Curves and ECDHParameters

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Curves and ECDHParameters

Joseph Christopher Sible
What exactly are each of "Curves" and "ECDHParameters" used for, as
documented by https://www.openssl.org/docs/man1.0.2/ssl/SSL_CONF_cmd.html?

My understanding of elliptic curves in TLS is that they're used in two
places: as ECDSA key pairs used in certificates, and in ECDHE for key
exchange. (Are there more uses I'm not aware of?)

I know the curve used for ECDSA is a property of the key pair associated
with the certificate, so it doesn't make sense to be a setting controlled
at runtime. My best guess is that the curve for ECDHE is controlled by
ECDHParameters. Given all of this, I can't figure out what's left for the
"Curves" parameter to control.

Are my above assumptions right? If so, what does "Curves" control?

Joseph C. Sible
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Re: Curves and ECDHParameters

Viktor Dukhovni
> On Sep 11, 2018, at 4:55 PM, Joseph Christopher Sible <[hidden email]> wrote:
>
> What exactly are each of "Curves" and "ECDHParameters" used for, as
> documented by https://www.openssl.org/docs/man1.0.2/ssl/SSL_CONF_cmd.html?

The documentation of OpenSSL 1.1.x does not mention "ECDHParameters",
only "Curves" is documented as a synonym of "Groups".

The 1.0.2 documentation for "ECDHParameters" explains that this is
server-side setting to select a particular *fixed* ECDHE curve.
This is a legacy feature that predates negotiation of the curve
used based on the client's extension.

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        Viktor.

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Re: Curves and ECDHParameters

Viktor Dukhovni


> On Sep 11, 2018, at 6:20 PM, Viktor Dukhovni <[hidden email]> wrote:
>
>
> The 1.0.2 documentation for "ECDHParameters" explains that this is
> server-side setting to select a particular *fixed* ECDHE curve.
> This is a legacy feature that predates negotiation of the curve
> used based on the client's extension.

That said, in 1.0.2, it may be necessary to set "ECDHParameters" to
"Automatic" in order to enable ECDHE with Curve negotiation based
on the (separately specified) Curves.  I am not sure whether
automatic ECDHE is on by default in 1.0.2, IIRC it may not be.

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        Viktor.

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