Hi Mohit, that are good points. > The reality however is that in most group PSK scenarios, the nodes don't have > any strong identities that can be verified. I assume blocking all identities that are not the nodes own identity would mitigate the attacks, but lead to availability problems as two nodes with the same chosen identity cannot talk to each other.
>What you should instead (or additionally) say in the text is that a node >should check that the client_mac and server_mac (or any >other identities >used) are never the same. I think checking ClientHello.random is enough. Below is what we wrote in https://tools.ietf.org/html/draft-selander-ace-cose-ecdhe-14 “If two nodes unintentionally initiate two simultaneous EDHOC message exchanges with each other even if they only want to complete a single EDHOC message exchange, they MAY terminate the exchange with the lexicographically smallest G_X. If the two G_X values are equal, the received message_1 MUST be discarded to mitigate reflection attacks. Note that in the case of two simultaneous EDHOC exchanges where the nodes only complete one and where the nodes have different preferred cipher suites, an attacker can affect which of the two nodes' preferred cipher suites will be used by blocking the other exchange.” John From: TLS <tls-boun...@ietf.org> on behalf of Mohit Sethi M <mohit.m.sethi=40ericsson....@dmarc.ietf.org> Date: Tuesday, 8 October 2019 at 11:57 To: Christopher Wood <c...@heapingbits.net>, "TLS@ietf.org" <tls@ietf.org> Subject: Re: [TLS] Selfie attack Hi Chris, For the benefit of the list, let me summarize that the selfie attack is only relevant where multiple parties share the same PSK and use the same PSK for outgoing and incoming connections. These situations are rather rare, but I accept that TLS is widely used (and sometimes misused) in many places. The Selfie attack only happens when an entity with the PSK acts maliciously. So I think the fact that you write in the draft: "each node must be trusted not to impersonate another node's role" does not take into account that there must be a malicious node for the selfie attack to happen in the first place. Drucker and Gueron's paper recommends that "every participating party gets (during the setup of the network) a unique identity" and "the client and the server must verify the validity of the claimed identities.". The reality however is that in most group PSK scenarios, the nodes don't have any strong identities that can be verified. What you should instead (or additionally) say in the text is that a node should check that the client_mac and server_mac (or any other identities used) are never the same. Without this check, the selfie attack would still be possible. And at least this does not require strong identity verification for preventing selfie attacks. --Mohit On 10/5/19 2:41 PM, Christopher Wood wrote: Hi Feng, For what it's worth, the latest version of the PSK importers draft includes a "context" field into which identity information can be fed: https://tools.ietf.org/html/draft-ietf-tls-external-psk-importer-01#appendix-B Best, Chris On Tue, Sep 24, 2019, at 9:19 AM, Hao, Feng wrote: 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. Cheers, Feng On 24/09/2019, 16:17, "John Mattsson" <john.matts...@ericsson.com><mailto:john.matts...@ericsson.com> wrote: Hi, 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 http://www.cs.bham.ac.uk/~tpc/cwi/Teaching/Files/Lecture4_6up.pdf 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). Cheers, John -----Original Message----- From: TLS <tls-boun...@ietf.org><mailto:tls-boun...@ietf.org> on behalf of "Hao, Feng" <feng....@warwick.ac.uk><mailto:feng....@warwick.ac.uk> Date: Tuesday, 24 September 2019 at 16:09 To: Mohit Sethi M <mohit.m.sethi=40ericsson....@dmarc.ietf.org><mailto:mohit.m.sethi=40ericsson....@dmarc.ietf.org>, "Owen Friel (ofriel)" <ofr...@cisco.com><mailto:ofr...@cisco.com>, Jonathan Hoyland <jonathan.hoyl...@gmail.com><mailto:jonathan.hoyl...@gmail.com> Cc: "TLS@ietf.org"<mailto:TLS@ietf.org> <tls@ietf.org><mailto:tls@ietf.org> 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" <tls-boun...@ietf.org on behalf of <mailto:tls-bounces@ietf.orgonbehalfofmohit.m.sethi=40ericsson....@dmarc.ietf.org> mohit.m.sethi=40ericsson....@dmarc.ietf.org><mailto:tls-bounces@ietf.orgonbehalfofmohit.m.sethi=40ericsson....@dmarc.ietf.org> wrote: Hi all, On the topic of external PSKs in TLS 1.3, I found a publication on the Selfie attack: https://protect2.fireeye.com/url?k=dd432f13-81c9e5ad-dd436f88-869a17b5b21b-dc6c6f0a5dd21faf&q=1&u=https%3A%2F%2Feprint.iacr.org%2F2019%2F347 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. [1] https://protect2.fireeye.com/url?k=5a822513-0608efad-5a826588-869a17b5b21b-eb260151f78b0718&q=1&u=https%3A%2F%2Feprint.iacr.org%2F2014%2F585.pdf [2] https://arxiv.org/abs/1802.04900 [3] https://protect2.fireeye.com/url?k=d5bf88ff-89354241-d5bfc864-869a17b5b21b-0e9b3bf58e104f32&q=1&u=https%3A%2F%2Feprint.iacr.org%2F2010%2F136.pdf _______________________________________________ TLS mailing list TLS@ietf.org<mailto:TLS@ietf.org> https://www.ietf.org/mailman/listinfo/tls _______________________________________________ TLS mailing list TLS@ietf.org<mailto:TLS@ietf.org> https://www.ietf.org/mailman/listinfo/tls _______________________________________________ TLS mailing list TLS@ietf.org<mailto:TLS@ietf.org> https://www.ietf.org/mailman/listinfo/tls
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