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NAME
ssh-keygen - authentication key pair generation
SYNOPSIS
ssh-keygen [-b bits] [-f file] [-N new_passphrase]
[-C comment]
ssh-keygen -p [-P old_passphrase] [-N new_passphrase]
ssh-keygen -c [-P passphrase] [-C comment]
ssh-keygen -u [-f file] [-P passphrase]
DESCRIPTION
Ssh-keygen generates and manages authentication keys for
ssh(1). Normally each user wishing to use ssh with RSA
authentication runs this once to create the authentication
key in $HOME/.ssh/identity. Additionally, the system
administrator may use this to generate host keys.
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 pro-
gram 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 sim-
ple 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 a passphrase
is defined in ssh.h. Using the -u option, keys encrypted in
any supported cipher can be updated to use this default
cipher.
OPTIONS
-b bits
Specifies the number of bits in the key to create.
Minimum is 512 bits. Generally 1024 bits is con-
sidered sufficient, and key sizes above that no longer
improve security but make things slower. The default
is 1024 bits.
-c Requests changing the comment in the private and pub-
lic key files. The program will prompt for the file
containing the private keys, for passphrase if the key
has one, and for the new comment.
-f Specifies the file name in which to load/store the
key.
-p Requests changing the passphrase of a private key file
instead of creating a new private key. The program
will prompt for the file containing the private key,
for the old passphrase, and twice for the new
passphrase.
-u Requests that the key's cipher is changed to the
current default cipher (determined at compile-time -
currently 3DES).
-C Provides the new comment.
-N Provides the new passphrase.
-P Provides the (old) passphrase.
FILES
$HOME/.ssh/random_seed
Used for seeding the random number generator. This
file should not be readable by anyone but the user.
This file is created the first time the program is
run, and is updated every time.
$HOME/.ssh/identity
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.
$HOME/.ssh/identity.pub
Contains the public key for authentication. The con-
tents of this file should be added to
$$HHOOMMEE//..sssshh//aauutthhoorriizzeedd_kkeeyyss on all machines where you
wish to log in using RSA authentication. There is no
need to keep the contents of this file secret.
AUTHOR
Tatu Ylonen <ylo@ssh.fi>
SEE ALSO
ssh(1), sshd(8), ssh-agent(1), ssh-add(1)
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