What should the next steps be here? Is this a bunch of errata, or something else?
On Wed, Apr 17, 2024 at 10:08 AM David Benjamin <david...@chromium.org> wrote: > > Sender implementations should already be able to retransmit messages > with older epochs due to the "duplicated" post-auth state machine > > The nice thing about option 7 is that the older epochs retransmit problem > becomes moot in updated senders, I think. If the sender doesn't activate > epoch N+1 until KeyUpdate *and prior messages* are ACKed and if KeyUpdate > is required to be the last handshake message in epoch N, then the previous > epoch is guaranteed to be empty by the time you activate it. > > On Wed, Apr 17, 2024, 09:27 Marco Oliverio <ma...@wolfssl.com> wrote: > >> Hi David, >> >> Thanks for pointing this out. I also favor solution 7 as it's the simpler >> approach and it doesn't require too much effort to add in current >> implementations. >> Sender implementations should already be able to retransmit messages with >> older epochs due to the "duplicated" post-auth state machine. >> >> Marco >> >> On Tue, Apr 16, 2024 at 3:48 PM David Benjamin <david...@chromium.org> >> wrote: >> >>> Thanks, Hannes! >>> >>> Since it was buried in there (my understanding of the issue evolved as I >>> described it), I currently favor option 7. I.e. the sender-only fix to the >>> KeyUpdate criteria. >>> >>> At first I thought we should also change the receiver to mitigate >>> unfixed senders, but this situation should be pretty rare (most senders >>> will send NewSessionTicket well before they KeyUpdate), DTLS 1.3 isn't very >>> widely deployed yet, and ultimately, it's on the sender implementation to >>> make sure all states they can get into are coherent. >>> >>> If the sender crashed, that's unambiguously on the sender to fix. If the >>> sender still correctly retransmits the missing messages, the connection >>> will perform suboptimally for a blip but still recover. >>> >>> David >>> >>> >>> On Tue, Apr 16, 2024, 05:19 Tschofenig, Hannes < >>> hannes.tschofe...@siemens.com> wrote: >>> >>>> Hi David, >>>> >>>> >>>> >>>> this is great feedback. Give me a few days to respond to this issue >>>> with my suggestion for moving forward. >>>> >>>> >>>> >>>> Ciao >>>> >>>> Hannes >>>> >>>> >>>> >>>> *From:* TLS <tls-boun...@ietf.org> *On Behalf Of *David Benjamin >>>> *Sent:* Saturday, April 13, 2024 7:59 PM >>>> *To:* <tls@ietf.org> <tls@ietf.org> >>>> *Cc:* Nick Harper <nhar...@chromium.org> >>>> *Subject:* Re: [TLS] Issues with buffered, ACKed KeyUpdates in DTLS 1.3 >>>> >>>> >>>> >>>> Another issues with DTLS 1.3's state machine duplication scheme: >>>> >>>> >>>> >>>> Section 8 says implementation must not send new KeyUpdate until the >>>> KeyUpdate is ACKed, but it says nothing about other post-handshake >>>> messages. Suppose KeyUpdate(5) in flight and the implementation decides to >>>> send NewSessionTicket. (E.g. the application called some >>>> "send NewSessionTicket" API.) The new epoch doesn't exist yet, so naively >>>> one would start sending NewSessionTicket(6) in the current epoch. Now the >>>> peer ACKs KeyUpdate(5), so we transition to the new epoch. But >>>> retransmissions must retain their original epoch: >>>> >>>> >>>> >>>> > Implementations MUST send retransmissions of lost messages using the >>>> same epoch and keying material as the original transmission. >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-4.2.1-3 >>>> >>>> >>>> >>>> This means we must keep sending the NST at the old epoch. But the peer >>>> may have no idea there's a message at that epoch due to packet loss! >>>> Section 8 does ask the peer to keep the old epoch around for a spell, but >>>> eventually the peer will discard the old epoch. If NST(6) didn't get >>>> through before then, the entire post-handshake stream is now wedged! >>>> >>>> >>>> >>>> I think this means we need to amend Section 8 to forbid sending *any* >>>> post-handshake message after KeyUpdate. That is, rather than saying you >>>> cannot send a new KeyUpdate, a KeyUpdate terminates the post-handshake >>>> stream at that epoch and all new post-handshake messages, be they KeyUpdate >>>> or anything else, must be enqueued for the new epoch. This is a little >>>> unfortunate because a TLS library which transparently KeyUpdates will then >>>> inadvertently introduce hiccups where post-handshake messages triggered by >>>> the application, like post-handshake auth, are blocked. >>>> >>>> >>>> >>>> That then suggests some more options for fixing the original problem. >>>> >>>> >>>> >>>> *7. Fix the sender's KeyUpdate criteria* >>>> >>>> >>>> >>>> We tell the sender to wait for all previous messages to be ACKed too. >>>> Fix the first paragraph of section 8 to say: >>>> >>>> >>>> >>>> > As with other handshake messages with no built-in response, >>>> KeyUpdates MUST be acknowledged. Acknowledgements are used to both control >>>> retransmission and transition to the next epoch. Implementations MUST NOT >>>> send records with the new keys until the KeyUpdate *and all preceding >>>> messages* have been acknowledged. This facilitates epoch >>>> reconstruction (Section 4.2.2) and avoids too many epochs in active use, by >>>> ensuring the peer has processed the KeyUpdate and started receiving at the >>>> new epoch. >>>> >>>> > >>>> >>>> > A KeyUpdate message terminates the post-handshake stream in an epoch. >>>> After sending KeyUpdate in an epoch, implementations MUST NOT send any new >>>> post-handshake messages in that epoch. Note that, if the implementation has >>>> sent KeyUpdate but is waiting for an ACK, the next epoch is not yet active. >>>> In this case, subsequent post-handshake messages may not be sent until >>>> receiving the ACK. >>>> >>>> >>>> >>>> And then on the receiver side, we leave things as-is. If the sender >>>> implemented the old semantics AND had multiple post-handshake transactions >>>> in parallel, it might update keys too early and then we get into the >>>> situation described in (1). We then declare that, if this happens, and the >>>> sender gets confused as a result, that's the sender's fault. Hopefully this >>>> is not rare enough (did anyone even implement 5.8.4, or does everyone just >>>> serialize their post-handshake transitions?) to not be a serious protocol >>>> break? That risk aside, this option seems the most in spirit with the >>>> current design to me. >>>> >>>> >>>> >>>> *8. Decouple post-handshake retransmissions from epochs* >>>> >>>> >>>> >>>> If we instead say that the same epoch rule only applies for the >>>> handshake, and not post-handshake messages, I think option 5 (process >>>> KeyUpdate out of order) might become viable? I'm not sure. Either way, this >>>> seems like a significant protocol break, so I don't think this is an option >>>> until some hypothetical DTLS 1.4. >>>> >>>> >>>> >>>> >>>> >>>> On Fri, Apr 12, 2024 at 6:59 PM David Benjamin <david...@chromium.org> >>>> wrote: >>>> >>>> Hi all, >>>> >>>> >>>> >>>> This is going to be a bit long. In short, DTLS 1.3 KeyUpdates seem to >>>> conflate the peer *receiving* the KeyUpdate with the peer *processing* the >>>> KeyUpdate, in ways that appear to break some assumptions made by the >>>> protocol design. >>>> >>>> >>>> >>>> *When to switch keys in KeyUpdate* >>>> >>>> >>>> >>>> So, first, DTLS 1.3, unlike TLS 1.3, applies the KeyUpdate on the ACK, >>>> not when the KeyUpdate is sent. This makes sense because KeyUpdate records >>>> are not intrinsically ordered with app data records sent after them: >>>> >>>> >>>> >>>> > As with other handshake messages with no built-in response, >>>> KeyUpdates MUST be acknowledged. In order to facilitate epoch >>>> reconstruction (Section 4.2.2), implementations MUST NOT send records with >>>> the new keys or send a new KeyUpdate until the previous KeyUpdate has been >>>> acknowledged (this avoids having too many epochs in active use). >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-8-1 >>>> >>>> >>>> >>>> Now, the parenthetical says this is to avoid having too many epochs in >>>> active use, but it appears that there are stronger assumptions on this: >>>> >>>> >>>> >>>> > After