git-annex/doc/design/assistant/telehash.mdwn

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[Telehash](http://telehash.org/) for secure P2P communication between
git-annex (assistant) repositories.
## telelhash implementation status
* node.js version seems most complete
* C version currently lacks channel support and seems buggy (13 Jan 2014)
* No pure haskell implementation of telelhash v2. There was one of
telehash v1 (even that seems incomplete). I have pinged its author
to see if he anticipates updating it.
* Rapid development, situation may change in a month or 2.
## implementation basics
* Add a telehash.log that maps between uuid and telehash address.
* On startup, assistant creates a new telehash keypair if not already
present; stores this locally and generates a telehash address from it,
stored in telehash.log.
* Use telehash for notifications of changes to the repository
* Do git push over telehash. (Pretty easy, may need rate limiting in
situations involving relays.)
* Remove git push over XMPP (which has several problems including
XMPP being an unreliable transport, requiring a separate XMPP account per
repo, and XMPP not being end-to-end encrypted)
## telehash address discovery
* Easy way is any set of repos that are already connected can communicate
them via telehash.log.
* Local pairing can be used for telehash address discovery. Could be made
to work without ssh (with content transfer over telehash discussed
below).
* XMPP pairing can also be used for telehash address discovery. (Note that
MITM attacks are possible.) Is it worth keeping XMPP in git-annex just
for this?
* Telelhash addresses of repoitories can be communicated out of band (eg,
via an OTR session or gpg signed mail), and pasted into the webapp to
initiate a repository pairing that then proceeds entirely over telehash.
Once both sides do this, the pairing can proceed automatically.
## content transfer over telehash
* In some circumstances, it would be ok to do annexed content transfer
over telehash.
Need to check if there are MTU problems with large data bodies in
telelhash messages.
Probably not when a bridge is being used, due to required rate
limiting in bridging over telehash. Cloud transfer remotes still needed for
those situations.
* On a LAN, telehash can be used to determine the current local IP address
of another computer on the LAN. The 2 could then determine if either uses
ssh and if so use regular git-annex-shell for transfers. Or could do
annexed content transfer directly over telelhash.
## generic git-remote-telehash
This might turn out to be easy to split off from git-annex, so `git pull`
and `git push` can be used at the command line to access telehash remotes.
Allows using general git entirely decentralized and with end-to-end
encryption.