ssh-keygen defaults to RSA (for SSH protocol 2) and a 2048 bit key.

At this description of [ssh](http://unixhelp.ed.ac.uk/CGI/man-cgi?ssh%201)

```
         The supported ciphers
         are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr,
         aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow-
         fish-cbc, and cast128-cbc.  The default is:

           aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
           arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
           aes192-ctr,aes256-ctr

```

Thus it appears ciphers are either 128 or 256 bit, if I infer from unitless numbers. What is the justification for the ciphers being shorter than the key?