Re: [dns-privacy] Spencer Dawkins' Yes on draft-ietf-dprive-edns0-padding-02: (with COMMENT)

[Tim Wicinski <[e-mail]>]

Shane's explanation on padding is a great summary of the mailing list 
discussion during the document review.

tim
(as DocumentShepherd)

On 3/1/16 5:39 AM, Shane Kerr wrote:
> Spencer,
>
> At 2016-02-29 20:38:34 -0800
> "Spencer Dawkins" <[e-mail]> wrote:
>> Thanks for producing this draft. I do have one question about this text:
>>
>>    The PADDING octets SHOULD be set to 0x00.  Other values MAY be used;
>>    for example, in cases where there is a concern that the padded
>>    message could be subject to compression before encryption.  PADDING
>>    octets of any value MUST be accepted in messages received.
>>
>> I'm not entirely sure I understand the point of "SHOULD be set to 0x00".
>> I'm not questioning the SHOULD (you explain why choosing another value
>> would be a good idea, thank you ), but I'm wondering why there's a
>> normative requirement at all.
>>
>> I was trying to guess, and one hypothesis was that if the padding is
>> consistently 0x00, it's less likely to provide a covert channel (or at
>> least you'd be more likely to notice packets using different padding),
>> but the security considerations section didn't mention that, so I'm still
>> lost.
>>
>> If there's a reason to zero the padding bytes in the uncompressed case, a
>> sentence of explanation might be useful.
>
> There was a lot of discussion on the list about this very topic,
> actually. I'm not sure if it can be reduced to a sentence or two.
>
> IIRC, the idea is that we should recommend some value there to give
> guidance for implementors to avoid common mistakes (such as using
> uninitialized memory, which might contain left-over values).
>
> Someone did raise the possibility of having a covert channel, but it
> was recognized that there are so many places for putting covert data
> into DNS that this is not really a concern. For example, one could
> simply use any unregistered EDNS(0) option and stick data in there, or
> encode it into the QNAME, or put values in the query ID, or...
>
> Putting random data in the padding was considered, but it did not seem
> to add any real value beyond zero-padding.
>
> Cheers,
>
> --
> Shane "not an editor on the draft" Kerr
>