SSH-KEYGEN(1) UNIX System V (September 25, 1999) SSH-KEYGEN(1)
NAME
ssh-keygen - authentication key generation, management and
conversion
SYNOPSIS
ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C
comment] [-f output_keyfile]
ssh-keygen-p [-P old_passphrase] [-N new_passphrase] [-f
keyfile]
ssh-keygen-i [-f input_keyfile]
ssh-keygen-e [-f input_keyfile]
ssh-keygen-y [-f input_keyfile]
ssh-keygen-c [-P passphrase] [-C comment] [-f keyfile]
ssh-keygen-l [-f input_keyfile]
ssh-keygen-B [-f input_keyfile]
ssh-keygen-D reader
ssh-keygen-U reader [-f input_keyfile]
ssh-keygen-r hostname [-f input_keyfile] [-g]
ssh-keygen-G output_file [-v] [-b bits] [-M memory] [-S
start_point]
ssh-keygen-T output_file -f input_file [-v] [-a num_trials]
[-W generator]
DESCRIPTION
ssh-keygen generates, manages and converts authentication
keys for ssh(1). ssh-keygen can create RSA keys for use by
SSH protocol version 1 and RSA or DSA keys for use by SSH
protocol version 2. The type of key to be generated is
specified with the -t option.
ssh-keygen is also used to generate groups for use in
Diffie-Hellman group exchange (DH-GEX). See the MODULI
GENERATION section for details.
Normally each user wishing to use SSH with RSA or DSA
authentication runs this once to create the authentication
key in $HOME/.ssh/identity, $HOME/.ssh/id_dsa or
$HOME/.ssh/id_rsa. Additionally, the system administrator
may use this to generate host keys, as seen in
/etc/init.d/sshd.
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 an
empty passphrase), or it may be a string of arbitrary
length. A passphrase is similar to a password, except it
can be a phrase with a series of words, punctuation,
numbers, whitespace, or any string of characters you want.
Good passphrases are 10-30 characters long, are not simple
sentences or otherwise easily guessable (English prose has
Page 1 (printed 10/26/05)
SSH-KEYGEN(1) UNIX System V (September 25, 1999) SSH-KEYGEN(1)
only 1-2 bits of entropy per character, and provides very
bad passphrases), and contain a mix of upper and lowercase
letters, numbers, and non-alphanumeric characters. 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, a new key must be generated
and copied to the corresponding public key to other
machines.
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
``user@host'' when the key is created, but can be changed
using the -c option.
After a key is generated, instructions below detail where
the keys should be placed to be activated.
The options are as follows:
-a trials
Specifies the number of primality tests to perform when
screening DH-GEX candidates using the -T command.
-b bits
Specifies the number of bits in the key to create.
Minimum is 512 bits. Generally, 1024 bits is
considered sufficient. The default is 1024 bits.
-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.
-e This option will read a private or public OpenSSH key
file and print the key in a `SECSH' Public Key File
Format to stdout. This option allows exporting keys
for use by several commercial SSH implementations.
-g Use generic DNS format when printing fingerprint
resource records using the -r command.
-f filename
Specifies the filename of the key file.
-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.
ssh-keygen also reads the `SECSH' Public Key File
Page 2 (printed 10/26/05)
SSH-KEYGEN(1) UNIX System V (September 25, 1999) SSH-KEYGEN(1)
Format . This option allows importing keys from
several commercial SSH implementations.
-l Show fingerprint of specified public key file. Private
RSA1 keys are also supported. For RSA and DSA keys
ssh-keygen tries to find the matching public key file
and prints its fingerprint.
-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.
-q Silence ssh-keygen. Used by /etc/init.d/sshd when
creating a new key.
-y This option will read a private OpenSSH format file and
print an OpenSSH public key to stdout.
-t type
Specifies the type of the key to create. The possible
values are ``rsa1'' for protocol version 1 and ``rsa''
or ``dsa'' for protocol version 2.
-B Show the bubblebabble digest of specified private or
public key file.
