Re: [TLS] 0-RTT and Anti-Replay

Eric Rescorla <ekr@rtfm.com> Sun, 22 March 2015 22:30 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 22 Mar 2015 15:30:10 -0700
Message-ID: <CABcZeBOAuraUEHbi1c1qJuBhZMgkSfx1ukB4ihtabNRE73KVRQ@mail.gmail.com>
To: Mohamad Badra <mbadra@gmail.com>
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Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] 0-RTT and Anti-Replay
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On Sun, Mar 22, 2015 at 3:26 PM, Mohamad Badra <mbadra@gmail.com>; wrote:

> Hi Eric,
>
>
>> Client                        Attacker                          Server
>>
>> ClientHello [+0-RTT] ------------>
>> "POST /buy-something" ----------->
>>
>>                               ClientHello [+0-RTT] --------------->
>>                               "POST /buy-something" -------------->
>>
>>                                                    [Processes purchase]
>>                                  <---------- ServerHello [accept 0-RTT]
>>                                                   (+ rest of handshake)
>>
>>
>> The attacker just discards the server's response and then the server
>> somehow forces the server to reboot (this is the hard part).
>>
>> Once the server has rebooted, the attacker re-sends the client's
>> initial flight then just forwards data back and forth to the client.
>>
>> Client                        Attacker                          Server
>>                               ClientHello [+0-RTT] --------------->
>>                               "POST /buy-something" -------------->
>>
>>    <--------------------------------------  ServerHello [reject 0-RTT]
>>                                                  (+ rest of handshake)
>>
>> Finished --------------------------------------------------------->
>> "Post /buy-something" -------------------------------------------->
>>                                                   [Processes purchase]
>>
>> At this point, you've mounted a successful replay attack. Obviously
>> this is not that great an attack because it requires you to be able to
>> force a reboot and also get in under the client's timeout window.
>>
>
>
> According to draft-rescorla-tls13-new-flows-01:
>
>    The EncryptedExtensions MUST include an ART indicator that matches
>    the client's ART so that the client knows that the 0-RTT handshake
>    was successful and that the client's application_data was accepted.
>
>
>
> I wonder why the Server doesn't include an indicator in his reject
> (encrypted indicator) so the client will avoid sending its request again.
>

This would basically be option #3, namely that this data is unreliable.

Best,
-Ekr

(BTW, based on what a server can reject after a reboot)
>
> Best regards
> Mohamad
>