Hi,
- I cannot find anything in RFC 8446 or draft-ietf-tls-esni-15 on reuse of key
shares but my understanding from earlier discussions in the TLS WG is that this
is allowed as summarized in draft-ietf-tls-hybrid-design: "TLS 1.3 does not
require that ephemeral public keys be used only in a single key exchange
session; some implementations may reuse them"
Server reuse of key shares seem very problematic with ECH and seems to leak the
server identity to an passive or active attacker.
Client Attacker Server
ClientHello
+ ech ------>
ServerHello
+ key_share
<-------
(intercept)
...
ClientHello ------->
ServerHello
+ key_share
<-------
(compare key shares)
Figure X: Active attacker identifying server resuing key share
Client1 Client2 Attacker Server
ClientHello
+ ech ------>
ServerHello
+ key_share
(intercept)
...
ClientHello
------> (intercept SNI)
ServerHello
+ key_share
(compare key shares)
Figure Y: Passive attacker identifying server resuing key share
My current understanding is that draft-ietf-tls-esni should require that the
server MUST NOT reuse a key shares. Unless I miss something I suggest adding
that and one or two of the above figures to the draft. An alternative solution
would be to extend the ECH encryption to also cover ServerHello. If I
understand ECH correctly, the ServerHello is still unencrypted.
- As a main goal of ECH is to hide the server name, I think the draft should
explicitly mention padding of the server certificate message. Some web servers
are using very large certificate messages. If padding is not used, they can be
identified with a quite high probability just by looking at the size.
Cheers,
John
From: TLS <[email protected]> on behalf of Christopher Wood
<[email protected]>
Date: Monday, 3 October 2022 at 16:12
To: [email protected] <[email protected]>
Subject: Re: [TLS] New Version Notification for draft-ietf-tls-esni-15.txt
This is mainly a keep-alive update, with some additional details summarizing
the results from an upcoming paper to appear at ACM CCS 2022.
Best,
Chris, for the editors
On Mon, Oct 3, 2022, at 7:56 AM, [email protected] wrote:
> A new version of I-D, draft-ietf-tls-esni-15.txt
> has been successfully submitted by Christopher Wood and posted to the
> IETF repository.
>
> Name: draft-ietf-tls-esni
> Revision: 15
> Title: TLS Encrypted Client Hello
> Document date: 2022-10-03
> Group: tls
> Pages: 48
> URL:
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-43f8a48e256f1fa7&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fwww.ietf.org%2Farchive%2Fid%2Fdraft-ietf-tls-esni-15.txt
> Status:
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-d964cabe684fd6b8&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fdraft-ietf-tls-esni%2F
> Html:
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-bd77e6206604f594&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fwww.ietf.org%2Farchive%2Fid%2Fdraft-ietf-tls-esni-15.html
> Htmlized:
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-9a2e632545b77f65&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fhtml%2Fdraft-ietf-tls-esni
> Diff:
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-cb651e94574b6bd5&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fwww.ietf.org%2Frfcdiff%3Furl2%3Ddraft-ietf-tls-esni-15
>
> Abstract:
> This document describes a mechanism in Transport Layer Security (TLS)
> for encrypting a ClientHello message under a server public key.
>
> Discussion Venues
>
> This note is to be removed before publishing as an RFC.
>
> Source for this draft and an issue tracker can be found at
>
> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-e32d7d92817bd444&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fgithub.com%2Ftlswg%2Fdraft-ietf-tls-esni
>
> (https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-e32d7d92817bd444&q=1&e=665b2e5c-6b5e-43fa-8eaa-4cdc64ce6ae3&u=https%3A%2F%2Fgithub.com%2Ftlswg%2Fdraft-ietf-tls-esni).
>
>
>
>
> The IETF Secretariat
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