sftp server does not accept pubkey auth

[gdt]

The docs suggest that one can put a public key in the ftp accounts file, but the same docs file says this is not yet implemented. Having to configure and enter a password makes it harder to do a full fuse mount automatically.

This is related to #1353.

[davidsarah]

<tarcieri>: is the sftp server gonna support pubkey auth any time soon?

<warner>: tarcieri: patches welcome :)

<warner>: to be honest, I forget how large or small a project that one is

<davidsarah>: I suspect that twisted conch does support client pubkey auth, we're just not exposing that

<warner>: yeah, I vaguely remember it being in there, just a bit hard to reach

<davidsarah>: so, convince us this is a really useful thing to add (shouldn't be hard since we hate passwords)

[warner]

Looks like the easy half is done: frontends/auth.py parses the pubkey lines from the account file and stashes them in the right place.

The hard half will involve code from twisted/conch/checkers.py, probably from SSHPublicKeyDatabase._cbRequestAvatarId, specifically this bit:

pubKey = keys.Key.fromString(credentials.blob)
if pubKey.verify(credentials.signature, credentials.sigData):
    return credentials.username

plus some code to make sure we're using the right key for the requested username.

[davidsarah]

Yes, it does look easy. There's a full example at ​http://www.devshed.com/c/a/Python/SSH-with-Twisted/2/. (I don't know why that example has the server knowing the client's private key. It doesn't need it; that's the whole point of public key auth! I think you can just omit the sshFactory.privateKeys = ... line.)

[pyhedgehog]

sftp-auth-key-1411.patch​ contains diff for src/allmydata/frontends/auth.py and enables to setup several keys each with it's own rootcap.

[zooko]

Needs test before further review.