ssh-keygen -p [-P old_passphrase] [-N new_passphrase]
ssh-keygen -c [-P passphrase] [-C comment]
ssh-keygen -u [-f file] [-P passphrase]
Normally this program generates the key and asks for a file in which to store the private key. The public key is stored in a file with the same name but ".pub" appended. The program also asks for a passphrase. The passphrase may be empty to indicate no passphrase (host keys must have empty passphrase), or it may be a string of arbitrary length. Good passphrases are 10-30 characters long and are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per word, and provides very bad passphrases). The passphrase can be changed later by using the -p option.
There is no way to recover a lost passphrase. If the passphrase is lost or forgotten, you will have to generate a new key and copy the corresponding public key to other machines.
USING GOOD, UNGUESSABLE PASSPHRASES IS STRONGLY RECOMMENDED. EMPTY PASSPHRASES SHOULD NOT BE USED UNLESS YOU UNDERSTAND WHAT YOU ARE DOING.
There is also a comment field in the key file that is only for convenience to the user to help identify the key. The comment can tell what the key is for, or whatever is useful. The comment is initialized to user@host when the key is created, but can be changed using the -c option.
The cipher to be used when encrypting keys with passphrase is defined in ssh.h. Using the -u option, keys encrypted in any supported cipher can be updated to use this default cipher.
Contains the RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file using 3DES. This file is not automatically accessed by ssh-keygen, but it is offered as the default file for the private key.
Contains the public key for authentication. The contents of this file should be added to
%HOME%/.ssh/authorized_keys on all machines where you
wish to log in using RSA authentication. There is
no need to keep the contents of this file secret.