[Standards] OMEMO Key Agreement

Sebastian Verschoor sebastian.verschoor at gmail.com
Tue Jun 6 23:35:52 UTC 2017


Thanks, I that clears some things up for me, not so worrisome after all, I
now believe that the users are properly authenticated.  I have thought
about the Matrix protocol with seperated DH/signing keys some more and
discussed it with a colleague and we believe it is fine.  A very minor
points springs to my mind, but I think you can judge for yourself if you
think this is an acceptable risk: Eve can impersonate Bob when she
compromises a private Curve25519 key that is signed by Bob's Ed25519 key
without compromising the Ed25519 private key itself.  This should not be
possible under normal circumstances, but I can think of two scenario's: (1)
Eve tricks Bob into signing a Curve25519 key generated by her.  This should
never happen as long as the only thing that Bob signs is his own locally
generated public keys; (2) Bob's random number generator is weak during
some identity-key generation, so Eve can guess/bruteforce his private
Curve25519 key.  Compare this to X3DH where a RNG failure results in the
loss of security for a session, not an identity.

I don't know how likely either scenario is in your context: you can better
judge that for yourself.

On 6 June 2017 at 09:14, Richard van der Hoff <richard at matrix.org> wrote:

> At the risk of turning the standards list into a forum discussing the
> finer points of cryptographic key exchange, I thought it might be useful
> weigh in on some of the issues raised, in the hope of clearing up some
> confusion.
>
> As Matthew has previously said: it's certainly not our intention to
> attempt to railroad the decisions of the XMPP community - just trying to
> make sure that the information on which such decisions are made is sound.
>
> Sebastian Verschoor wrote:
>
> TBH: I don't know enough about Olm to answer that. From what I read in the
>> specs they do 3DH: s = DH(IK_A, OTK_B) || DH(OTK_A, IK_B) || DH(OTK_A,
>> OTK_B).
>>
>>
> That's correct, as far as it goes. It's worth reiterating that, in the
> libolm implementation, IK_A and IK_B have previously been signed by the
> relevant user's (long-term) Ed25519 key.
>
> We don't consider that part of the core Olm protocol (which assumes you
> are starting with identity keys you trust, and focusses on the triple-DH
> and double ratchet); indeed an alternative implementation is possible in
> which you don't bother with the Ed25519 keys and treat the IKs as long-term
> instead, but since the only implementation of Olm I know of uses the
> Ed25519 key as the long-term key, we may as well forget it.
>
> In Matrix, at least, we also have Bob sign OTK_B which avoids the
> potential loss of forward secrecy if Eve later compromises IK_B, though it
> does come with a trade-off in deniability.
>
> I've tried to explain both of these points at
> https://matrix.org/git/olm/about/docs/signing.rst. (Sebastian previously
> described that document as 'worrisome' [1] but I wonder if that was based
> on some misunderstandings which have since been cleared up. If not, I'd be
> very glad to discuss your worries.)
>
> One other point here: although we refer to Alice's short-term key in this
> exchange as a one-time-key (hence OTK_A here), it is never "published"
> outside the Olm setup, so that makes it analogous to an ephemeral key in
> the Signal terminology. so E_A might be a better symbol for it. (In general
> the Olm spec is, regrettably, somewhat inconsistent in the use of the terms
> 'ephemeral key' vs 'one-time key' vs 'single-use key' and tends to treat
> them all interchangeably.)
>
> I don't understand yet what the 4th DH does, though. Given that I_b is
>>> signed (and can be replaced from time to time), I was assuming this
>>> corresponds to the signed prekey instead of the identity keys from X3DH.
>>>
>>
> This is a valid analogy.
>
> This would mean that Olm's 3DH doesn't use a long-standing identity key in
>>> its key agreement (it's only used to sign the prekey), and that Olm's 3DH
>>> involves signed prekeys from both parties (does this impact
>>> deniability?).
>>>
>>>
>> So there is no long-term key involved in the key agreement? I don't think
>> that such a protocol authenticates the other party... If that is true the
>> protocol has bigger problems than just deniability...
>>
>
> It's true that such an exchange doesn't identify the other party by
> itself. Assuming you're using a system whereby you use the Ed25519 key as
> the long-term key and periodically rotate the Curve25519 olm 'identity'
> key, this works as follows:
>
> 1. Alice and Bob exchange and verify each other's public Ed25519 keys
> out-of-band - or exchange them in-band and TOFU.
>
> 2. Alice publishes a message including her (current) public Curve25519
> identity key IK_A and signs it with her Ed25519 key. In doing so she is
> claiming control of the private part of IK_A (though there is no evidence
> of this yet). In other words: this signature prevents an eavesdropper Eve
> masquerading as Alice and publishing a key she controls in place of
> Alice's; it does *not* prevent Alice publishing someone else's Curve25519
> key as her own.
>
> 3. Bob downloads Alice's signed key, and likewise publishes his own signed
> key IK_B - again, claiming but not proving control of the private part of
> that key.
>
> 4. Alice now downloads Bob's signed key (along with one of his one-time
> keys), and initiates the 3DH exchange. She uses it to encrypt a pre-key
> message which *must* include her public Ed25519 key, as well as Bob's user
> ID.
>
> 5. Bob receives the pre-key message, and decrypts it. He must now check
> that the Ed25519 key embedded within the message matches that he has
> verified for Alice. This check prevents Alice publishing someone else's
> Curve25519 key as her own.
>
> (Bob also checks the user ID, to mitigate an unknown key-share attack
> whereby Mallory publishes Bob's IK_B as her own, and Alice thinks she is
> talking to Mallory, but Mallory is forwarding the messages to Bob, and Bob
> thinks the message was intended for him.)
>
> 6. Bob replies to the pre-key message including his own Ed25519 key, to
> allow Alice to finally verify that Bob controls IK_B.
>
>
> Regards
>
> Richard
>
>
> [1] https://mail.jabber.org/pipermail/standards/2017-June/032866.html
>
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