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thoughts on encryption

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Joey Hess 2013-06-13 21:38:07 -04:00
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doc/design/assistant/xmpp.mdwn
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@ -9,6 +9,28 @@ who share a repository, that is stored in the [[cloud]].
See <http://git-annex.branchable.com/design/assistant/blog/day_114__xmpp/#comment-aaba579f92cb452caf26ac53071a6788> See <http://git-annex.branchable.com/design/assistant/blog/day_114__xmpp/#comment-aaba579f92cb452caf26ac53071a6788>
* Support registering with XMPP provider from within the webapp, * Support registering with XMPP provider from within the webapp,
as clients like pidgin are able to do. as clients like pidgin are able to do.
* Add an encryption layer that does not rely on trusting the XMPP server's
security (currently it's encrypted only to the server, not end-to-end).
There are a few options for how to generate the key for eg,
AES encryption:
* Do a simple Diffie-Hellman shared key generation when starting each XMPP
push session. Would not protect the users from active MITM by the
XMPP server, but would prevent passive data gathering attacks from
getting useful data. Since the key is ephemeral, would provide
Forward Security.
* Do D-H key generation, but at pairing, not push time. Allows for an
optional confirmation step, using eg, BubbleBabble to compare the
keys out of band. ("I see xebeb-dibyb-gycub-kacyb-modib-pudub-sefab-vifuc-bygoc-daguc-gohec-kuxax .. do you too?")
* Prompt both users for a passphrase when XMPP pairing, and
use SPEKE (or similar methods like J-PAKE) to generate a shared key.
Avoids active MITM attacks. Makes pairing harder, especially pairing
between one's own devices, since the passphrase has to be entered on
all devices. Also problimatic when pairing more than 2 devices,
especially when adding a device to the set later, since there
would then be multiple different keys in use.
* Rely on the user's gpg key, and do gpg key verification during XMPP
pairing. Problimatic because who wants to put their gpg key on their
phone? Also, require the users be in the WOT and be gpg literate.
## design goals ## design goals
@ -109,9 +131,10 @@ status with a chat message, directed at the sender:
### security ### security
Data git-annex sends over XMPP will be visible to the XMPP Data git-annex sends over XMPP will be visible to the XMPP account's
account's buddies, to the XMPP server, and quite likely to other interested buddies, and to the XMPP server (and any attacker who has access to the
parties. So it's important to consider the security exposure of using it. XMPP server). So it's important to consider the security exposure of using
it.
Even if git-annex sends only a single bit notification, this lets attackers Even if git-annex sends only a single bit notification, this lets attackers
know when the user is active and changing files. Although the assistant's other know when the user is active and changing files. Although the assistant's other
@ -121,9 +144,11 @@ As soon as git-annex does anything unlike any other client, an attacker can
see how many clients are connected for a user, and fingerprint the ones see how many clients are connected for a user, and fingerprint the ones
running git-annex, and determine how many clients are running git-annex. running git-annex, and determine how many clients are running git-annex.
If git-annex sent the UUID of the remote it pushed to, this would let An attacker can observe the UUIDs used for pushes and pairing, and determine
attackers determine how many different remotes are being used, how many different remotes are being used.
and map some of the connections between clients and remotes.
An attacker could replay push notification messages, reusing UUIDs it's An attacker could replay push notification messages, reusing UUIDs it's
observed. This would make clients pull repeatedly, perhaps as a DOS. observed. This would make clients pull repeatedly, perhaps as a DOS.
The XMPP server, or an attacker with access to it can reconstruct the git
repository from data sent in pushes, in part or in whole.