Re: [TLS] secdir review of draft-ietf-tls-ecdhe-psk-aead-03

mrex@sap.com (Martin Rex) Fri, 19 May 2017 14:17 UTC

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In-Reply-To: <20170519043827.GL39245@kduck.kaduk.org>
To: Benjamin Kaduk <kaduk@mit.edu>
Date: Fri, 19 May 2017 16:17:12 +0200 (CEST)
CC: iesg@ietf.org, secdir@ietf.org, draft-ietf-tls-ecdhe-psk-aead.all@ietf.org, tls@ietf.org, ietf@ietf.org
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From: mrex@sap.com (Martin Rex)
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/ipaVUWS_K6aCrylgzmzar4gD6I0>
Subject: Re: [TLS] secdir review of draft-ietf-tls-ecdhe-psk-aead-03
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Benjamin Kaduk wrote:
> 
> Some other editorial nits follow.
> 
> In section 4, "these cipher suites MUST NOT be negotiated in TLS
> versions prior to 1.2" should probably clarify that "these" cipher
> suites are the new ones specified by this document.


This reminds me of the specification goofs in several TLSv1.2-related
documents about AEAD cipher suites which are responsible for the viability
of the POODLE attack and other exploitable fallback hacks.

It would be much preferable to avoid/fix those problems and facilitate
the migration to and use of TLSv1.2 without failing TLS handshakes and
band aids such as TLS_FALLBACK_SCSV


Suggested improvement:

   The cipher suites defined in this document make use of the
   authenticated encryption with additional data (AEAD) defined in TLS
   1.2 [RFC5246] and DTLS 1.2 [RFC6347].  Earlier versions of TLS do not
   have support for AEAD and consequently, these cipher suites MUST NOT
-  be negotiated in TLS versions prior to 1.2.  Clients MUST NOT offer
-  these cipher suites if they do not offer TLS 1.2 or later.  Servers,
-  which select an earlier version of TLS MUST NOT select one of these
-  cipher suites.  A client MUST treat the selection of these cipher
-  suites in combination with a version of TLS that does not support
-  AEAD (i.e., TLS 1.1 or earlier) as an error and generate a fatal
-  'illegal_parameter' TLS alert.
+                                               A client that offers
+  the cipher suites from this document in ClientHello.cipher_suites
+  in combination with (3,1) "TLSv1.0" or (3,2) "TLSv1.1" in
+  ClientHello.client_version MUST support TLSv1.2 and MUST accept
+  the server to negotiate TLSv1.2 for the current session.  If the
+  client does not support TLSv1.2 or is not willing to negotiate TLSv1.2,
+  then this client MUST NOT offer any of these cipher suites with a
+  lower protocol version than (3,3) "TLSv1.2" in ClientHello.client_version.
+  A server receiving a ClientHello and a client_version indicating
+  (3,1) "TLSv1.0" or (3,2) "TLSv1.1" and any of the cipher suites from
+  this document in ClientHello.cipher_suites can safely assume that the
+  client supports TLSv1.2 and is willing to use it.  The server MUST
+  NOT negotiate these cipher suites with TLS protocol versions earlier
+  than TLSv1.2.
+
+  Not requiring clients to indicate their support for TLSv1.2 cipher
+  suites exclusively through ClientHello.client_hello improves the
+  interoperability in the installed base and use of TLSv1.2 AEAD
+  cipher suites without upsetting the installed base of version-intolerant
+  TLS servers, results in more TLS handshakes succeeding and obviates
+  fallback mechanisms.


-Martin