ssh-keygen.1

.\"  -*- nroff -*-
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
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.Dd September 25, 1999
.Dt SSH-KEYGEN 1
.Os
.Sh NAME
.Nm ssh-keygen
.Nd authentication key generation
.Sh SYNOPSIS
.Nm ssh-keygen
.Op Fl dq
.Op Fl b Ar bits
.Op Fl N Ar new_passphrase
.Op Fl C Ar comment
.Op Fl f Ar output_keyfile
.Nm ssh-keygen
.Fl p
.Op Fl P Ar old_passphrase
.Op Fl N Ar new_passphrase
.Op Fl f Ar keyfile
.Nm ssh-keygen
.Fl x
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl X
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl y
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl c
.Op Fl P Ar passphrase
.Op Fl C Ar comment
.Op Fl f Ar keyfile
.Nm ssh-keygen
.Fl l
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl R
.Sh DESCRIPTION
.Nm
generates and manages authentication keys for
.Xr ssh 1 .
.Nm
defaults to generating an RSA key for use by protocols 1.3 and 1.5;
specifying the
.Fl d
flag will create a DSA key instead for use by protocol 2.0.
.Pp
Normally each user wishing to use SSH
with RSA or DSA authentication runs this once to create the authentication
key in
.Pa $HOME/.ssh/identity
or
.Pa $HOME/.ssh/id_dsa .
Additionally, the system administrator may use this to generate host keys,
as seen in
.Pa /etc/rc .
.Pp
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
.Dq .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
.Fl p
option.
.Pp
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.
.Pp
For RSA, 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
.Dq user@host
when the key is created, but can be changed using the
.Fl c
option.
.Pp
After a key is generated, instructions below detail where the keys
should be placed to be activated.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b Ar bits
Specifies the number of bits in the key to create.
Minimum is 512 bits.
Generally 1024 bits is considered sufficient, and key sizes
above that no longer improve security but make things slower.
The default is 1024 bits.
.It Fl c
Requests changing the comment in the private and public 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.
.It Fl f
Specifies the filename of the key file.
.It Fl l
Show fingerprint of specified private or public key file.
.It Fl 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.
.It Fl q
Silence
.Nm ssh-keygen .
Used by
.Pa /etc/rc
when creating a new key.
.It Fl C Ar comment
Provides the new comment.
.It Fl N Ar new_passphrase
Provides the new passphrase.
.It Fl P Ar passphrase
Provides the (old) passphrase.
.It Fl R
If RSA support is functional, immediately exits with code 0.  If RSA
support is not functional, exits with code 1.  This flag will be
removed once the RSA patent expires.
.It Fl x
This option will read a private
OpenSSH DSA format file and print a SSH2-compatible public key to stdout.
.It Fl X
This option will read a unencrypted
SSH2-compatible private (or public) key file and
print an OpenSSH compatible private (or public) key to stdout.
.It Fl y
This option will read a private
OpenSSH DSA format file and print an OpenSSH DSA public key to stdout.
.El
.Sh FILES
.Bl -tag -width Ds
.It Pa $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
.Nm
but it is offered as the default file for the private key.
.Xr sshd 8
will read this file when a login attempt is made.
.It Pa $HOME/.ssh/identity.pub
Contains the public key for authentication.
The contents of this file should be added to
.Pa $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.
.It Pa $HOME/.ssh/id_dsa
Contains the DSA 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
.Nm
but it is offered as the default file for the private key.
.Xr sshd 8
will read this file when a login attempt is made.
.It Pa $HOME/.ssh/id_dsa.pub
Contains the public key for authentication.
The contents of this file should be added to
.Pa $HOME/.ssh/authorized_keys2
on all machines
where you wish to log in using DSA authentication.
There is no need to keep the contents of this file secret.
.El
.Sh AUTHOR
Tatu Ylonen <ylo@cs.hut.fi>
.Pp
OpenSSH
is a derivative of the original (free) ssh 1.2.12 release, but with bugs
removed and newer features re-added.
Rapidly after the 1.2.12 release,
newer versions bore successively more restrictive licenses.
This version of OpenSSH
.Bl -bullet
.It
has all components of a restrictive nature (i.e., patents, see
.Xr crypto 3 )
directly removed from the source code; any licensed or patented components
are chosen from
external libraries.
.It
has been updated to support ssh protocol 1.5.
.It
contains added support for
.Xr kerberos 8
authentication and ticket passing.
.It
supports one-time password authentication with
.Xr skey 1 .
.El
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr sshd 8 ,
.Xr crypto 3