ssh-keygen
SSH-KEYGEN(1) BSD General Commands Manual 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 -F hostname [-f known_hosts_file]
ssh-keygen -H [-f known_hosts_file]
ssh-keygen -R hostname [-f known_hosts_file]
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 admin-
istrator may use this to generate host keys, as seen in /etc/rc.
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
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 correspond-
ing 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 screen-
ing 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 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/rc 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 proto-
col 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.
-F hostname
Search for the specified hostname in a known_hosts file, listing
any occurances found. This option is useful to find hashed host
names or addresses and may also be used in conjunction with the
-H option to print found keys in a hashed format.
-H Hash a known_hosts file, printing the result to standard output.
This replaces all hostnames and addresses with hashed represen-
tations. These hashes may be used normally by ssh and sshd, but
they do not reveal identifying information should the file’s
contents be disclosed. This option will not modify existing
hashed hostnames and is therefore safe to use on files that mix
hashed and non-hashed names.
-R hostname
Removes all keys belonging to hostname from a known_hosts file.
This option is useful to delete hashed hosts (see the -H option
above).
-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 generat-
ing 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 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 gener-
ation. 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 pro-
cess: first, candidate primes are generated using a fast, but memory
intensive process. These candidate primes are then tested for suitabil-
ity (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:
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:
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 option. The DH generator value will
be chosen automatically for the prime under consideration. If a spe-
cific 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/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 authentica-
tion. 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 authentica-
tion. 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 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 authentica-
tion. 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/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)
J. Galbraith and R. Thayer, 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 pro-
tocol versions 1.5 and 2.0.
BSD September 25, 1999 BSD
