Re: [TLS] encrypted content type and padding

Watson Ladd <watsonbladd@gmail.com> Mon, 21 September 2015 11:43 UTC

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Date: Mon, 21 Sep 2015 07:43:27 -0400
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From: Watson Ladd <watsonbladd@gmail.com>
To: noloader@gmail.com
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Cc: IETF TLS Working Group <tls@ietf.org>
Subject: Re: [TLS] encrypted content type and padding
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On Mon, Sep 21, 2015 at 3:19 AM, Jeffrey Walton <noloader@gmail.com> wrote:
> On Mon, Sep 21, 2015 at 3:02 AM, Daniel Kahn Gillmor
> <dkg@fifthhorseman.net> wrote:
>> Hey TLS folks--
>>
>> apologies for the delay in sending these pull requests.
>>
>> encrypted content type:
>> -----------------------
>>
>> https://github.com/tlswg/tls13-spec/pull/51
>>
>> This should be uncontroversial, and just needed freshening against the
>> current draft.
>
> :)
>
>> padding:
>> --------
>>
>> We're now proposing that handshake padding should be offered by
>> introducing a new HandshakePadding message.  I've avoided any sort of
>> padding negotiation in the handshake in favor of making padding always
>> available in TLS 1.3 and up.  Life is simpler this way.
>
> Padding can create oracles and complicates proofs.
>
> When sending data that needs to be padded during key exchange, then
> fill it up with random data and digest it in HKDF- like fashion.
>
> When sending data that needs to be padded during bulk transfer, then
> don't do it. Use GCM, CTR, etc.
>
> Why make life more complicated then it needs to be?

Is this actually true in the second pull request? No: a moment of
actually reading reveals that the string is inside an AEAD encrypted
packet. There is no way in which this padding could be modified for
use in a side-channel attack.


>
> Jeff
>
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