Re: [TLS] Alexey Melnikov's Discuss on draft-ietf-tls-tls13-26: (with DISCUSS and COMMENT)

[Eric Rescorla <[e-mail]>]

On Wed, Mar 7, 2018 at 5:29 AM, Alexey Melnikov <[e-mail]>
wrote:

> Alexey Melnikov has entered the following ballot position for
> draft-ietf-tls-tls13-26: Discuss
>
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>
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
>
>
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-tls-tls13/
>
>
>
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
>
> Thank you for a well written document. I will be switching to Yes once the
> following is addressed/discussed:
>
> Relationship to TLS 1.2 needs to be clarified. The document is adding
> requirements on TLS 1.2 clients. Implementors of TLS 1.2 are not going to
> (or
> very unlikely to) read this document. This looks fishy to me. Two examples
> on
> page 37:
>
>   TLS 1.2 clients SHOULD also check that the last eight bytes  are not
> equal to
>   the second value if the ServerHello indicates TLS  1.1 or below
>
> and
>
>  A legacy TLS client performing renegotiation with TLS 1.2 or prior  and
> which
>  receives a TLS 1.3 ServerHello during renegotiation MUST  abort the
> handshake
>  with a "protocol_version" alert.  Note that  renegotiation is not possible
>  when TLS 1.3 has been negotiated.
>
> There are similar statements on page 45:
>
>   TLS 1.2 implementations SHOULD also process this extension.
>
> and on page 48:
>
>   However, the old semantics did not constrain the signing
>   curve.  If TLS 1.2 is negotiated, implementations MUST be prepared
>   to accept a signature that uses any curve that they advertised in
>   the "supported_groups" extension.
>
> I think you need to clarify whether these normative requirements apply to
> pure
> TLS 1.2 clients or TLS clients that implement both 1.2 and 1.3 and choose
> to
> use 1.2 for some reason. Or maybe you need to say in the
> Abstract/Introduction
> that although this document obsoletes TLS 1.2 it also specifies new
> requirements on TLS 1.2 implementations. (So it is sort of both
> "Obsoletes" and
> "Updates" TLS 1.2 RFC.)
>


The intent is that these affect old TLS 1.2 implementations as well. S 1.4
tries
to be clear about this, but maybe it fails.

I suggest we:

(1) Add the following sentence to the abstract:
"This document also specifies new requirements for TLS 1.2 implementations.

(2) Rewrite the first sentence of S 1.4 to say:
   This document defines several changes that optionally affect
   implementations of TLS 1.2, including those which do not also
   support TLS 1.3

(3) Strike the following graf:

   An implementation of TLS 1.3 that also supports TLS 1.2 might need to
   include changes to support these changes even when TLS 1.3 is not in
   use.  See the referenced sections for more details.




>
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> 1) On page 47:
>  The length of the salt MUST be equal to the length of the digest
> algorithm
>
> Length of algorithm?
>

Right. Output of.



>
> 2) DER need a Normative Reference on first use. There are some references
> on
> 2nd/3rd use.
>

Thanks.


>
> 3) On page 57:
>
>    The
>    server then ignores early data by attempting to decrypt received
>    records in the handshake traffic keys until it is able to receive
>    the client's second flight and complete an ordinary 1-RTT
>    handshake, skipping records that fail to decrypt, up to the
>    configured max_early_data_size.
>
> I read this several times and still don't understand what this is saying.
> It is
> saying "ignores ... until it is able to receive". I think you either don't
> mean
> "ignore" (as in discard the rest) or I misunderstood. I clarifying example
> or a
> reference to another section (e.g. with the diagram) would be very helpful
> here.
>

We do mean discard. The idea here is that you try to decrypt those records
using the handshake keys and if that fails you ignore them.


> 4) On page 82:
>
>    When record protection has not yet been engaged, TLSPlaintext
> structures
>    are written directly onto the wire.  Once record  protection has
> started,
>    TLSPlaintext records are protected and sent   as described in the
> following
>    section
>
> Just to double check: are you saying that before the handshake TLS
> application
> layer effectively results in plain text messages (with some extra octets to
> signal record type)?
>

No, you can't write application data prior to this point, as stated in S 2.

   Application data MUST NOT be sent prior to sending the Finished
   message and until the record layer starts using encryption keys.
   Note that while the server may send application data prior to
   receiving the client's Authentication messages, any data sent at that
   point is, of course, being sent to an unauthenticated peer.

It's non-application data (handshake, alerts, acks) which is sent in this
fashion
prior to the handshake.



5) I am pretty sure that [RFC5116] is a Normative reference. It is required
> to
> be understood to implemented TLS 1.3. Also, you have additional
> requirements on
> AEADs, which again implies understanding of what they are:
>
> On page 84:
>
>    An AEAD algorithm used in TLS 1.3 MUST NOT produce an expansion  greater
>    than 255 octets
>
> and
>
>    An AEAD algorithm where N_MAX is less than 8  bytes MUST NOT be used
> with TLS
>

I concur. Thanks.



6) The diagram in section 7.1 was a bit cryptic. Maybe explain that when you
> use '0' you mean as many bytes of 0s as needed for various functions.
>

This is in the first paragraph after the diagram. Would you prefer it
elsewhere?

   If a given secret is not available, then the 0-value consisting of a
   string of Hash.length bytes set to zeros is used.  Note that this

-Ekr