Re: [TLS] 0RTT?

Watson Ladd <watsonbladd@gmail.com> Sun, 03 August 2014 17:32 UTC

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Date: Sun, 3 Aug 2014 10:32:30 -0700
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From: Watson Ladd <watsonbladd@gmail.com>
To: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
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Subject: Re: [TLS] 0RTT?
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On Sun, Aug 3, 2014 at 10:25 AM, Karthikeyan Bhargavan
<karthik.bhargavan@gmail.com> wrote:
>
>> The PMS that gets used is PMS_interim= HMAC(old_PMS, nonce || refresh
>> key): data encrypted as usual under the old ciphersuite follows.
>
> I guess I don’t understand what this means. Is the new application data encrypted
> under (a) the old MS? (b) a new MS derived from PMS_interim (and the old randoms)?

I was thinking new MS.

>
>
>>
>> On receiving this a server looks up the anti-replay nonce and checks
>> it is fresh. To make this easier we include UTC time since the epoch (
>> midnight of 1 January 1970) and mandate some degree of synchronization
>> and a window. To avoid tagging with clock drift we truncate some low
>> order bits after adding a random small offset.
>>
>> If the nonce is not previously seem the server can send Application
>> Data as normal.
>>
>> We now introduce a new handshake type: Rekey Finish, containing a new
>> ticket and server key. CCS follows afterwards. These are sent
>> encrypted.
>>
>> The new PMS will be HMAC(PMS_interim, ECDH(server, client keys)).
>>
>> The big limitation is ticket keys are going to need rotation. This
>> also doesn't address the desire to put extra data in DNS to give some
>> degree of forward secrecy, but I don't think you can change DNS that
>> quickly without some problems.
>>
>> Furthermore, if we want rekeys without renegotiation, we can reuse
>> Rekey Finish and add a Rekey Initiate handshake type that will send
>> the client key.
>>
>> Open questions: how secure is this? We certainly need to hash the
>> anti-replay nonce into the keys: is that all we need? (Not if we want
>> to avoid attackers manipulating which method we use) Does noisy
>> truncation work to prevent fingerprinting while reducing storage
>> requirements?
>>
>> Sincerely,
>> Watson Ladd
>>
>> _______________________________________________
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>> TLS@ietf.org
>> https://www.ietf.org/mailman/listinfo/tls
>



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