the handshake is complete, if the epoch bits do not match those >>>> from the current epoch, implementations SHOULD use the most recent * >>>> *past** epoch which has matching bits, and then reconstruct the >>>> sequence number for that epoch as described above. >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-4.2.2-3 >>>> >>>> (emphasis mine) >>>> >>>> >>>> >>>> > After the handshake, implementations MUST use the highest available >>>> sending epoch [to send ACKs] >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-7-7 >>>> >>>> >>>> >>>> These two snippets imply the protocol wants the peer to definitely have >>>> installed the new keys before you start using them. This makes sense >>>> because sending stuff the peer can't decrypt is pretty silly. As an aside, >>>> DTLS 1.3 retains this text from DTLS 1.2: >>>> >>>> >>>> >>>> > Conversely, it is possible for records that are protected with the >>>> new epoch to be received prior to the completion of a handshake. For >>>> instance, the server may send its Finished message and then start >>>> transmitting data. Implementations MAY either buffer or discard such >>>> records, though when DTLS is used over reliable transports (e.g., SCTP >>>> [RFC4960]), they SHOULD be buffered and processed once the handshake >>>> completes. >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-4.2.1-2 >>>> >>>> >>>> The text from DTLS 1.2 talks about *a* handshake, which presumably >>>> refers to rekeying via renegotiation. But in DTLS 1.3, the epoch >>>> reconstruction rule and the KeyUpdate rule mean this is only possible >>>> during the handshake, when you see epoch 4 and expect epoch 0-3. The steady >>>> state rekeying mechanism never hits this case. (This is a reasonable change >>>> because there's no sense in unnecessarily introducing blips where the >>>> connection is less tolerant of reordering.) >>>> >>>> >>>> >>>> *Buffered handshake messages* >>>> >>>> >>>> >>>> Okay, so KeyUpdates want to wait for the recipient to install keys, >>>> except we don't seem to actually achieve this! Section 5.2 says: >>>> >>>> >>>> >>>> > DTLS implementations maintain (at least notionally) a >>>> next_receive_seq counter. This counter is initially set to zero. When a >>>> handshake message is received, if its message_seq value matches >>>> next_receive_seq, next_receive_seq is incremented and the message is >>>> processed. If the sequence number is less than next_receive_seq, the >>>> message MUST be discarded. If the sequence number is greater than >>>> next_receive_seq, the implementation SHOULD queue the message but MAY >>>> discard it. (This is a simple space/bandwidth trade-off). >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-5.2-7 >>>> >>>> >>>> >>>> I assume this is intended to apply to post-handshake messages too. (See >>>> below for a discussion of the alternative.) But that means that, when you >>>> receive a KeyUpdate, you might not immediately process it. Suppose >>>> next_receive_seq is 5, and the peer sends NewSessionTicket(5), >>>> NewSessionTicket(6), and KeyUpdate(7). 5 is lost, but 6 and 7 come in, >>>> perhaps even in the same record which means that you're forced to ACK both >>>> or neither. But suppose the implementation is willing to buffer 3 messages >>>> ahead, so it ACKs the 6+7 record, by the rules in section 7, which permits >>>> ACKing fragments that were buffered and not yet processed. >>>> >>>> >>>> >>>> That means the peer will switch keys and now all subsequent records >>>> from them will come from epoch N+1. But the sender is not ready for N+1 >>>> yet, so we contradict everything above. We also contradict this >>>> parenthetical in section 8: >>>> >>>> >>>> >>>> > Due to loss and/or reordering, DTLS 1.3 implementations may receive a >>>> record with an older epoch than the current one (the requirements above >>>> preclude receiving a newer record). >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-8-2 >>>> >>>> >>>> >>>> I assume then that this was not actually what was intended. >>>> >>>> >>>> >>>> *Options (and