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 pkcs11
ssh-keygen -F hostname [-f known_hosts_file] [-l]
ssh-keygen -H [-f known_hosts_file]
ssh-keygen -R hostname [-f known_hosts_file]
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]
ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals]
[-O constraint] [-V validity_interval] file ...
ssh-keygen -L [-f input_keyfile]
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.
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 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 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.
Generally, 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
program will prompt for the file containing the private keys, for
the passphrase if the key has one, and for the new comment.
-D pkcs11
Download the RSA public keys provided by the PKCS#11 shared
library pkcs11.
-e This option will read a private or public OpenSSH key file and
print the key in RFC 4716 SSH 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.
-h When signing a key, create a host certificate instead of a user
certificate. Please see the CERTIFICATES section for details.
-I certificate_identity
Specify the key identity when signing a public key. Please see
the CERTIFICATES section for details.
-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 RFC 4716
SSH Public Key File Format. This option allows importing keys
from several commercial SSH implementations.
-L Prints the contents of a certificate.
-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. If
combined with -v, an ASCII art representation of the key is
supplied with the fingerprint.
-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.
-n principals
Specify one or more principals (user or host names) to be
included in a certificate when signing a key. Multiple
principals may be specified, separated by commas. Please see the
CERTIFICATES section for details.
-O constraint
Specify a certificate constraint when signing a key. This option
may be specified multiple times. Please see the CERTIFICATES
section for details. The constraints that are valid for user
certificates are:
clear Clear all enabled permissions. This is useful for
clearing the default set of permissions so permissions
may be added individually.
force-command=command
Forces the execution of command instead of any shell or
command specified by the user when the certificate is
used for authentication.
no-agent-forwarding
Disable ssh-agent(1) forwarding (permitted by default).
no-port-forwarding
Disable port forwarding (permitted by default).
no-pty Disable PTY allocation (permitted by default).
no-user-rc
Disable execution of ~/.ssh/rc by sshd(8) (permitted by
default).
no-x11-forwarding
Disable X11 forwarding (permitted by default).
permit-agent-forwarding
Allows ssh-agent(1) forwarding.
permit-port-forwarding
Allows port forwarding.
permit-pty
Allows PTY allocation.
permit-user-rc
Allows execution of ~/.ssh/rc by sshd(8).
permit-x11-forwarding
Allows X11 forwarding.
source-address=address_list
Restrict the source addresses from which the certificate
is considered valid. The address_list is a comma-
separated list of one or more address/netmask pairs in
CIDR format.
At present, no constraints are valid for host keys.
-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 system administration scripts 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.
-s ca_key
Certify (sign) a public key using the specified CA key. Please
see the CERTIFICATES section for details.
-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.
-V validity_interval
Specify a validity interval when signing a certificate. A
validity interval may consist of a single time, indicating that
the certificate is valid beginning now and expiring at that time,
or may consist of two times separated by a colon to indicate an
explicit time interval. The start time may be specified as a
date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a
relative time (to the current time) consisting of a minus sign
followed by a relative time in the format described in the TIME
FORMATS section of sshd_config(5). The end time may be specified
as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
starting with a plus character.
For example: “+52w1d” (valid from now to 52 weeks and one day
from now), “-4w:+4w” (valid from four weeks ago to four weeks
from now), “20100101123000:20110101123000” (valid from 12:30 PM,
January 1st, 2010 to 12:30 PM, January 1st, 2011), “-1d:20110101”
(valid from yesterday to midnight, January 1st, 2011).
-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.
-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
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 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/ssh/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.
CERTIFICATES
ssh-keygen supports signing of keys to produce certificates that may be
used for user or host authentication. Certificates consist of a public
key, some identity information, zero or more principal (user or host)
names and an optional set of constraints that are signed by a
Certification Authority (CA) key. Clients or servers may then trust only
the CA key and verify its signature on a certificate rather than trusting
many user/host keys. Note that OpenSSH certificates are a different, and
much simpler, format to the X.509 certificates used in ssl(8).
ssh-keygen supports two types of certificates: user and host. User
certificates authenticate users to servers, whereas host certificates
authenticate server hosts to users. To generate a user certificate:
$ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
The resultant certificate will be placed in /path/to/user_key-cert.pub.
A host certificate requires the -h option:
$ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
The host certificate will be output to /path/to/host_key-cert.pub. In
both cases, key_id is a "key identifier" that is logged by the server
when the certificate is used for authentication.
Certificates may be limited to be valid for a set of principal
(user/host) names. By default, generated certificates are valid for all
users or hosts. To generate a certificate for a specified set of
principals:
$ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
$ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub
Additional limitations on the validity and use of user certificates may
be specified through certificate constraints. A constrained certificate
may disable features of the SSH session, may be valid only when presented
from particular source addresses or may force the use of a specific
command. For a list of valid certificate constraints, see the
documentation for the -O option above.
Finally, certificates may be defined with a validity lifetime. The -V
option allows specification of certificate start and end times. A
certificate that is presented at a time outside this range will not be
considered valid. By default, certificates have a maximum validity
interval.
For certificates to be used for user or host authentication, the CA
public key must be trusted by sshd(8) or ssh(1). Please refer to those
manual pages for details.
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 128-bit AES. 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
authentication. 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 128-bit AES. 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
authentication. 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 128-bit AES. 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
authentication. 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/ssh/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), ssh-vulnkey(1), moduli(5), sshd(8)
The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
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.