bug#49714: 28.0.50; TRAMP burns CPU and has insufficient user reporting when using xxxx-sk SSH keys

[Dima Kogan]

Hi.

I have a yubikey, a hardware encryption device. For the purposes of SSH,
the SSH private key is split betweek a file on disk (.ssh/id_ed25519_sk)
and some data on the yubikey. When logging into an SSH server, the
interaction generally looks like this:

  client$ ssh ADDRESS

  Enter passphrase for key '/home/dima/.ssh/id_ed25519_sk':
  [the user enters the passphrase; not echoed to the console]

  Confirm user presence for key ED25519-SK XXXXXXXXXXXXXXX
  [the user physically touches the yubikey with their finger]

  server$
  [we are logged-in]

In TRAMP it looks like this:

   C-x C-f
   /ssh:ADDRESS:

   [Emacs asks for the passphrase. This is good]

   [Now we have a problem]

Emacs now spins the CPU, saying nothing. Internally, ssh is asking for
the user to touch the yubikey, but TRAMP is not communicating this
information. I know I should touch the yubikey, and if I do that, TRAMP
succeeds. There are two bugs here:

1. Emacs should not be spinning the CPU. This is actually an old bug. I
   usually see this with normal SSH connections, but it only happens
   during the login sequence, which isn't very long. In THIS case the
   login sequence could be longer. In any case, we shouldn't be
   spinning.

2. Emacs should relay whatever ssh is saying (i.e. "Confirm user
   presence...") to the user

Thanks!





I'm using emacs built from source on 2021-07-16. I'm pretty sure the