Bits are cheap! Don’t sell yourself short on key length

Using SSH keys to perform password-free login is quite common in Unix hosts and in  Appliances that have embedded Unix (like Storwize products).

You effectively have a public key which is shared  and a private key (usually with a PPK extension) that is not shared.  Think of the public key like the lock in your front door, that  everyone can see.   Think of the private key like the door key in your pocket or hand-bag.   If you keep your private key secure, your door is relatively secure.  If you lose your keys, your door is most likely no longer secure (unless they are down the back of the couch).

Sticking with the door analogy, the risk with a door lock is that someone could still just try to kick your door in (brute force attack) or pick your lock.   The bit length of the key can make this harder to achieve: the longer the bit length the harder it is to crack.

It is not unusual to see instructions that suggest you use a command like this to generate keys, where a bit length of 1024 is specified with an RSA key:

ssh-keygen –b 1024 -t rsa -f ~/.ssh/id_rsa

Of if using PuTTYgen to create the keys, to see instructions like this:

  1. Start PuTTYgen by clicking Start > Programs > PuTTY > PuTTYgen. The PuTTY Key Generator panel is displayed.
  2. Click SSH-2 RSA as the type of key to generate.
    Note: Leave the number of bits in a generated key value at 1024.

The problem is that these instructions are all old.  In fact using the ssk-keygen command syntax example shown above would represent a down-grade in what is now the default setting.   The wiki and man pages for ssh-keygen both confirm that for RSA, the default length is now 2048 bits (not 1024 bits).

To confirm what key length you get by default, simply make a test key and then read it back.  In this example I create a new public/private key pair called testkey  without specifying a bit length (there is no -b 1024):

anthony$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/Users/anthony/.ssh/id_rsa): testkey
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in testkey.
Your public key has been saved in
The key fingerprint is:
SHA256:or3Yhykd0W569QcHtGk4ZMSdQDYlaM9ko+TiWQZ7pp4 anthony@Anthonys-Actifio-MacBook-Pro.local
The key's randomart image is:
+---[RSA 2048]----+
| =B+.. |
| . +.Bo+ |
| B O + o |
| + O = = |
| ..@S o . |
| o=.o . . . |
| .o.B . . o |
| .oE.o . . |
| ..oo . |

I then read the file back using the -l and -f params (specifying the name of the file) and confirm the bit length, which in this case is 2048 bits as highlighted by the red text:

anthony$ ssh-keygen -l -f testkey
2048 SHA256:or3Yhykd0W569QcHtGk4ZMSdQDYlaM9ko+TiWQZ7pp4 anthony@Anthonys-Actifio-MacBook-Pro.local (RSA)

When using PuTTYGen, if you use a recent version you will note that the default bit length is now 2048 (as indicated by the red circle).   If you load a key you should see the bit length of the loaded key as indicated by the orange circle.


So if you see instructions specifying the creation of a 1024 bit key, I suggest you ignore them and use 2048 bits or at the least question this with your vendor.   Equally if you are using older keys, it is well worth checking their bit length and generating new keys, since this will give you the now default bit length of 2048, but also renew them, reducing the risk of someone using an older (and potentially leaked) key inappropriately.





About Anthony Vandewerdt

I am an IT Professional who lives and works in Melbourne Australia. This blog is totally my own work. It does not represent the views of any corporation. Constructive and useful comments are very very welcome.
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