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 proto-
col 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 cre-
ate 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/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 pro-
gram 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, punc-
tuation, 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 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 com-
mand.
-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 sup-
ported. 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 proto-
col 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.
-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 representations. These hashes may be used nor-
mally 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 use-
ful 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 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 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 gener-
ated 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:
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 stan-
dard 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 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/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 pri-
vate 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 con-
tents 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 pri-
vate 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 by anyone but the user. It is possible to specify a
passphrase when generating the key; that passphrase will be used to encrypt the pri-
vate 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/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 protocol versions 1.5 and 2.0.
BSD September 25, 1999 BSD
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