View and changing the SSH HostKey files

With all of the NSA leaks in the past few months, I figured it was a good time to go look at the SSH keys that we use on the servers and decide whether we want to re-key things. Naturally, this is a bit of a PITA because you'll have to let all clients know that the SSH host key changed and users will have to edit their ~/.ssh/known_hosts file.

First off, let's look at the current key information (using the "-l" option to display the signature, and the "-f filename" option to look at an existing file):

# /usr/bin/ssh-keygen -l -f /etc/ssh/ssh_host_dsa_key
1024 86:72:0c:d8:47:ce:c4:4a:79:25:9b:ad:22:1b:de:87 /etc/ssh/ssh_host_dsa_key.pub (DSA)
# /usr/bin/ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 
3072 2b:3d:27:77:49:bf:05:09:ee:b7:74:68:e8:f3:fc:3f /etc/ssh/ssh_host_rsa_key.pub (RSA)

This displays a few useful pieces of information:

#1 - The key size is 1024 for the DSA key. All DSA keys are 1024 bits in size due to FIPS 186-2 (Federal Information Processing Standard 186-2). While there is a newer FIPS 186-3 and FIPS 186-4 standard that allows larger DSA keys, I'm not sure how well supported it is in OpenSSH.

My RSA key is 3072 bits in size instead of the default 2048 bits in CentOS 6. Older releases had a default of only RSA/1024 bits, which is considered to be a bit weak today. The current recommended minimum is 2048 bits and the maximum in common use is 4096 bits.

A good read is Anatomy of a change - Google announces it will double its SSL key sizes.

#2 - The key signature, which should be communicated to your users via out-of-band communications.

To re-key, I suggest using the following for DSA keys:

# /usr/bin/ssh-keygen -N '' -C 'servername SSH host key Sep 2013' -t dsa -f /etc/ssh/ssh_host_dsa_key
Generating public/private dsa key pair.
/etc/ssh/ssh_host_dsa_key already exists.
Overwrite (y/n)? y
Your identification has been saved in /etc/ssh/ssh_host_dsa_key.
Your public key has been saved in /etc/ssh/ssh_host_dsa_key.pub.
The key fingerprint is:
86:72:0c:d8:47:ce:c4:4a:79:25:9b:ad:22:1b:de:87 severname SSH host key Sep 2013

For RSA keys, you need to change "-t dsa" to "-t rsa", change the filename, and add a "-b 2048" option before the "-f filename" option. Suggested key sizes are 2048 for short-term, 3172 for 1-2 decades, and 4096 for keys that will be in use past 2030. The downside is that as key length doubles, performance drops by a factor of 6-7x.