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blog for the day

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Joey Hess 2012-09-11 16:16:11 -04:00
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Tons of pairing work, which culminated today in pairing fully working for
the very first time. And it works great! Type something like "my
hovercraft is full of eels" into two git annex webapps on the same LAN
and the two will find each other, automatically set up ssh keys, and sync
up, like magic. Magic based on math.
* Revert changes made to `authorized_keys` when the user cancels
a pairing response. Which could happen if the machine that sent the
pairing request originally is no longer on the network.
* Some fixes to handle lossy UDP better. Particularly tricky at the end
of the conversation -- how do both sides reliably know when a
conversation is over when it's over a lossy wire? My solution is just
to remember some conversatons we think are over, and keep saying
"this conversation is over" if we see messages in that conversation.
Works.
* Added a UUID that must be the same in related pairing messages.
This has a nice security feature: It allows detection of brute-force
attacks to guess the shared secret, after the first wrong guess!
In which case the pairing is canceled and a warning printed.
* That led to a thorough security overview, which I've added to
the [[pairing]] page. Added some guards against unusual attacks,
like console poisioning attacks. I feel happy with the security
of pairing now, with the caveats that only I have reviewed it (and
reviewing your own security designs is never ideal), and that the
out-of-band shared secret communication between users is only as good
as they make it.
* Found [a bug](https://github.com/yesodweb/yesod/issues/421)
in Yesod's type safe urls. At least, I think it's a bug. Worked around it.
* Got very stuck trying to close the sockets that are opened to send
multicast pairing messages. Nothing works, down to and including calling
C `close()`. At the moment I have a socket leak. :(
I need to understand the details of multicast sockets better to fix this.
Emailed the author of the library I'm using for help.

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.. seems it just doesn't support IPv6. The pairing code in git-annex
does support ipv6, apart from this, it's just broadcasting the messages
that fails. (Pairing over mixed networks is fine.)
* If there are three assistants on the network, and 2 pair, the third is
left displaying a "Pair request from foo" alert, until it's close.
Or, if the user clicks the button to pair, it'll get to the
"Pairing in progress" alert, which will show forever (until canceled).
It should be possible for third parties to tell when pairing is done,
but it's actually rather hard since they don't necessarily share the secret.