Re: [TLS] What would make TLS cryptographically better for TLS 1.3

Watson Ladd <watsonbladd@gmail.com> Fri, 01 November 2013 22:40 UTC

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From: Watson Ladd <watsonbladd@gmail.com>
To: "rransom. 8774" <rransom.8774@gmail.com>
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Subject: Re: [TLS] What would make TLS cryptographically better for TLS 1.3
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On Nov 1, 2013 2:21 PM, "Robert Ransom" <rransom.8774@gmail.com> wrote:
>
> On 10/31/13, Watson Ladd <watsonbladd@gmail.com> wrote:
>
> > TLS 1.3 should significantly reduce the number of round trips
> > required. To this end I propose the following obviously secure scheme:
> > the client sends a point on a curve in ClientHello and the server
> > responds with certificates (or some other authentication thing) and a
> > point on a curve, so that when the client speaks again, it is with a
> > negotiated, authenticated shared secret. Before everyone screams about
> > needing one signature per connection, note the server can use a time
> > based secret key, so only has to do one exponentiation per client.
>
> This protocol should be designed to allow future extension to use
> post-quantum encryption and/or key encapsulation cryptosystems.  (I
> don't believe that quantum computers will become a threat, but some PQ
> cryptosystems may be faster than ECDH.)
>
The PQ transition involves reworking the PKI as well. We also don't know if
NTRU or something else will win. A proffered version byte to avoid
downgrades could work, but I would not want to commit to the future.
> It also needs to either allow session resumption without the
> possibility of reusing any key used to encrypt or authenticate
> application-level data, or explicitly forbid session resumption.
>
>
> > Renegotiation should be killed: it serves no purpose.
>
> Renegotiation is a critical feature of TLS, which serves multiple
purposes.
>
> * Renegotiation allows rekeying of a session.  This is absolutely
> required for any ciphersuite based on a block cipher with a 128-bit or
> smaller block, because block cipher modes' security properties degrade
> after they are used for more than some number of blocks.
After one zettabyte according to a simple calculation.
>
> * Applications can also use renegotiation-based rekeying to improve
> forward secrecy; for example, the Mixminion specification
> (<
https://github.com/nmathewson/mixminion-doc/blob/a661212831d2afc3200339b2634ca16452e3aeec/spec/minion-spec.txt
>,
> section 4, line 1040) requires that relay-to-relay TLS connections be
> rekeyed using renegotiation every 15 minutes for this purpose.
If reconnecting is cheap enough that works. Designers need to deal with
connection death anyway.
>
> * A TLS connection can be established by a fully trusted device which
> knows a password or other application-layer authorization credential,
> authorized to perform some operations using messages within the TLS
> connection, and then transferred with the help of renegotiation to a
> less trusted device to actually perform those operations.  This is
> similar to the preceding use, but to provide 'sideways secrecy' rather
> than forward secrecy.
What does renegotiation get here?
The untrusted device can do everything unless it is being monitored by a
trusted device.
>
> * One version of the Tor 'link protocol' (Tor's term for its outer
> TLS-based connection protocol) uses renegotiation to provide secrecy
> for the server's certification chain against purely passive attackers.
>  The purposes above could be served by applying a one-way function to
> the originally derived key material, then discarding the old keys;
> this purpose cannot.
TOR could send a temporary outer signature and an inner proof. I agree this
is a weakness, but CurveCP like mechanisms only work thanks to some known
key.
>
>
> Robert Ransom