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

Nico Williams <nico@cryptonector.com> Fri, 01 November 2013 23:13 UTC

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Date: Fri, 1 Nov 2013 18:13:45 -0500
From: Nico Williams <nico@cryptonector.com>
To: Dan Harkins <dharkins@lounge.org>
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Subject: Re: [TLS] What would make TLS cryptographically better for TLS 1.3
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On Fri, Nov 01, 2013 at 03:38:06PM -0700, Dan Harkins wrote:
> On Fri, November 1, 2013 2:34 pm, Robert Ransom wrote:
> > On 10/31/13, Nico Williams <nico@cryptonector.com> wrote:
> >
> >>  - Many fewer nonce bytes and random IVs where possible.  Nonce payloads
> >>    should be sent when needed, if needed.  For example, to derive a
> >>    session key from an DHE shared secret one does not really need
> >>    nonces.  This means that counter modes are better, for example, than
> >>    CBC modes.
> >
> > If the server sends a nonce during a DHE/ECDHE key exchange, the
> > server can safely reuse its DH keypair for multiple clients with no
> > further design or implementation considerations.
> 
>   I don't believe that's true. If the server reuses its ephemeral D-H key
> then caveat emptor applies-- it should validate the client's public key,

Agreed.  A small nonce should suffice for key derivation if either party
reuses their supposedly-ephemeral DH keys.  By "small" I mean "not
nearly as large as 32 bytes, i.e., not enough to suffice for a
Dual_EC-type backdoored RNG attack".  The party reusing a key should
send some such small nonce, possibly a 64-bit nonce.  In any case, the
client ought not be reusing ephemeral DH keys, and any server that does
should be rotating them often (like SSHv1 used to).

> regardless of whether a nonce is sent or not.

For some curves there's no need to validate the client's public key.

Nico
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