Subject: [TLS] Comments on draft-ietf-tls-tls13-18

From: Watson Ladd <[email-masked]>
Date: Mon, 31 Oct 2016 11:41:46 -0700

Hello,

Looking at the history of TLS implementation attacks we see that many
result from the standard not strictly enough prescribing behavior,
particularly of the state machine. This draft does not actually fix
this problem, but continues to present example flows without
explicitly requiring them to be the only possible flows.

For example, consider HelloRetryRequest. Do servers only send one of
these per connection, or can it be resent multiple times? Obviously
there is a DoS possibility here, but I do not see where this behavior
is explicitly defined. I think we should require that the server only
ever sends one HelloRetryRequest, and then terminates the connection
if the ClientHello is unacceptable. At no point is it stated that only
the example flows should be supported. I would prefer more clarity
about what messages are to be expected when, especially with alerts.

ECDSA cannot be used with x25519 or x448, but the draft seems to
require support in TLS 1.2 for this as a consequence of its
description of the fallback mode. The mandated use of uncompressed
points invites easy wrong-curve attacks: mandating support and use for
compressed points would solve this problem. I think at minimum we
should fix the text to make clear what we mean about TLS 1.2.

ALPN, resumption, and 0-RTT remain problematic. For instance we see
that 0-RTT data is sent with the same ALPN state when the PSK was
derived, but this could be different from the ALPN transmitted and
negotiated during the handshake, which is explicitly allowed later in
the document. I do not understand what is supposed to happen in this
scenario.

Appendix B removes the text about upper-layer protocol interactions
with 0-RTT I provided, merely discussing the API. I think this is a
mistake: how 0-RTT should be used safely depends on the upper-layer
protocol, and can be complex. API guidance is not enough.

There is still a note about needing a channel binding mechanism in the
text. I think this should be resolved soon so it can be analyzed,
probably built on top of the exporter mechanism. Either that, or we
consciously punt and remove the note and replace with something else.

As for process, I support the idea of having a last call on November
20th, and then completing the security analysis by January 20th (or
whatever date was decided). This will prevent a flurry of changes
potentially breaking things.

Sincerely,
Watson Ladd

---

From: Ilari Liusvaara <[email-masked]>
Date: Mon, 31 Oct 2016 21:28:13 +0200

> should fix the text to make clear what we mean about TLS 1.2.

Just as note: Point compression does not completely fix wrong-curve
attacks.

The reason is that for Weierstrass curves and the usual square square
root algorithm, the "square roots" for QNRs, if the code does not check,
don't result points on the twist, but all sorts of strange points, on
variety of different curves.

(With one-coordinate Montgomery ladder on montgomery curves, the invalid
points all map to the twist, which is a single curve and can be arranged
to have a large prime factor in its order).

Oh, and thanks to different message order in TLS 1.3 compared to TLS
1.2, wrong-curve attacks look more exploitable.

> the document. I do not understand what is supposed to happen in this

Well, the spec prohibits accepting 0-RTT data if ALPN does not match,
but this looks bit error prone to me.

I proposed forcing the ALP to the previous value, and not allowing it
to be negotiated via any method. This would eliminate this sort of
edge case check.

> consciously punt and remove the note and replace with something else.

I think this is a replacement for the tls-unique channel binding. While
the definition of tls-unique does still make sense for TLS 1.3, the
definition has security problems with TLS 1.2.

Thus, need for replacment.

-Ilari

---

From: Eric Rescorla <[email-masked]>
Date: Mon, 31 Oct 2016 14:28:00 -0700

Watson,

Thanks for your comments!

> if the ClientHello is unacceptable.

The server is forbidden to send multiple HRRs and the client is required
to enforce it. See:

  https://tlswg.github.io/tls13-spec/#hello-retry-request

I agree that we don't require the server to behave this way. I can fix the
draft to say this.

At no point is it stated that only
> about what messages are to be expected when, especially with alerts.

Actually the text does say something here:
https://tlswg.github.io/tls13-spec/#handshake-protocol

"Protocol messages MUST be sent in the order defined below (and shown in
the diagrams in Section 2
<https://tlswg.github.io/tls13-spec/#protocol-overview>). A peer which
receives a handshake message in an unexpected order MUST abort the
handshake with an “unexpected_message” alert. Unneeded handshake messages
are omitted, however."

However, this text was also in 5246, so I think there's a fair argument
that it's not strong enough. I'm not quite sure how to make it better.
Suggestions?

> description of the fallback mode.

I don't *think* that that's true: can you point to the specific text that
you are concerned with?

> the document. I do not understand what is supposed to happen in this

Here's the relevant text:
https://tlswg.github.io/tls13-spec/#early-data-indication

"If any of these checks fail [ALPN is in the list above] the server MUST
NOT respond with the extension and must discard all the remaining first
flight data (thus falling back to 1-RTT). If the client attempts a 0-RTT
handshake but the server rejects it, it will generally not have the 0-RTT
record protection keys and must instead trial decrypt each record with the
1-RTT handshake keys until it finds one that decrypts properly, and then
pick up the handshake from that point."

Is there anything else you'd like to see here?

> protocol, and can be complex. API guidance is not enough.

This ended up in the main text:
https://tlswg.github.io/tls13-spec/#zero-rtt-data

"Protocols MUST NOT use 0-RTT data without a profile that defines its use.
That profile needs to identify which messages or interactions are safe to
use with 0-RTT. In addition, to avoid accidental misuse, implementations
SHOULD NOT enable 0-RTT unless specifically requested. Special functions
for 0-RTT data are RECOMMENDED to ensure that an application is always
aware that it is sending or receiving data that might be replayed."

Is this missing pieces you think we need?

> consciously punt and remove the note and replace with something else.

The idea here is that we need a separate draft that just says "use
exporters with
label X". I think we remove the note and see if we can get someone to write
that draft.

> potentially breaking things.

That's the idea.

-Ekr

> Sincerely,
> Watson Ladd
>
> _______________________________________________
> TLS mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/tls
>