[TLS] DTLS Handshake race condition

Michael Tuexen <Michael.Tuexen@lurchi.franken.de> Sun, 11 August 2013 13:24 UTC

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From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de>
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Date: Sun, 11 Aug 2013 15:17:54 +0200
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Subject: [TLS] DTLS Handshake race condition
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Dear all,

while fixing a bug in OpenSSL regarding the DTLS handshake, I thought about
the following scenario (both sides decide to renegotiate at about the same

* A DTLS connection is established.
* The server sends a HelloRequest(MsgSeqNo = 0) and starts the retransmission
  timer, since this is a flight.
* The HelloRequest is dropped by the network.
* The client sends a ClientHello(MsgSeqNo = 0) and start a retransmission timer,
  since it is its first flight.
* The server receives the ClientHello, stops the retransmission timer
  and sends the next flight starting with ServerHello(MsgSeqNo = 1) 
  since it considers the received ClientHello as an ack for the flight.
* The client doesn't process the ServerHello, since it expects the
  MsgSeqNo == 0.

Therefore the client retransmits its ClientHello and the server retransmits
its flight containing the ServerHello. Am I missing something?

The problem is that the server has no way to figure out if the received
ClientHello is a reaction to a HelloRequest or not.
The only way out I see is that the client accepts ServerHellos with
MsgSeqNo=0 and MsgSeqNo=1.
I don't think this is covered in http://tools.ietf.org/html/rfc6347

Any opinions?

Best regards