non-options)* >>>> >>>> >>>> >>>> Assuming I'm reading this right, we seem to have made a mess of things. >>>> The sender could avoid this by only allowing one active post-handshake >>>> transaction at a time and serializing them, at the cost of taking a >>>> round-trip for each. But the receiver needs to account for all possible >>>> senders, so that doesn't help. Some options that come to mind: >>>> >>>> >>>> >>>> *1. Accept that the sender updates its keys too early* >>>> >>>> >>>> >>>> Apart from contradicting most of the specification text, the protocol >>>> doesn't *break* per se if you just allow the peer to switch keys early >>>> in this buffered KeyUpdate case. We *merely* contradict all of the >>>> explanatory text and introduce a bunch of cases that the specification >>>> suggests are impossible. :-) Also the connection quality is poor. >>>> >>>> >>>> >>>> The sender will use epoch N+1 at a point when the peer is on N. But >>>> epoch reconstruction will misread it as N-3 instead of N+1, and either way >>>> you won't have the keys to decrypt it yet! The connection is interrupted >>>> (and with all packets discarded because epoch reconstruction fails!) until >>>> the peer retransmits 5 and you catch up. Until then, not only will you not >>>> receive application data, but you also won't receive ACKs. This also adds a >>>> subtle corner case on the sender side: the sender cannot discard the old >>>> sending keys because it still has unACKed messages from the previous epoch >>>> to retransmit, but this is not called out in section 8. Section 8 only >>>> discusses the receiver needing to retain the old epoch. >>>> >>>> >>>> This seems not great. Also it contradicts much of the text in the spec, >>>> including section 8 explicitly saying this case cannot happen. >>>> >>>> >>>> >>>> *2. Never ACK buffered KeyUpdates* >>>> >>>> >>>> >>>> We can say that KeyUpdates are special and, unless you're willing to >>>> process them immediately, you must not ACK the records containing them. >>>> This means you might under-ACK and the peer might over-retransmit, but >>>> seems not fatal. This also seems a little hairy to implement if you want to >>>> avoid under-ACKing unnecessarily. You might have message >>>> NewSessionTicket(6) buffered and then receive a record with >>>> NewSessionTicket(5) and KeyUpdate(7). That record may appear unACKable, but >>>> it's fine because you'll immediately process 5 then 6 then 7... unless your >>>> NewSessionTicket process is asynchronous, in which case it might not be? >>>> >>>> >>>> >>>> Despite all that mess, this seems the most viable option? >>>> >>>> >>>> >>>> *3. Declare this situation a sender error* >>>> >>>> >>>> >>>> We could say this is not allowed and senders MUST NOT send KeyUpdate if >>>> there are any outstanding post-handshake messages. And then the receiver >>>> should fail with unexpected_message if it ever receives KeyUpdate at a >>>> future message_seq. But as the RFC is already published, I don't know if >>>> this is compatible with existing implementations. >>>> >>>> >>>> >>>> *4. Explicit KeyUpdateAck message* >>>> >>>> >>>> >>>> We could have made a KeyUpdateAck message to signal that you've >>>> processed a KeyUpdate, not just sent it. But that's a protocol change and >>>> the RFC is stamped, so it's too late now. >>>> >>>> >>>> >>>> *5. Process KeyUpdate out of order* >>>> >>>> >>>> >>>> We could say that the receiver doesn't buffer KeyUpdate. It just goes >>>> ahead and processes it immediately to install epoch N+1. This seems like it >>>> would address the issue but opens more cans of worms. Now the receiver >>>> needs to keep the old epoch around for more than packet reorder, but also >>>> to pick up the retransmissions of the missing handshake messages. Also, by >>>> activating the new epoch, the receiver now allows the sender to KeyUpdate >>>> again, and again, and again. But, several epochs later, the holes in the >>>> message stream may remain unfilled, so we still need the old keys. Without >>>> further protocol rules, a sender could force the receiver