As of standardized DH parameters were added to mod_ssl. If I
understand docs correctly, the bit length is based on the RSA/DSA key. With the recent support
of multiple certificates per VirtualHost it is possible to use an RSA and ECC certificate.
> When using RSA and ECC, the DH bit length is dependent on the certificate configured
last, so:
> - if ECC is configured last, 1024 bit DH params are set (minimum)
> - if RSA is configured last, the DH params depend on the private keys bit length (it
> The ECC certificate should in any way be skipped and not taken into account when setting
DH params. 		 	   		  

I think that's a consequence of how SSL_get_certificate works. You get the
current certificate which is not necessarily the one the server actually uses.

Ugly workaround for ephemeral DH ciphersuites... if the cipher is of the form
*-RSA-* it's the RSA certificate and *-DSS-* it's the DSA one. No ciphersuites
exist using ephemeral DH and ECDSA.

We should probably have a way to set the current certificate in OpenSSL to the
one the server used.

Dr Stephen Henson. OpenSSL Software Foundation, Inc.
1829 Mount Ephraim Road
Adamstown, MD 21710
+1 877-673-6775

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