Re: [TLS] Doubts in draft-ietf-tls-rfc4492bis

I believe what Raja is pointing out is that a server which accepts an ECDSA
*client* certificate has no way to advertise to the client which curves it
accepts. The signature algorithm list (and before TLS 1.2, the certificate
types) do advertise ECDSA as a whole, but not curves. E.g., a client with
both a P-256 and P-384 certificate may send P-384 when the server only
accepted P-256. This issue existed in RFC 4492 as well.

Though I wouldn't say the implication is all curves must be implemented.
Rather I think you just reject those curves you don't accept and manage
your client certificate deployment so that all servers accept a curve
before starting to use those certificates.

That's not great, so TLS 1.3 fixes this by moving ECDSA curves into
signature algorithms. It's too late to change supported_groups to allow a
TLS 1.2 ServerHello acknowledgement since clients will unexpected server
extensions[*], so I would suggest we just leave this in the awkward state
for TLS 1.2 and say it is fixed in TLS 1.3.

David

[*] Although, glancing through ours, it seems we do accept and ignore a
ServerHello supported_groups in TLS 1.2. We apparently came across a server
implementation which sent it, contrary to the spec.

On Mon, Jul 24, 2017 at 10:58 AM Ilari Liusvaara <ilariliusvaara@welho.com>
wrote:

> On Mon, Jul 24, 2017 at 01:48:13PM +0000, Raja ashok wrote:
> > Hi Nir, Josefsson & Pegourie,
> >
> > As per section 5.2 server should send only "Supported Point Format"
> > extensions in server hello message. And it doesn't require to send
> > "Supported Elliptic Curve" extensions. Because of this if server is
> > supporting only few Curves and if it receives unsupported Elliptic
> > curve in client certificate message, then server might not be able
> > to continue the handshake.
>
> In TLS 1.2, the client sends the list of curves (and other groups
> and DHFs) it supports. The server picks one if it can.
>
> Thus if there is at least one common curve that both client and
> server support, then the group selection will succeed (if there
> is none, then no matter what one does things won't work).
>
> The actual curve server selected is transmitted in ServerKeyExchange
> message.
>
>
> In TLS 1.3, things get bit more complicated, since client can
> signal it supports a group without sending a share for it (if
> server selects such group, it needs to tell the client to retry
> using HelloRetryRequest message). The server group selection is
> in KeyShare extension in ServerHello message.
>
>
> -Ilari
>
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