blag/post/ssh-key-authentication-with-encrypted-home-directories.mdwn

   1 [[!meta title="SSH key authentication with encrypted home directories"]]
   2 [[!meta date="2010-10-09"]]
   3 [[!meta author="rohieb"]]
   4 [[!meta license="CC-BY-SA 3.0"]]
   5 [[!flattrthing id=80700]]
   6
   7 Yesterday, I ran into an interesting problem: I tried to set up [SSH
   8 public key authentication][0] between two of my machines, `c3po` and
   9 `r2d2`, so I could log in from `rohieb@r2d2` to `rohieb@c3po` without a
  10 passphrase. However, everytime I tried to login to `c3po`, I was
  11 prompted to enter the passwort for `rohieb@c3po`, and the debug output
  12 mentioned something that the key could not be verified.  More
  13 astonishing, when I established a second SSH connection while the first
  14 was still running, I was *not* prompted for a password, and debug output
  15 said that key authentication had been sucessful. I googled a bit, and
  16 after a while got to [this comment][1] on Launchpad, mentioning problems
  17 when the user on the remote machine had its home directory encrypted
  18 through ecryptfs – which was the case for me. Of course, since ecryptfs
  19 only encrypts the user’s home *after* he has been authenticated, the SSH
  20 daemon cannot read his `~/.ssh/authorized_keys` at the first time, and
  21 falls back to password authentication.
  22
  23 [0]: http://www.openbsd.org/cgi-bin/man.cgi?query=ssh&sektion=1#AUTHENTICATION
  24 [1]: https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/362427/comments/12
  25
  26 The Launchpad comment proposes to first unmount the ecryptfs filesystem,
  27 then store `~/.ssh/authorized_keys` unencrypted, and then mount the
  28 encrypted home again (**note** that no program should be running that
  29 could try to access your home directory):
  30
  31     $ ecryptfs-umount-private
  32     $ cd $HOME
  33     $ chmod 700 .
  34     $ mkdir -m 700 .ssh
  35     $ chmod 500 .
  36     $ echo $YOUR_REAL_PUBLIC_KEY > .ssh/authorized_keys
  37     $ ecryptfs-mount-private
  38
  39 This works indeed, but has the drawback that key authentication only
  40 works for the *first* login, because ecryptfs hides the unencrypted
  41 files when it mounts the encrypted directory on login; and you had to
  42 synchronize the encrypted and the unencrypted version of
  43 `authorized_keys` everytime you add a new key. To circumvent that, I
  44 simply moved the file to `/etc/ssh/authorized_keys/rohieb` (with the
  45 file only readable and writable by me, and `/etc/ssh/authorized_keys`
  46 writeable for all users) and adjusting `/etc/ssh/sshd_config`
  47 appropriately:
  48
  49     $ sudo vi /etc/ssh/sshd_config  # or use your favorite editor instead of vi
  50     [... some lines ...]
  51     AuthorizedKeysFile /etc/ssh/authorized_keys/%u
  52     [... some more lines ...]
  53     $ sudo /etc/init.d/ssh restart
  54
  55 ## Update
  56
  57 There is yet a better approach instead, which doesn’t need the SSHd
  58 config to be edited at all:
  59
  60 1. login to the user on the remote machine
  61 2. create `/home/.ecryptfs/$USER/.ssh` and put your `authorized_hosts` there
  62 3. symlink your encrypted version there:
  63
  64        $ ln -s /home/.ecryptfs/$USER/.ssh/authorized_hosts ~/.ssh/authorized_hosts
  65
  66 4. symlink your unencrypted version there (as above, **make sure** no
  67    process wants to write to your home directory in the meantime):
  68
  69        $ ecryptf-umount-private
  70        $ mkdir ~/.ssh
  71        $ ln -s /home/.ecryptfs/$USER/.ssh/authorized_hosts ~/.ssh/authorized_hosts
  72        $ ecryptfs-mount-private
  73
  74 The paths are for Ubuntu 9.10 (Karmic Koala) and later. On other
  75 systems, you might want to replace `/home/.ecryptfs` with
  76 `/var/lib/ecryptfs`.
  77
  78 [[!tag Debian howto Ubuntu workaround eCryptfs encrypted_home Linux
  79     OpenSSH]]