Tor for bootstrapping

[nothingmuch]

Summary

1. directories and relays should be available over a hidden service
   • Onion-Location HTTP header can be provided
     • but this is not an alternative for TLS based bootstrap, so onion addresses should be configured
     • unfortunately no mechanism to specify an equivalent clearnet address seems to exist
2. relays should support connecting to hidden service directories
   • senders that do not support tor can still access directories that are only available through tor
   • for example always online receivers can self host directories, allowing both v1 and v2 payjoins to be fully end to end encrypted, still accepting payjoins from v2 only senders that cannot access tor or from v1 only senders that can
3. A TLS free (i.e. Bitcoin core) implementation can bootstrap and therefore still operate as a secure receiver (senders can rely on OHTTP configuration included in the BIP 21 URI) so long as tor is available (e.g. via the SOCKS interface)

Discussion

OHTTP encapsulation requires a public key, which can be obtained by sending a GET request to the directory's /.well-known/ohttp-gateway resource, but normally this bootstrap requires TLS in order to authenticate the directory's response. Since certificate authorities these days are mainly domain validation signing oracles, they can be removed as trusted parties by using DANE to authenticate, but this still necessitates a TLS software stack.

After bootstrap, the relay is normally accessed using HTTPS over clearnet, and it connects to the directory over clearnet, allowing clients to access the directory using OHTTP.

In environments where Tor is available, relays and directories accessible via Tor hidden services and the hidden service addresses are authentic, this protects end to end encryption on the Tor network, and therefore makes it possible to bootstrap the OHTTP configuration as well as access the relay without trusting DNS or certificate authorities, and without directly requiring TLS (Tor still uses TLS, so it's still an indirect dependency).

If a directory is available over a hidden service, OHTTP requests can be submitted to it without going through an OHTTP relay first, and the target resources could even be made available directly since OHTTP is redundant at this point. Note that if this is distinguishable by the directory, whereas OHTTP requests submitted by relays are not. This is primarily a censorship concern.

Conversely, if a client can access a relay but not Tor, a relay with Tor connectivity can serve as a bridge allowing such clients to communicate with directories that are only available via an onion address, such as a self hosted directory that is not publicly reachable on clearnet.