Re: [Cfrg] [TLS] Closing out tls1.3 "Limits on key usage" PRs (#765/#769).

"Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk> Wed, 01 March 2017 14:38 UTC

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From: "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>
To: "Dang, Quynh (Fed)" <quynh.dang@nist.gov>, Aaron Zauner <azet@azet.org>
Thread-Topic: [TLS] [Cfrg] Closing out tls1.3 "Limits on key usage" PRs (#765/#769).
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Date: Wed, 1 Mar 2017 14:38:04 +0000
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Cc: IRTF CFRG <cfrg@irtf.org>, "<tls@ietf.org>" <tls@ietf.org>
Subject: Re: [Cfrg] [TLS] Closing out tls1.3 "Limits on key usage" PRs (#765/#769).
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Hi,

On 01/03/2017 14:31, "TLS on behalf of Dang, Quynh (Fed)"
<tls-bounces@ietf.org on behalf of quynh.dang@nist.gov> wrote:
>From: Aaron Zauner <azet@azet.org>
>Date: Wednesday, March 1, 2017 at 9:24 AM
>To: 'Quynh' <Quynh.Dang@nist.gov>
>Cc: Sean Turner <sean@sn3rd.com>om>, "<tls@ietf.org>" <tls@ietf.org>rg>, IRTF
>CFRG <cfrg@irtf.org>
>Subject: Re: [Cfrg] Closing out tls1.3 "Limits on key usage" PRs
>(#765/#769).
>
>
>
>>
>>
>>>On 01 Mar 2017, at 13:18, Dang, Quynh (Fed) <quynh.dang@nist.gov> wrote:
>>>From: Aaron Zauner <azet@azet.org>
>>>Date: Wednesday, March 1, 2017 at 8:11 AM
>>>To: 'Quynh' <Quynh.Dang@nist.gov>
>>>Cc: Sean Turner <sean@sn3rd.com>om>, "<tls@ietf.org>" <tls@ietf.org>rg>, IRTF
>>>CFRG <cfrg@irtf.org>
>>>Subject: Re: [Cfrg] Closing out tls1.3 "Limits on key usage" PRs
>>>(#765/#769).
>>>>>On 25 Feb 2017, at 14:28, Dang, Quynh (Fed) <quynh.dang@nist.gov>
>>>>>wrote:
>>>>>Hi Sean, Joe, Eric and all,
>>>>>I would like to address my thoughts/suggestions on 2 issues in option
>>>>>a.
>>>>>1) The data limit should be addressed in term of blocks, not records.
>>>>>When the record size is not the full size, some user might not know
>>>>>what to do. When the record size is 1 block, the limit of 2^24.5
>>>>>blocks (records) is way too low unnecessarily for
>>>>> the margin of 2^-60.  In that case, 2^34.5 1-block records is the
>>>>>limit which still achieves the margin of 2^-60.
>>>>I respectfully disagree. TLS deals in records not in blocks, so in the
>>>>end any semantic change here will just confuse implementors, which
>>>>isn't a good idea in my opinion.
>>>Over the discussion of the PRs, the preference was blocks.
>>
>>
>>I don't see a clear preference. I see Brian Smith suggested switching to
>>blocks to be more precise in a PR. But in general it seems to me that
>>"Option A" was preferred in this thread anyhow - so these PRs aren't
>>relevant? I'm not sure that text on key-usage
>> limits in blocks in a spec that fundamentally deals in records is less
>>confusing, quite the opposite (at least to me). As I pointed out
>>earlier: I strongly recommend that any changes to the spec are as clear
>>als possible to engineers (non-crypto/math people)
>> -- e.g. why the spec is suddenly dealing in blocks instead of records
>>et cetera. Again; I really don't see any reason to change text here - to
>>me all suggested changes are even more confusing.
>>
>>
>
>
>Hi Aaron,
>
>
>The  technical reasons I explained are reasons for using records. I don’t
>see how that is confusing.
>
>
>If you like records, then the record number = the total blocks / the
>record size in blocks: this is simplest already.
>

That formula does not correctly compute how many records have been sent on
a connection, because the record size in blocks is variable, not constant.
You can modify it to get bounds on the total number of records sent, but
the bounds are sloppy because some records only consume 2 blocks (one for
encryption, one for masking in GHASH) while some consume far more.

It's simpler for an implementation to count how many records have been
sent on a connection .... by using the connection's sequence number. This
puts less burden on the implementation/implementer.

Cheers

Kenny