Re: [TLS] CPU cost of 1RTT handshake

Eric Rescorla <ekr@rtfm.com> Mon, 11 August 2014 04:37 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 10 Aug 2014 21:36:47 -0700
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To: Watson Ladd <watsonbladd@gmail.com>
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Subject: Re: [TLS] CPU cost of 1RTT handshake
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On Sun, Aug 10, 2014 at 9:28 PM, Watson Ladd <watsonbladd@gmail.com> wrote:
>
>  > 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.


Yes, one could do that, but it has different properties:

1. It doesn't allow the server to encrypt its handshake data on
the first flight and therefore does not conceal it from passive
observers [0]

2. It doesn't allow the server to send data to the client on its first
flight (if it doesn't care about the client's identity).

-Ekr

[0] Or active observers in cases like WebRTC where the server
doesn't respond to any client.