| SSH-KEYGEN(1) | General Commands Manual | SSH-KEYGEN(1) |
ssh-keygen —
ssh-keygen |
[-q]
[-b bits] -t
type
[-N new_passphrase]
[-C comment]
[-f output_keyfile]
|
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 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-T command.-B-b
bits-C
comment-c-D
pkcs11-e-F
hostname-H option to print
found keys in a hashed format.-f
filename-G
output_file-T option) before use.-g-r command.-Hssh 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-I
certificate_identity-issh-keygen also reads the RFC 4716
SSH Public Key File Format. This option allows importing keys from several
commercial SSH implementations.-L-lssh-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-N
new_passphrase-n
principals-O
constraintno-x11-forwardingno-agent-forwardingno-port-forwardingno-ptyno-user-rcclearpermit-x11-forwardingpermit-port-forwardingpermit-ptypermit-user-rcforce-command=commandsource-address=address_listAt present, no constraints are valid for host keys.
-P
passphrase-p-qssh-keygen. Used by
/etc/rc when creating a new key.-R
hostname-H option above).-r
hostname-S
start-s
ca_key-T
output_file-G option) for safety.-t
type-V
validity_intervalFor 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).
-vssh-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-yssh-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 2048By 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.candidatesBy 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.
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.pubThe 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.pubThe 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.pubAdditional 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.
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-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-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.The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
| March 4, 2010 | BSD |