Re: [TLS] DTLS 1.3 AEAD additional data

[Eric Rescorla <[e-mail]>]

I think there are two potential resolutions to your CID issue:

1. Cryptographically protect it as in
https://github.com/tlswg/dtls13-spec/pull/143
2. Forbid implicit CIDs (my preference) see:
https://github.com/tlswg/dtls13-spec/issues/144

Would like to hear what others in the WG think.

-Ekr


On Tue, Apr 21, 2020 at 10:59 AM Eric Rescorla <[e-mail]> wrote:

>
>
> On Tue, Apr 21, 2020 at 8:39 AM Hanno Becker <[e-mail]> wrote:
>
>> Hi all,
>>
>> To my understanding, DTLS 1.3 defines AEAD additional data for record
>> protection
>> as the record header as seen on the wire. Quoting Draft 37, Section 4:
>>
>> ```
>>    The entire header value shown in Figure 4 (but prior to record number
>>    encryption) is used as as the additional data value for the AEAD
>>    function.  For instance, if the minimal variant is used, the AAD is 2
>>    octets long.  Note that this design is different from the additional
>>    data calculation for DTLS 1.2 and for DTLS 1.2 with Connection ID.
>> ```
>>
>> I would like to suggest that DTLS 1.3 uses a structured representation
>> of the record header instead, as do all other versions of [D]TLS as
>> far as I understand.
>>
>
> I am not in favor of this change as proposed. I think it is better to
> protect the data that is actually on the wire than to allow for changes in
> the on-the-wire representation that are not reflected in the integrity
> check.
>
>
> The reasons for this are as follows, in decreasing order of
>> my perception of importance:
>>
>> - Omission of Connection ID
>>
>>   Regarding the presence of Connection IDs in multiple records within
>>   a single datagram, Draft 37 says:
>>
>> ```
>>    Implementations which send multiple records in the same datagram
>>    SHOULD omit the connection id from all but the first record;
>>    receiving implementations MUST assume that any subsequent records
>>    without connection IDs belong to the same assocatiation.
>> ```
>>
>>   This means that the Connection ID for non-initial records in a
>>   datagram containing multiple records is _not_ part of the AEAD
>>   additional data for those records, which seems wrong. Concretely,
>>   one could inject such non-initial records into other datagrams
>>   using different CIDs, and the record protection wouldn't notice it.
>>
>
> This seems like a reasonable point, though it's not clear to me that there
> is an actual problem here. I'd be in favor of explicitly including the CID
> in the AD as well as the header.
>
>>
>>
>>   One might argue that CID shouldn't be part of the AEAD in the first
>>   place, but in any case, I believe the treatment should be uniform
>>   and not distinguish between initial and non-initial records in
>>   a datagram.
>>
>
> We're not distinguishing it. The AD is protecting the record on the wire.
>
>
> - Modularity
>>
>>   Decoupling the wire-presentation of the record header from
>>   record protection allows to implement record protection and
>>   the choice of record header independently: One piece of
>>   the implementation can take care of record protection -
>>   using the structured presentation of the record header - while
>>   another takes care of the wire-encoding. It is even possible
>>   to change the record header format in transit.
>>
>
> This seems like a defect, not a feature.
>
>
> - Simplicity
>>
>>   At first it seems that using the record header as an
>>   unstructured binary blob for AEAD makes things simpler,
>>   but I don't think this is the case: Prior to record
>>   decryption, the record sequence number needs to be
>>   decrypted, and for that purpose, the record header already
>>   has to be parsed. Hence, at the time of record decryption,
>>   the record header is already be present a modified, structured
>>   form, and retaining the corresponding modified binary form
>>   appears to create additional complexity which would be
>>   avoided if record protection would use the structured
>>   header presentation.
>>
>
> I've implemented this for QUIC (I can't remember who at Mozilla did it for
> DTLS) and it's not particularly difficult.
>
>
> - Uniformity with other [D]TLS versions
>>
>
> I don't find this argument at all persuasive. To the contrary: we should
> break with  DTLS 1.2 in any case where it's an improvement and not too
> onerous.
>
> -Ekr
>
>
>>
>>
>> Let me know what you think,
>>
>> Best,
>> Hanno
>>
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