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. If invoked without any
arguments, ssh-keygen will generate an RSA key for use in SSH protocol 2
connections.
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 ~/.ssh/identity,
~/.ssh/id_dsa or ~/.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 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 char‐
acters 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 initial‐
ized 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 Show the bubblebabble digest of specified private or public key
file.
-b bits
Specifies the number of bits in the key to create. For RSA keys,
the minimum size is 768 bits and the default is 2048 bits. Gen‐
erally, 2048 bits is considered sufficient. DSA keys must be
exactly 1024 bits as specified by FIPS 186-2.
-C comment
Provides a new comment.
-c Requests changing the comment in the private and public key
files. This operation is only supported for RSA1 keys. The pro‐
gram will prompt for the file containing the private keys, for
the passphrase if the key has one, and for the new comment.
-D reader
Download the RSA public key stored in the smartcard in reader.
-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.
-F hostname
Search for the specified hostname in a known_hosts file, listing
any occurrences 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.
-f filename
Specifies the filename of the key file.
-G output_file
Generate candidate primes for DH-GEX. These primes must be
screened for safety (using the -T option) before use.
-g Use generic DNS format when printing fingerprint resource records
using the -r command.
-H Hash a known_hosts file. This replaces all hostnames and
addresses with hashed representations within the specified file;
the original content is moved to a file with a .old suffix.
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.
-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.
-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.
-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.
-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).
-r hostname
Print the SSHFP fingerprint resource record named hostname for
the specified public key file.
-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.
-t type
Specifies the type of key to create. The possible values are
“rsa1” for protocol version 1 and “rsa” or “dsa” for protocol
version 2.
-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 genera‐
tion. Multiple -v options increase the verbosity. The maximum
is 3.
-W generator
Specify desired generator when testing candidate moduli for DH-
GEX.
-y This option will read a private OpenSSH format file and print an
OpenSSH public key to stdout.
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 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 speci‐
fies 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 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
~/.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.
~/.ssh/identity.pub
Contains the protocol version 1 RSA public key for authentica‐
tion. The contents of this file should be added to
~/.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.
~/.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.
~/.ssh/id_dsa.pub
Contains the protocol version 2 DSA public key for authentica‐
tion. The contents of this file should be added to
~/.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.
~/.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.
~/.ssh/id_rsa.pub
Contains the protocol version 2 RSA public key for authentica‐
tion. The contents of this file should be added to
~/.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).
ENVIRONMENT
SSH_USE_STRONG_RNG
The reseeding of the OpenSSL random generator is usually done
from /dev/urandom. If the SSH_USE_STRONG_RNG environment vari‐
able is set to value other than 0 the OpenSSL random generator is
reseeded from /dev/random. The number of bytes read is defined
by the SSH_USE_STRONG_RNG value. Minimum is 6 bytes. This set‐
ting is not recommended on the computers without the hardware
random generator because insufficient entropy causes the connec‐
tion to be blocked until enough entropy is available.
SEE ALSOssh(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 cre‐
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
BSD September 25, 1999 BSD
