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

[Hanno Becker <[e-mail]>]

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]>
Sent: Monday, March 30, 2020 11:57 AM
To: Hanno Becker <[e-mail]>; [e-mail] <[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]> On Behalf Of Hanno Becker
Sent: Saturday, March 28, 2020 11:31 PM
To: [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.

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.