Re: [TLS] Reducing record expansion overhead allowance

Eric Rescorla <ekr@rtfm.com> Sun, 20 July 2014 16:00 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 20 Jul 2014 08:59:18 -0700
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To: "StJohns, Michael" <msj@nthpermutation.com>
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Subject: Re: [TLS] Reducing record expansion overhead allowance
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On Sun, Jul 20, 2014 at 7:52 AM, StJohns, Michael <msj@nthpermutation.com>
wrote:

> The only other thing that hasn't been mentioned that was discussed at the
> interim in Denver is padding for traffic analysis resistance. That could
> hit 1k in size.  But since there's no proposal that's just a guess on size.
>


I believe the consensus here was to have padding be done separately.

-Ekr


>
> On Saturday, July 19, 2014, Eric Rescorla <ekr@rtfm.com> wrote:
>
>>  https://github.com/tlswg/tls13-spec/issues/55
>>
>> In TLS 1.2, we had the following maximum values:
>>
>> TLSPlaintext: 2^{14}
>> TLSCompressed: 2^{14} + 1024
>> TLSCiphertext: 2^{14} + 2048
>>
>> These overhead values allow for expansion in these transforms
>> due to potential bad compression overhead or padding, etc.
>>
>> Wan-Teh Chang points out that we no longer have compression
>> so there's no need to allow for 1024 bytes of expansion there.
>>
>> Minimally we should reduce the TLSCiphertext overhead to
>> 2^{14} + 1024. Do people believe that we will have AEAD
>> ciphers with 1024 bytes of expansion or should we reduce
>> it further? I'm inclined to not re-judge that and just leave it
>> at 2^{14} + 1024.
>>
>> Thoughts?
>> -Ekr
>>
>>