[TLS] TLS ECH, how much can the hint stick out?

[Christian Huitema <[e-mail]>]

The ECH proposal for Encrypted SNI is almost ready, but for a very small
debate. The original proposal was using trial description to distinguish
between ECH aware responses to the encrypted inner Client-Hello from non
ECH aware response to the "cover" outer CH. This is problematic in the
QUIC use case. The latest proposal is to embed a "hint" in 8 bytes of
the Server Random. The proposal was for ECH-aware servers to use eight
bytes of a hash of the inner Client Random as a hint. Analysis shows
that this enables two attacks:

1) If the Client Hello is replayed, the same hint will be present in the
response to the original CH and to the response to the copy, providing
observers with a clear indication that ECH was used.

2) If the Client Hello is intercepted by an MITM attacker, the attacker
can rewrite the server's response before presenting it to the client.
The attacker formats its own Server Hello that reuses the Server Random
from the original server response, but use its own key share, etc. The
hint will cause the ECH aware client to create an handshake key using
the inner CH. In a QUIC set up, the MITM attacker can easily detect
that, before even transmitting the server's certificate. Thus, the MITM
attacker can detect usage of ECH.

We have a simple proposal that solves the replay attack: set the hint as
a hash of both the inner Client Random and the "non-hint" bytes of the
Server Random. That's clearly a good idea, but it does not solve the
active MITM attack. Solving that requires tying the hint with the
handshake key derived by the original server, for example by hashing the
inner Client Random, the "non-hint" bytes of the Server Random, and the
server key share. That's harder to implement, so the question is about
cost and opportunity.

This all relates to how much ECH is "sticking out". The current stance
is that a passive attacker cannot distinguish between a client using ECH
to access a hidden SNI and a client merely greasing the ECH extension.
The observation is that there may be many potential active attacks,
especially if the server shares its ESNI/ECH configuration publicly. If
there are many such attacks, and if defense of the hint against MITM is
hard to implement, implementing the defense might make the code more
fragile, with little actual gain. But I am not convinced by that
argument, because it smells a lot like "the other side of the boat is
leaking too, why should I plug this particular leak?"

And so, at Chris Wood's request, I am sending this message to the list.

-- Christian Huitema