Re: [TLS] Possible deadlock when ACKing KeyUpdate messages?

[Hanno Becker <[e-mail]>]

Hi Hannes,

Thank for opening a PR!

I think the underlying issue here is the conception that all that needs to be ensured is the ability to ACK
retransmissions of the KeyUpdate, and the modified version still gives that impression. If that was true,
the use of 'canned ACKs' as in Draft 37 would be fine. However, the example shows that there's more to
retaining old keying material than just ACKing retransmitted KeyUpdates, and I'd therefore opt for a more
intrusive change of wording, such as:

Due to the possibility of an ACK message for a KeyUpdate being lost and thereby
preventing the sender of the KeyUpdate from updating its keying material,
receivers MUST retain the pre-update keying material until receipt and successful
decryption of a message using the new keys. Receivers MUST NOT delete the
pre-update keying material immediately and use canned ACK messages for
retransmitted KeyUpdate messages, since situations can occur in which receivers
still need to be able to decrypt messages secured with pre-update keying material.


Maybe the second part is better treated as a note or implementation warning, though.

Best,
Hanno
________________________________
From: Hannes Tschofenig <[e-mail]>
Sent: Tuesday, March 31, 2020 7:28 AM
To: Hanno Becker <[e-mail]>; [e-mail] <[e-mail]>
Subject: RE: [TLS] Possible deadlock when ACKing KeyUpdate messages?


Hi Hanno,



I believe the part of the paragraph that talks about “canned ACKs” needs to be clarified.



In https://github.com/tlswg/dtls13-spec/pull/135 I changed the text  from



“

   Although KeyUpdate MUST be acknowledged, it is possible for the ACK

   to be lost, in which case the sender of the KeyUpdate will retransmit

   it.  Implementations MUST retain the ability to ACK the KeyUpdate for

   up to 2MSL.  It is RECOMMENDED that they do so by retaining the pre-

   update keying material, but they MAY do so by responding to messages

   which appear to be out-of-epoch with a canned ACK message; in this

   case, implementations SHOULD rate limit how often they send such

   ACKs.

"



To:



“

Although KeyUpdates MUST be acknowledged, it is possible for the ACK to be

lost, in which case the sender of the KeyUpdate will retransmit it.

The receiver of the KeyUpdate MUST retain the ability to ACK the KeyUpdate for

up to 2MSL. It is RECOMMENDED that they do so by retaining the

pre-update keying material, but they MAY remove the pre-update

keying material only upon receipt and successful decryption of a message

using the new keys.

“



Ciao

Hannes





From: Hanno Becker <[e-mail]>
Sent: Monday, March 30, 2020 8:02 PM
To: Hannes Tschofenig <[e-mail]>; [e-mail]
Subject: Re: [TLS] Possible deadlock when ACKing KeyUpdate messages?



Hi Hannes,



>  In your example below, the sender of the initial KeyUpdate has to re-send it because of the lost ACK. In order to resubmit it, it has to use the old keying material (or cache the message). The receiver cannot immediately delete keying material after processing the initial KeyUpdate message because it does not know whether the ACK will subsequently get lost.



My point is that the paragraph cited at the top of my post appears to say that receivers MAY immediately delete keying material after

receiving a KeyUpdate IF they blindly ACK retransmissions of the KeyUpdate (even though they can't decrypt it anymore). The example

shows that this doesn't work, unless I've made a mistake.



Best,

Hanno

________________________________

From: Hannes Tschofenig <[e-mail]<mailto:[e-mail]>>
Sent: Monday, March 30, 2020 11:57 AM
To: Hanno Becker <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Subject: RE: [TLS] Possible deadlock when ACKing KeyUpdate messages?



Hi Hanno, Hi all,



I believe it would be useful to add some extra sentences to the draft to retaining the old key material.



In your example below, the sender of the initial KeyUpdate has to re-send it because of the lost ACK. In order to resubmit it, it has to use the old keying material (or cache the message). The receiver cannot immediately delete keying material after processing the initial KeyUpdate message because it does not know whether the ACK will subsequently get lost.



Ciao

Hannes



From: TLS <[e-mail]<mailto:[e-mail]>> On Behalf Of Hanno Becker
Sent: Saturday, March 28, 2020 11:31 PM
To: [e-mail]<mailto:[e-mail]>
Subject: [TLS] Possible deadlock when ACKing KeyUpdate messages?



In relation to ACKs for KeyUpdate messages, DTLS 1.3 Draft 37 states:



   Although KeyUpdate MUST be acknowledged, it is possible for the ACK

   to be lost, in which case the sender of the KeyUpdate will retransmit

   it.  Implementations MUST retain the ability to ACK the KeyUpdate for

   up to 2MSL.  It is RECOMMENDED that they do so by retaining the pre-

   update keying material, but they MAY do so by responding to messages

   which appear to be out-of-epoch with a canned ACK message; in this

   case, implementations SHOULD rate limit how often they send such

   ACKs.



This seems to allow implementations to remove old incoming keys immediately

after ACKing the KeyUpdate, which appears to open the door for the following

situation leading to deadlock:





  +-------------------------+

  |   KeyUpdate, epoch N    |-------------> received

  +-------------------------+

                                          +------------------------+

                               lost x-----|     ACK,   epoch M     |

                                          +------------------------+



                                          [ new incoming epoch N+1,

                                            remove keys for epoch N ]



-                                         +------------------------+

                  received  <-------------|   KeyUpdate, epoch M   |

                                          +------------------------+

  +-------------------------+

  |       ACK, epoch N      |-------[ irrelevant whether it goes through - see below ]

  +-------------------------+



  [ new incoming epoch M+1,

    remove keys for epoch M ]



Note: This isn't an entirely unlikely situation, since a KeyUpdate with update_requested flag

will result in a subsequent KeyUpdate from the other side, and the only unlucky thing that

needs to happen is for the original ACK to be lost while both KeyUpdate messages go through.



At this point, both sides have updated their incoming key material but

not their outgoing key material, since they're still awaiting the ACK -

however, it turns out that they can't actually read those ACKs anymore:



After some time, the peers resend the KeyUpdate messages, which will be

blindly ACKed by the peer according to the recommendation in the spec;

however, the ACKs will be encrypted with the wrong keys and cannot

be parsed on either side:



  +---------------------------+

  | resent KeyUpdate, epoch N |-------------> received, but can't be read

  +---------------------------+               because incoming epoch N+1



                                                send 'blind' ACK



  received, but can't be read                +------------------------+

  because incoming epoch M+1  <--------------|     ACK,   epoch M     |

                                             +------------------------+



The same will happen to KeyUpdate retransmission from the other side.



It seems that this results in a deadlock. Am I missing / misunderstanding something?



A possible mitigation would be to force retaining the old key material for 2MSL,

or alternatively, to mandate that old key material must only be removed upon

receipt and successful decryption of a message using the new keys.

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