to keep keys >>>> arbitrarily many records back. All this is, at best, a difficult case that >>>> is unlikely to be well-tested, and at worst get the implementation into >>>> some broken state and then misbehave badly. >>>> >>>> >>>> >>>> *6. Post-handshake transactions aren't ordered at all* >>>> >>>> >>>> >>>> It could be that my assumption above was wrong and the next_receive_seq >>>> discussion in 5.2 only applies to the handshake. After all, section 5.8.4 >>>> discusses how every post-handshake transaction duplicates the "state >>>> machine". Except it only says to duplicate the 5.8.1 state machine, and >>>> it's unclear ambiguous whether that includes the message_seq logic. >>>> >>>> >>>> >>>> However, going this direction seems to very quickly make a mess. If >>>> each post-handshake transaction handles message_seq independently, you >>>> cannot distinguish a retransmission from a new transaction. That seems >>>> quite bad, so presumably the intent was to use message_seq to distinguish >>>> those. (I.e. the intent can't have been to duplicate the message_seq >>>> state.) Indeed, we have: >>>> >>>> >>>> >>>> > However, in DTLS 1.3 the message_seq is not reset, to allow >>>> distinguishing a retransmission from a previously sent post-handshake >>>> message from a newly sent post-handshake message. >>>> >>>> https://www.rfc-editor.org/rfc/rfc9147.html#section-5.2-6 >>>> >>>> >>>> >>>> But if we distinguish with message_seq AND process transactions out of >>>> order, now receivers need to keep track of fairly complex state in case >>>> they process messages 5, 7, 9, 11, 13, 15, 17, ... but then only get the >>>> even ones later. And we'd need to define some kind of sliding window for >>>> what happens if you receive message_seq 9000 all of a sudden. And we import >>>> all the cross-epoch problems in option 5 above. None of that is in the >>>> text, so I assume this was not the intended reading, and I don't think we >>>> want to go that direction. :-) >>>> >>>> >>>> * Digression: ACK fate-sharing and flow control* >>>> >>>> >>>> >>>> All this alludes to another quirk that isn't a problem, but is a little >>>> non-obvious and warrants some discussion in the spec. Multiple handshake >>>> fragments may be packed into the same record, but ACKs apply to the whole >>>> record. If you receive a fragment for a message sequence too far into the >>>> future, you are permitted to discard the fragment. But if you discard >>>> *any* fragment, you cannot ACK the record, *even if there were >>>> fragments which you did process*. During the handshake, an >>>> implementation could avoid needing to make this decision by knowing the >>>> maximum size of a handshake flight. After the handshake, there is no >>>> inherent limit on how many NewSessionTickets the peer may choose to send in >>>> a row, and no flow control. >>>> >>>> >>>> >>>> QUIC ran into a similar issue here and said an implementation can >>>> choose an ad-hoc limit, after which it can choose to either wedge the >>>> post-handshake stream or return an error. >>>> >>>> https://github.com/quicwg/base-drafts/issues/1834 >>>> https://github.com/quicwg/base-drafts/pull/2524 >>>> >>>> >>>> >>>> I suspect the most practical outcome for DTLS (and arguably already >>>> supported by the existing text, but not very obviously), is to instead say >>>> the receiver just refuses to ACK stuff and, okay, maybe in some weird edge >>>> cases the receiver under-ACKs and then the sender over-retransmits, until >>>> things settle down. Whereas ACKs are a bit more tightly integrated with >>>> QUIC, so refusing to ACK a packet due to one bad frame is less of an >>>> option. Still, I think this would have been worth calling out in the text. >>>> >>>> >>>> >>>> >>>> >>>> So... did I read all this right? Did we indeed make a mess of this, or >>>> did I miss something? >>>> >>>> >>>> >>>> David >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>> TLS mailing list >>> TLS@ietf.org >>> https://www.ietf.org/mailman/listinfo/tls >>> >>
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