Re: [TLS] Pre_shared_key Extension Question

[Benjamin Kaduk <[e-mail]>]

On 08/18/2016 10:29 AM, Eric Rescorla wrote:
>
>
> On Thu, Aug 18, 2016 at 8:18 AM, Benjamin Kaduk <[e-mail]
> <mailto:[e-mail]>> wrote:
>
>     On 08/17/2016 05:17 PM, Eric Rescorla wrote:
>>     It would be a fairly significant simplification to say you could
>>     only have one PSK, because then we could easily require the
>>     client to prove knowledge of the key, for instance by stuffing a
>>     MAC at the end of the ClientHello as we discussed in Berlin.
>>
>>     So:
>>     Is there any demand for multiple identities? I do not believe
>>     there is any in the Web context. If not, we should remove this
>>     feature.
>>
>
>     Then at PSK rollover time, clients are expected to fall back to a
>     new TLS connection using the other PSK?
>
>
> I'm not sure I follow. Can you say more?
>

With caveat that I don't deploy PSK-based systems and this is before my
morning coffee ... suppose I have a related group of IoT devices that
use TLS+PSK to communicate.  But, I don't want the long-term
cryptographic secret (the PSK) to be permanent and unchanging, since
that's not best practices and leaves me in a tough spot if it ever
leaks.  So, I generate a new PSK every three months, provide a device on
the network that lets the devices retrieve the new one, and everyone is
supposed to expire the old one a month after the new one is available. 
My PSK identities could then be something like "2016Q2" (or something
more complicated; it doesn't really matter), but since the devices are
autonomous and not synchronized, I can't have a "flag day" transition
from using the 2016Q1 to 2016Q2 key.  At some point, a device will try
to use the 2016Q2 key as a client against a server device that only has
the 2016Q1 key, and the handshake fails.  In the rollover scenario I
describe, the client can then try a new handshake with the 2016Q1 key
instead of the 2016Q2 key, which should succeed.

Given the benefits of only allowing one PSK identity from the client, I
am fine with requiring the second hanshake in this (contrived?)
scenario; I just wanted to mention it so that we know what we're getting
into.

-Ben