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What this command does is create a directory named 
.ssh
 in your 
home
 directory, if it 
doesn't already exist. Inside that directory, it will create two files, 
id_rsa
 and 
id_rsa.
pub
. The 
id_rsa
 file is your private key. It should never leave your machine, be given 
to another user, or be stored on any external media. If your private key leaks out, 
your keys can no longer be trusted. By default, the private key is owned by the user 
that created it, with 
rw
 permissions given only to its owner.
The public key, on the other hand, can leave your computer and doesn't need to 
be secured as much. Its permissions are more lenient, being readable by everyone 
and writeable by the owner. You can see this yourself by executing 
ls -l /
home//.ssh
:
Figure 10.9: Listing the contents of the .ssh directory, showing the permissions of the newly created keys
The public key is the key that actually gets copied to other servers to facilitate your 
being able to log in via such a key-pair. When you log in to a server that has your 
key, it checks that it's a mathematical match to your private key, and if it is, it will let 
you log in. You'll also be asked for your passphrase, if you chose to set one when you 
first created it. But before we can actually use our key, we need to copy it over to the 
server we intend to connect to.