Re: [TLS] Selfie attack

[John Mattsson <[e-mail]>]

I think things mitigating the Selfie attack should be done in an errata to RFC 8446 or in a draft updating RFC 8446. If draft-ietf-tls-external-psk-importer is the mitigation against selfie attacks I think it needs to update RFC 8446.

John

-----Original Message-----
From: TLS <[e-mail]> on behalf of Mohit Sethi M <mohit.m.sethi=[e-mail]>
Date: Friday, 11 October 2019 at 14:32
To: Christopher Wood <[e-mail]>, Mohit Sethi M <[e-mail]>, "[e-mail]" <[e-mail]>
Subject: Re: [TLS] Selfie attack

    Hi,
    
    I created a pull request for the draft to improve the section on 
    addressing selfie attacks. For the benefit of the list, here is the 
    proposed text:
    
    > The TLS external PSK authentication makes the implicit assumption that 
    > each PSK is known only to one client and one server, which do not 
    > switch roles with the same PSK.
    >
    > If multiple clients or multiple servers share a PSK, TLS only 
    > authenticates the entire group. Not only can a compromised group 
    > member impersonate another group member, but a malicious non-member 
    > can reroute handshakes between honest group members to connect them in 
    > unintended ways. This rerouting is a type of identity misbinding 
    > attack [Krawczyk][Sethi].
    >
    > Selfie attack [Selfie] is a special case of the rerouting attack 
    > against a group member that can act both as TLS server and client. In 
    > the selfie attack, a malicious non-member reroutes a connection from 
    > the client to the server on the same endpoint.
    >
    > Rerouting and selfie attacks can be detected by binding the TLS 
    > handshake to globally unique node identifiers using the following 
    > context string:
    >
    > struct {
    >        opaque client_id<0..2^16-1>;
    >        opaque server_id<0..2^16-1>;
    >      } Context;
    >
    > When the PSK is shared by only two endpoints, it is not necessary to 
    > know the identifier(s) of the other endpoint. Instead, it is 
    > sufficient to check that the identifier of the other endpoint in 
    > context is not equal to any of one’s own identifiers. To simplify 
    > implementation of this check, it is RECOMMENDED that each endpoint 
    > selects one globally unique identifier and uses it in all PSK 
    > handshakes. The unique identifier can, for example, be one of its MAC 
    > addresses or a 32-byte random number.
    >
    > When the PSK is a group key, the comparison with one’s own identifiers 
    > will only prevent selfie attacks but not malicious rerouting of the 
    > connection to another group member. To prevent malicious rerouting in 
    > groups, each endpoint needs to know the identifier of the other 
    > endpoint with which they want to connect and compare it with the other 
    > endpoint’s identifier in context. Of course, this only prevents 
    > attacks by non-members; the endpoints that share the group key can 
    > always impersonate each other.
    
    --Mohit
    
    On 10/8/19 7:46 PM, Christopher Wood wrote:
    > On Tue, Oct 8, 2019, at 2:55 AM, Mohit Sethi M wrote:
    >>   
    >> 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*.
    > This seems implicit in the text as written. Could I ask you to submit a PR against [1] to clarify?
    >
    > Thanks,
    > Chris
    >
    > [1] https://protect2.fireeye.com/url?k=bb68fbe7-e7e2d928-bb68bb7c-0cc47ad93ea4-ddab33427b4471c9&q=1&u=https%3A%2F%2Fgithub.com%2Ftlswg%2Fdraft-ietf-tls-external-psk-importer
    >
    >> 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" <[e-mail]> 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 <[e-mail]> on behalf of "Hao, Feng"
    >>>> <[e-mail]>
    >>      Date: Tuesday, 24 September 2019 at 16:09
    >>      To: Mohit Sethi M <mohit.m.sethi=[e-mail]>,
    >> "Owen Friel (ofriel)" <[e-mail]>, Jonathan Hoyland
    >>>> <[e-mail]>
    >>      Cc: "[e-mail]" <[e-mail]>
    >>      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"
    >>>> <[e-mail] on behalf of
    >> mohit.m.sethi=[e-mail]>
    >> <mailto:[e-mail]=[e-mail]> 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
    >>          
    >>          
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