ssh-keygen.1

.\"	$OpenBSD: ssh-keygen.1,v 1.50 2001/10/25 21:14:32 markus Exp $
.\"
.\"  -*- nroff -*-
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
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.Dd September 25, 1999
.Dt SSH-KEYGEN 1
.Os
.Sh NAME
.Nm ssh-keygen
.Nd authentication key generation, management and conversion
.Sh SYNOPSIS
.Nm ssh-keygen
.Op Fl q
.Op Fl b Ar bits
.Op Fl t Ar type
.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 i
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl e
.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 B
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl D Ar reader
.Nm ssh-keygen
.Fl U Ar reader
.Op Fl f Ar input_keyfile
.Sh DESCRIPTION
.Nm
generates, manages and converts authentication keys for
.Xr ssh 1 .
.Nm
defaults to generating a RSA1 key for use by SSH protocol version 1.
Specifying the
.Fl t
option instead creates a key for use by SSH protocol version 2.
.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 ,
.Pa $HOME/.ssh/id_dsa
or
.Pa $HOME/.ssh/id_rsa .
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 an 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 character, 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, a new key must be generated and copied to the
corresponding public key to other machines.
.Pp
For RSA1 keys,
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.
This operation is only supported for RSA1 keys.
The program will prompt for the file containing the private keys, for
the passphrase if the key has one, and for the new comment.
.It Fl e
This option will read a private or public OpenSSH key file and
print the key in a
.Sq SECSH Public Key File Format
to stdout.
This option allows exporting keys for use by several commercial
SSH implementations.
.It Fl f Ar filename
Specifies the filename of the key file.
.It Fl i
This option will read an unencrypted private (or public) key file
in SSH2-compatible format and print an OpenSSH compatible private
(or public) key to stdout.
.Nm
also reads the
.Sq SECSH Public Key File Format .
This option allows importing keys from several commercial
SSH implementations.
.It Fl l
Show fingerprint of specified public key file.
Private RSA1 keys are also supported.
For RSA and DSA keys
.Nm
tries to find the matching public key file and prints its fingerprint.
.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 y
This option will read a private
OpenSSH format file and print an OpenSSH public key to stdout.
.It Fl t Ar type
Specifies the type of the key to create.
The possible values are
.Dq rsa1
for protocol version 1 and
.Dq rsa
or
.Dq dsa
for protocol version 2.
The default is
.Dq rsa1 .
.It Fl B
Show the bubblebabble digest of specified private or public key file.
.It Fl C Ar comment
Provides the new comment.
.It Fl D Ar reader
Download the RSA public key stored in the smartcard in
.Ar reader .
.It Fl N Ar new_passphrase
Provides the new passphrase.
.It Fl P Ar passphrase
Provides the (old) passphrase.
.It Fl U Ar reader
Upload an existing RSA private key into the smartcard in
.Ar reader .
.El
.Sh FILES
.Bl -tag -width Ds
.It Pa $HOME/.ssh/identity
Contains the protocol version 1 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 ssh 1
will read this file when a login attempt is made.
.It Pa $HOME/.ssh/identity.pub
Contains the protocol version 1 RSA public key for authentication.
The contents of this file should be added to
.Pa $HOME/.ssh/authorized_keys
on all machines
where the user wishes 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 protocol version 2 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 ssh 1
will read this file when a login attempt is made.
.It Pa $HOME/.ssh/id_dsa.pub
Contains the protocol version 2 DSA public key for authentication.
The contents of this file should be added to
.Pa $HOME/.ssh/authorized_keys
on all machines
where the user wishes to log in using public key authentication.
There is no need to keep the contents of this file secret.
.It Pa $HOME/.ssh/id_rsa
Contains the protocol version 2 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 ssh 1
will read this file when a login attempt is made.
.It Pa $HOME/.ssh/id_rsa.pub
Contains the protocol version 2 RSA public key for authentication.
The contents of this file should be added to
.Pa $HOME/.ssh/authorized_keys
on all machines
where the user wishes to log in using public key authentication.
There is no need to keep the contents of this file secret.
.El
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr sshd 8
.Rs
.%A J. Galbraith
.%A R. Thayer
.%T "SECSH Public Key File Format"
.%N draft-ietf-secsh-publickeyfile-01.txt
.%D March 2001
.%O work in progress material
.Re