On Sat, Apr 27, 2024, 8:03 AM David Benjamin <david...@chromium.org> wrote:
> What should the next steps be here? Is this a bunch of errata, or > something else? > Errata at a minimum but this might be big enough for a small RFC describing the fix. > > 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 >>>> >>> _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls >
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