Re: New(ish) draft: Secure Messaging in XMPP

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> On 23 okt. 2015, at 23:18, Adam Roach <[email protected]> wrote:
>=20
> XMPP folks:
>=20
> Martin and I put together a proposal for an approach that allows for =
end-to-end encrypted XMPP conversations, including in the presence of =
MUC. Although not a completely implementable spec, this should give a =
good idea about the direction we have in mind:
>=20
> https://tools.ietf.org/html/draft-thomson-xmpp-secure-00
>=20
> Anyone interested in this work should give it a read and provide =
feedback. In particular, I'm curious if anyone interested in =
implementing this kind of thing has requirements can't be addressed with =
the high-level approach we're describing.
>=20
> Thanks!
>=20
> /a

Hi Adam,

I've read through the draft, and there are still some things unclear to =
me.

Forward secrecy is never mentioned. =46rom the design, I can't tell if =
it was a
requirement. The symmetric key that is generated in =C2=A76.2 can be =
decrypted by
anyone who has a log of all stanzas and the private Diffie-Hellman key =
of either
party. The symmetric key has a limited lifetime, but that is not enough =
if the
DH keys do not expire and I don't see the document mention that they do.

The support for offline messaging is also a bit unclear to me. =C2=A76.4 =
suggests
that a client can send a Symmetric Key Advertisement to a client that is
currently offline. But how would the sender obtain the DH public key for =
the
recipient if that key is advertised using presence (as stated in =C2=A76.1=
)? The
receiving client would also need to wait for the sender to be online to =
obtain
its DH public key before it can decrypt the key. (And the sender would =
be unable
to change their DH key until all advertisements have been acknowledged,
destroying forward secrecy.)

To be honest, I think the current OMEMO draft, while not perfect, =
matches my
requirements a lot better.

Regards,
Thijs

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