-C comment
Provides the new comment.
-D reader
Download the RSA public key stored in the smartcard in
reader.
-G output_file
Generate candidate primes for DH-GEX. These primes
must be screened for safety (using the -T option)
before use.
-M memory
Specify the amount of memory to use (in megabytes) when
generating candidate moduli for DH-GEX.
-N new_passphrase
Provides the new passphrase.
-P passphrase
Provides the (old) passphrase.
-S start
Specify start point (in hex) when generating candidate
Page 3 (printed 10/26/05)
SSH-KEYGEN(1) UNIX System V (September 25, 1999) SSH-KEYGEN(1)
moduli for DH-GEX.
-T output_file
Test DH group exchange candidate primes (generated
using the -G option) for safety.
-W generator
Specify desired generator when testing candidate moduli
for DH-GEX.
-U reader
Upload an existing RSA private key into the smartcard
in reader.
-v Verbose mode. Causes ssh-keygen to print debugging
messages about its progress. This is helpful for
debugging moduli generation. Multiple -v options
increase the verbosity. The maximum is 3.
-r hostname
Print the SSHFP fingerprint resource record named
hostname for the specified public key file.
MODULI GENERATION
ssh-keygen may be used to generate groups for the Diffie-
Hellman Group Exchange (DH-GEX) protocol. Generating these
groups is a two-step process: first, candidate primes are
generated using a fast, but memory intensive process. These
candidate primes are then tested for suitability (a CPU-
intensive process).
Generation of primes is performed using the -G option. The
desired length of the primes may be specified by the -b
option. For example:
Dl ssh-keygen-G moduli-2048.candidates -b 2048
By default, the search for primes begins at a random point
in the desired length range. This may be overridden using
the -S option, which specifies a different start point (in
hex).
Once a set of candidates have been generated, they must be
tested for suitability. This may be performed using the -T
option. In this mode ssh-keygen will read candidates from
standard input (or a file specified using the -f option).
For example:
Dl ssh-keygen-T moduli-2048 -f moduli-2048.candidates
By default, each candidate will be subjected to 100
primality tests. This may be overridden using the -a
Page 4 (printed 10/26/05)
SSH-KEYGEN(1) UNIX System V (September 25, 1999) SSH-KEYGEN(1)
option. The DH generator value will be chosen automatically
for the prime under consideration. If a specific generator
is desired, it may be requested using the -W option. Valid
generator values are 2, 3 and 5.
Screened DH groups may be installed in /etc/openssh/moduli.
It is important that this file contains moduli of a range of
bit lengths and that both ends of a connection share common
moduli.
FILES
$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
ssh-keygen but it is offered as the default file for
the private key. ssh(1) will read this file when a
login attempt is made.
$HOME/.ssh/identity.pub
Contains the protocol version 1 RSA public key for
authentication. The contents of this file should be
added to $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.
$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
ssh-keygen but it is offered as the default file for
the private key. ssh(1) will read this file when a
login attempt is made.
$HOME/.ssh/id_dsa.pub
Contains the protocol version 2 DSA public key for
authentication. The contents of this file should be
added to $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.
$HOME/.ssh/id_rsa
Contains the protocol version 2 RSA authentication
identity of the user. This file should not be readable
Page 5 (printed 10/26/05)
SSH-KEYGEN(1) UNIX System V (September 25, 1999) SSH-KEYGEN(1)
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. ssh(1) will read this file when a
login attempt is made.
$HOME/.ssh/id_rsa.pub
Contains the protocol version 2 RSA public key for
authentication. The contents of this file should be
added to $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.
/etc/openssh/moduli
Contains Diffie-Hellman groups used for DH-GEX. The
file format is described in moduli(5).
SEE ALSO
ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
R. Thayer and J. Galbraith, SECSH Public Key File Format,
draft-ietf-secsh-publickeyfile-01.txt, March 2001, work in
progress material.
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.
Page 6 (printed 10/26/05)
