The `root` password is ... irrelevant

Not the literal string irrelevant, but a correctly configured Unix host can be quite secure. To the point that knowing the root password gives you no advantage for getting in as root. I know the root password for this server, but I cannot login as root!

There is more on my page showing how to harden Linux and BSD, but the short version for BSD Unix is:

`[1]` Limit execution of `su` to members of group `wheel`

This is already the default on BSD!

But before you go any further, to prevent locking yourself out of your own system, make sure that your personal unprivileged account is a member of group wheel!

Once you are absolutely certain you can use an account that is a member of group wheel, you can do something like the following. First use the command:
# which su
to see if it is /usr/bin/su (probably the case on BSD, as shown below) or /bin/su (probably the case on Linux):

# chgrp wheel /usr/bin/su
# chmod 4550 /usr/bin/su
# ls -l /usr/bin/su
-r-sr-x--- 1 root root 22396 2008-01-11 23:16 /usr/bin/su

The meaning of those bits displayed as -r-sr-x--- is:

 Octal:      4       5       5       0
Binary:    1 0 0   1 0 1   1 0 1   0 0 0
Meaning:   ^ ^ ^   user    group   other
           | | |   r w x   r w x   r w x
           | | |
	   | | sticky   (not generally used on *nix files any more, see here for details if you care)
	   | setgid     (if 1, process has effective UID of file's group)
	   setuid       (if 1, process has effective UID of file's owner)

`[2]` Prevent `root` login on the console

The file /etc/ttys will contain several lines resembling these:

console "/usr/libexec/getty Pc"    vt220   off  secure 
ttyC0   "/usr/libexec/getty Pc"    vt220   on   secure 
ttyC1   "/usr/libexec/getty Pc"    vt220   on   secure 
[....]

Edit that file and delete every instance of the string secure, otherwise leaving those lines alone, so the file begins:

console "/usr/libexec/getty Pc"    vt220   off
ttyC0   "/usr/libexec/getty Pc"    vt220   on
ttyC1   "/usr/libexec/getty Pc"    vt220   on
[....]

If you are instead doing this on Linux or an SVR4 Unix, make the file /etc/securetty be an empty file.

`[3]` Set up SSH service cautiously

Put the following in /etc/ssh/sshd_config and restart the SSH daemon:

# Only run the SSH2 protocol, refuse SSH1
Protocol 2
# Do not allow root to login over SSH
PermitRootLogin no
# Only allow one user to login over SSH...
AllowUsers someuser
# ...and require that user to authenticate
#    using DSA/RSA cryptographic keys
PasswordAuthentication no

So how can I get in?

First, I need to know the username someuser. And no, it is not literally someuser!

Second, I have to start from a system that has the RSA/DSA private keys for someuser stored in my account's ~/.ssh/id_rsa and ~/.ssh/id_dsa. That would appear to require me to be either at my home or using my laptop. However, see this page for a description of how to put a Linux system and an emulator on a USB thumbdrive. That way, all I need is my thumbdrive and the use of some Internet-connected system.

Third, those keys are not stored as cleartext data, but they are encrypted with 3DES using a passphrase as the key. I must run the command ssh-add and type that passphrase. So, the loss or theft of my laptop or USB thumbdrive is limited to denial of service, not authentication spoofing.

But what if I had ....

The UNIX login password for someuser — If you got to the physical console, then yes, you could get in. But if you could get to the physical console, you could boot from removeable media! So you would just have a less effective method of doing what you could do anyway with physical access.

Because of the line PasswordAuthentication no in /etc/ssh/sshd_config, you cannot authenticate to SSH with a login password. So the UNIX login password for someuser is useless from a distance.

The UNIX login password for root — See the above for why this would be useless even if I had not removed the secure field for /dev/console in the file /etc/ttys. But I did, so this would be doubly useless.

The cryptographic keys for root — Uh, root doesn't have personal RSA/DSA keys on this machine. Sorry, but I hadn't mentioned that above.

The cryptographic keys for someuser — Well, yes, this would let you in. But to do that, you are going to have to break some serious cryptography. You will have to do one of the following:

Steal my laptop and break the 3DES encryption of the key files. I would prefer that OpenSSH used something stronger than 3DES (say, AES or Blowfish), but 3DES is going to be fairly strong. NIST and NSA say it has an equivalent strength of a 112-bit symmetric cipher for an attack against limited ciphertext, and 2¹¹² = 5,192,296,858,534,827,628,530,496,329,220,096
See "A known-plaintext attack on two-key triple encryption", van Oorschot, et al., EuroCrypt '90, and NIST Special Publication 800-57, "Recommendation for Key Management", August 2005, p 63, and: http://csrc.nist.gov/publications/nistpubs/800-57/SP800-57-Part1.pdf and:
http://csrc.nist.gov/publications/nistpubs/800-57/SP800-57-Part2.pdf
Intercept an SSH user authentication session and break the RSA or DSA encryption used for that session. It's a 1024-bit key space. Because of the mathematical differences between symmetric (3DES) and asymmetric (RSA, DSA) ciphers, that will be about the same amount of work as breaking 3DES.