[tor-dev] Attentive Otter: Usability issues with existing OTR clients

Aaron aagbsn at torproject.org
Thu Oct 17 09:35:14 UTC 2013

On Thu, Oct 17, 2013 at 1:03 AM, Mike Perry <mikeperry at torproject.org>wrote:

> Here's the major usability pain points I've run in to with various OTR
> clients over the years. Feel free to chime in with any additional issues
> you've noticed.
> 1. The multiple client problem: If you have two devices online
> simultaneously under the same account, they will fight and endlessly
> renegotiate.
> This issue may be specific to OTRv2 only. Arlo pointed out that this
> change in OTRv3 may solve a lot of these problems:
> "Both fragmented and unfragmented messages contain sender and recipient
> instance tags. This avoids an issue on IM networks that always relay all
> messages to all sessions of a client who is logged in multiple times. In
> this situation, OTR clients can attempt to establish an OTR session
> indefinitely if there are interleaving messages from each of the
> sessions."
>  -- https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html
> We should be sure we use these features properly, and make sure the
> js-ctypes wrappers are updated to support the new libotr v3 APIs with
> these instance tags. The current js-ctypes wrapper may still be v2 only.
> If for some reason these tags in v3 can only be used if both parties
> support v3, perhaps we can add some additional code to track OTR state
> using XMPP resource/device info and tell the OTR lib underneath that
> different
> XMPP resources are actually different users, so that at least XMPP
> accounts won't have this problem.
> 2. Better TOFU support: We should track only one OTR key for a given
> XMPP resource/device ID.
> Some people I communicate with are constantly rotating OTR keys for
> security and other reasons. However, this doesn't help me any: I have
> accumulated about 50 keys of unknown age and usage pattern in my OTR
> fingerprint list for them, and I have no idea which ones have been
> changed due to compromise, device loss, or simply due to reinstall.
> Worse, I have no idea which keys should correspond to which XMPP
> resources/devices.

+10. This is huge.

As you point out, you get a "Unverified/Verified" when using OTR. Verifying
fingerprints of anonymous chat is practically impossible -- every side
channel I can think of (telephone, meeting in person, email - ha) either
leaks the act of contacting, isn't practical or convenient, or requires
some kind of Internet reputation. I'd bet that most OTR users don't verify

TOFU is a must, along with a dialogue that explains in simple terms what is
going on, with a way to dismiss the dialogue (e.g. by correctly answering a
security question).

Perhaps replacing "Unverified" with "In use for: x days" ?

> If a user changes the key for a given XMPP resource ID, we should expire
> the previous key and loudly alert the user of the fact that the
> resource's key has changed, and ask them if they want to replace the old
> key with the new key. We should not simply accumulate more keys by
> default, especially for the same XMPP resource/device, and especially
> not for unverified keys.

I think you mean that the existing key is expired after the user has OK'd
using the new key. The UI should not be something you can just click away
or ignore; it should be slightly inconvenient, and perhaps bidirectional
(the user whose key changed gets prompted as well).

At the same time, we should allow a different OTR key for each unique
> XMPP resource ID, and perhaps allow non-XMPP keys to accumulate at user
> option.

I'd like the opposite -- I would like to be able to bind the nickname *to*
the OTR key, but change the XMPP or IRC id, or to put it another way, be
able to keep a persistent cryptographic identity but use whatever
underlying transport or throwaway account is convenient.

Admittedly it's not as good as doing away with centralized resources
completely, and I think a Hidden Service transport would be very
interesting. (And it looks like the Tor network load testing results are
looking good ;-) )

> 3. "Unreadable encrypted message" errors should be visible only to
> advanced users/in the error console, and the client should transparently
> renegotiate and resend.
> It looks like OTR has an ERROR_START_AKE flag for this, but for some
> reason, some states of OTR in Pidgin still cause me to get and send
> these errors and miss messages though, usually if one client has crashed
> or restarted without properly ending the conversation. We should try
> extra hard to force key re-negotiation in these cases.
> We probably also want an auto-responder after the renegotiation that
> informs the other party that N messages were missed (for clients that
> don't support OTR's built-in notifications).
> 4. OTR shouldn't get stuck in the "Finished" state when you end a
> conversation.

Yeah, I've seen this one too.

> I've noticed in the past that OTR seemed to refuse to automatically
> negotiate/re-negotiate if it is still "Finished" regardless of your
> "Require/Force OTR" setting. This is just bad, and often causes
> plaintext to leak.
> 5. It should be impossible for our client to send a message in plaintext
> by default, and the client should discourage counterparties from sending
> more than one plaintext message.

> Perhaps related to issue 3, I've not had very good results with the
> "Require OTR"/"Force OTR" options on many clients.
> It also seems like many people's clients will still send the first
> message unencrypted (to get the handshake hint), and on all clients OTR
> doesn't seem to try very hard to negotiate if an unencrypted message is
> received.
> On our side, we should hold those messages and send only the handshake
> initiation (ie our default OTR mode should be REQUIRE_ENCRYPTION).

I think it might be simpler to block input until the OTR handshake has
completed. That is, the text input area is simply unavailable (in Force OTR

> We may also want to consider a plaintext auto-reply for when we receive
> unencrypted messages. Something like "Please install and enable OTR
> before sending further messages" as a separate message in *addition* to
> attempting an OTR handshake.
It might be better to not fingerprint the client that you are using. OTOH
the pidgin-otr plugin does behave this way.

> 6. The UI should have per-message encryption indicators (send and receive)
> For many clients, the UI is not clear which messages were sent
> encrypted, especially if the OTR state has changed due to
> autonegotiation or "Finishing". We should have per-message icons for
> both sent and received messages that indicate encryption status for that
> message in the chat scrollback window.

That could be a little cluttered; I'd suggest background tinting or only
flagging the unencrypted messages with skulls and crossbones.

> 7. Logging should be off by default when OTR is on.

I guess people like logging for some reason, and I'll admit that pidgins
"History" mode really enables asynchronous chat. Rather than require you
*turn off* OTR to get this feature, an alternative would be to require a
passphrase and automatically expunge encrypted logs after a configurable
period of time. And, unlike other clients, perhaps telling the conversation
counterparty that logging is enabled  (e.g. the opposite of  gchat "OTR"
notifications ) at the start of the session would provide informed consent.


8. Require (or at least allow) a passphrase to decrypt the OTR private key.
Or, store the key on a hardware token.

The number of arbitrary file read bugs in pidgin makes me cry...

> --
> Mike Perry
> _______________________________________________
> tor-dev mailing list
> tor-dev at lists.torproject.org
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