Re: [TLS] Selfie attack was Re: Distinguishing between external/resumption PSKs

"Hao, Feng" <> Tue, 24 September 2019 16:19 UTC

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From: "Hao, Feng" <>
To: John Mattsson <>, Mohit Sethi M <>, "Owen Friel (ofriel)" <>, Jonathan Hoyland <>
CC: "" <>
Thread-Topic: [TLS] Selfie attack was Re: Distinguishing between external/resumption PSKs
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Date: Tue, 24 Sep 2019 16:19:07 +0000
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Subject: Re: [TLS] Selfie attack was Re: Distinguishing between external/resumption PSKs
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Hi John,

Reflection attacks are indeed older, but the selfie attack is a bit different. It's actually a variant of the unknown key share attack. A typical example of the UKS attack is the one reported on MQV by Kaliski in 2001 (see "An unknown key-share attack on the MQV key agreement protocol" in ACM TISSEC 2001). In that example, the adversary plays message between two users to cause confusion in the identity, but in Selfie, the adversary plays message with only one user and uses another instance of the user to cause confusion in the identity. When we reported this variant of UKS in [3], we were not aware of anything like that in the literature.


On 24/09/2019, 16:17, "John Mattsson" <> wrote:

    I think these reflection attacks are much older than this. I quick search for reflection attack security protocol gives a lot of old results, The description of reflection attack in the following lecture material from 2009 looks just like the "selfie attack" on TLS 1.3
    With multiple sections there are other things that change as well. If two nodes unintentionally initiate simultaneous ClientHello to each other, even if they only want a single secure connection (I have seen live systems where this happens in practice), an attacker can select which ClientHello to block (e.g. the one with the strongest cryptographic parameters). The following security property would then no longer hold :
      "Downgrade protection:  The cryptographic parameters should be the
          same on both sides and should be the same as if the peers had been
          communicating in the absence of an attack"
    (I have not looked at what the definitions in [BBFGKZ16] say).
    -----Original Message-----
    From: TLS <> on behalf of "Hao, Feng" <>
    Date: Tuesday, 24 September 2019 at 16:09
    To: Mohit Sethi M <>rg>, "Owen Friel (ofriel)" <>om>, Jonathan Hoyland <>
    Cc: "" <>
    Subject: Re: [TLS] Selfie attack was Re: Distinguishing between external/resumption PSKs
        On 23/09/2019, 18:50, "TLS on behalf of Mohit Sethi M" < on behalf of> wrote:
            Hi all,
            On the topic of external PSKs in TLS 1.3, I found a publication on the 
            Selfie attack:
            Perhaps this was already discussed on the list. I thought that sharing 
            it again wouldn't hurt while we discuss how servers distinguish between 
            external and resumption PSKs.
        I just read the paper with interest. It occurs to me that the selfie attack is consistent with the "impersonation attack" that we reported on SPEKE in 2014; see Sec 4.1 [1] and the updated version with details on how SPEKE is revised in ISO/IEC 11770-4 [2]. The same attack can be traced back to 2010 in [3] where a "worm-hole attack" (Fig. 5, [3]) is reported on the self-communication mode of HMQV. The essence of these attacks is the same: Bob tricks Alice into thinking that she is talking to authenticated Bob, but she is actually talking to herself. In [3], we explained that the attack was missed from the "security proofs" as the proofs didn't consider multiple sessions. 
        The countermeasure we proposed in [1-3] was to ensure the user identity is unique in key exchange processes: in case of multiple sessions that may cause confusion in the user identity, an extension should be added to the user identity to distinguish the instances. The underlying intuition is that one should know "unambiguously" whom they are communicating with, and perform authentication based on that. The discovery of this type of attacks and the proposed solution are inspired by the "explicitness principle" (Ross Anderson and Roger Needham, Crypto'95), which states the importance of being explicit on user identities and other attributes in a public key protocol; also see [3]. I hope it might be useful to people who work on TLS PSK.
        TLS mailing list