Keys with passphrases for SFTP

[josipl]

Currently ssh keys with passphareses raise following exception: twisted.conch.ssh.keys.EncryptedKeyError: encrypted key with no passphrase

Twisted has ​support for passphrases but currently there is no way in Tahoe-LAFS to acquire the passphrase from user.

The solution for now is just to generate keys without a passphrase, even though source:docs/frontends/FTP-and-SFTP.txt suggests otherwise.

[davidsarah]

FTP-and-SFTP.txt doesn't actually suggest otherwise, but I can see how the current wording could be confusing (it is actually referring to user passwords, not the passphrase of the server private key).

[warner]

Server-side keys don't generally have passphrases. When they do, things like sshd can't start up by themselves. The usual linux distributions creates passphraseless keys in /etc/ssh/ssh_host_dsa_key at install time.

I think it's perfectly fine to have Tahoe's SFTP server refuse to use passphrase'd server keys. And yeah, the docs should encourage this by showing an example of running 'ssh-keygen' without providing a passphrase.

[davidsarah]

Milestone 1.7 to make the docs clear that this isn't supported; then we can decide whether to leave the ticket open or wontfix it.

[davidsarah]

FTP-and-SFTP.txt: remove description of public key format that is not actually implemented. Document that SFTP does not support server private keys with passphrases, and that FTP cannot list directories containing mutable files.

[davidsarah]

Update to previous patch adding a 'Known Issues' section

[kevan]

The updated patch looks good to me; I like the known issues section.

[davidsarah]

Doc patches applied in e05c6c2c7d25db66 and 29a9059c94eef955.

[davidsarah]

Demoting this to minor; I have no plans to support server-side keys with passphrases, and there's lots more important stuff to do.