On Wed, Apr 22, 2020 at 7:31 AM Hanno Becker <hanno.bec...@arm.com> wrote:
> > Considering the effort spent on shaving off bytes in the DTLS header, > > I think re-introducing the explicit CID should be avoided. It seems > > perfectly acceptable to me to have implicit header data which is > > protected via AAD. > > > This is only relevant if there is a common useful case in which you would > need to put multiple > DTLS records in the same datagram. Are you aware of such a case? > > > I can see the following uses: > 1) Replying to KeyUpdate with Ack;;KeyUpdate, or replying to > RequestConnectionID with Ack;;NewConnectionId > 2) Sending multiple (short) app records if the application protocol > doesn't provide its own framing. > Neither of these seem particularly compelling to me. The first happens very infrequently, and I'm not really aware of a lot of cases of the second. > > 1. Cryptographically protect it as in > https://github.com/tlswg/dtls13-spec/pull/143 > > > This seems to be a mixture of logical and on-the-wire representation, which > > moreover duplicates the CID in case it is explicitly present in the header. > > > Yes, so? > > > Isn't this less efficient > Trivially. and undoes the arguable benefit of the current solution that there's > no need to piece together an AAD buffer manually, because now you'd have > to? > I don't recall making that argument. -Ekr > Best, > Hanno > > > Looking forward to hearing other WG member's views, > Hanno > ------------------------------ > *From:* Eric Rescorla <e...@rtfm.com> > *Sent:* Wednesday, April 22, 2020 2:23 AM > *To:* Hanno Becker <hanno.bec...@arm.com> > *Cc:* tls@ietf.org <tls@ietf.org> > *Subject:* Re: [TLS] DTLS 1.3 AEAD additional data > > 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...@rtfm.com> wrote: > > > > On Tue, Apr 21, 2020 at 8:39 AM Hanno Becker <hanno.bec...@arm.com> 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 > > IMPORTANT NOTICE: The contents of this email and any attachments are > confidential and may also be privileged. If you are not the intended > recipient, please notify the sender immediately and do not disclose the > contents to any other person, use it for any purpose, or store or copy the > information in any medium. Thank you. > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls > > IMPORTANT NOTICE: The contents of this email and any attachments are > confidential and may also be privileged. If you are not the intended > recipient, please notify the sender immediately and do not disclose the > contents to any other person, use it for any purpose, or store or copy the > information in any medium. Thank you. > > IMPORTANT NOTICE: The contents of this email and any attachments are > confidential and may also be privileged. If you are not the intended > recipient, please notify the sender immediately and do not disclose the > contents to any other person, use it for any purpose, or store or copy the > information in any medium. Thank you. >
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