Re: [TLS] CPU cost of 1RTT handshake

Watson Ladd <watsonbladd@gmail.com> Mon, 11 August 2014 04:28 UTC

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Date: Sun, 10 Aug 2014 21:28:30 -0700
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From: Watson Ladd <watsonbladd@gmail.com>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] CPU cost of 1RTT handshake
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On Sun, Aug 10, 2014 at 9:09 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>
>
>
> On Sun, Aug 10, 2014 at 1:35 PM, Watson Ladd <watsonbladd@gmail.com> wrote:
>>
>> Dear all,
>>
>> Right now, instead of the server defining the group to be used and
>> sending a key in the group, the client computes multiple keys, and the
>> server selects one. This is very bad for embedded devices with
>> constrained CPU, especially if they are connecting to a server over
>> high-latency, low-bandwidth links.
>>
>> The justification for the current behavior is similarity to 0-RTT. But
>> I'm not convinced that this actually makes the protocol or
>> implementations any simpler, and it has real costs for many devices
>> that will otherwise not adopt TLS or try to invent their own encrypted
>> protocols.
>
>
> I'm not sure I would phrase the protocol behavior exactly as you have
> above. Rather, the client sends a list of the groups it supports  (as in
> TLS 1.2). It can also send any number of (EC)DHE shares that match
> those groups. If the server selects a group that matches one of those
> shares, the handshake proceeds. Otherwise, the server tells the
> client which group it has selected and the client then adds a share
> from that group to the list.
>
> In other words:
>
> 1. The server does get to select the group as  it can reject all the
> client's shares (thougb of course it cannot force the client to use
> a group it does not accept or know about)..
>
> 2. The client is not forced to send any shares. It can simply send
> an empty ClientKeyExchange (See S 7.4.2) and then wait for the
> server to select a group. In other words, it can act as you suggest
> above, but at the cost of a round trip. It's a explicit computation/
> latency tradeoff.

It's a completely unnecessary one: the server can send its share, then
the client goes, which is followed by the remaining messages. Why are
we making 1-RTT unnecessarily expensive?

Sincerely,
Watson Ladd