Is it possible to allow key based authentication for sshd_config chroot sftp users?

Question

I'm running an Ubuntu 12.04.1 LTS (GNU/Linux 3.2.0-30-generic x86_64) Server, with OpenSSH_5.9p1 Debian-5ubuntu1, OpenSSL 1.0.1 14 Mar 2012.

I'd like to configure chroot for sftp access using settings in /etc/ssh/sshd_config, with key based authentication.

I've followed the tutorial here: http://www.thegeekstuff.com/2012/03/chroot-sftp-setup/

That tutorial helped me get sftp working with password authentication. I've modified the /etc/ssh/sshd_config file and copied the keys to try to get key based authentication working, but so far, no luck.

Relevant parts of my /etc/ssh/sshd_config file:

# Change to no to disable tunnelled clear text passwords
PasswordAuthentication yes
# Of course, I'd like to change this to no once key-based auth is working


# Commenting for chroot use
#Subsystem sftp /usr/lib/openssh/sftp-server
Subsystem       sftp    internal-sftp

Match Group sftpusers
        ChrootDirectory /sftp/%u
        ForceCommand internal-sftp
        PubkeyAuthentication yes
        AuthorizedKeysFile     %h/.ssh/authorized_keys

I've put the key in /sftp/testuser/incoming/.ssh/authorized_keys and experimented a bit by putting it in different directories. I've ensured that the rights are 600 on the authorized_keys file and that testuser owns it:

-rw------- 1 testuser sftpusers  616 Sep 21 09:20 authorized_keys

Here's the entry in /etc/passwd for the testuser:

testuser:x:1008:1010::/incoming:/usr/lib/openssh/sftp-server

Answer 1

Put your keys outside chroot (they are used before chroot any way). For example put your keys in /etc/ssh-pool/user.pub and change you config:

AuthorizedKeysFile /etc/ssh-pool/%u.pub