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NAME
ssh-keygen — authentication key generation, management and conversion
SYNOPSIS
ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa] [-N new_passphrase] [-C comment]
[-f output_keyfile]
ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
ssh-keygen -i [-m key_format] [-f input_keyfile]
ssh-keygen -e [-m key_format] [-f input_keyfile]
ssh-keygen -y [-f input_keyfile]
ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile]
ssh-keygen -l [-v] [-E fingerprint_hash] [-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 rounds] [-J num_lines] [-j start_line]
[-K checkpt] [-W generator]
ssh-keygen -s ca_key -I certificate_identity [-h] [-U] [-D pkcs11_provider] [-n principals]
[-O option] [-V validity_interval] [-z serial_number] file ...
ssh-keygen -L [-f input_keyfile]
ssh-keygen -A [-f prefix_path]
ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number] file ...
ssh-keygen -Q -f krl_file file ...
DESCRIPTION
ssh-keygen generates, manages and converts authentication keys for ssh(1). ssh-keygen can
create 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.
ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange (DH-
GEX). See the MODULI GENERATION section for details.
Finally, ssh-keygen can be used to generate and update Key Revocation Lists, and to test
whether given keys have been revoked by one. See the KEY REVOCATION LISTS section for
details.
Normally each user wishing to use SSH with public key authentication runs this once to
create the authentication key in ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 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 the corresponding public key copied to other machines.
For keys stored in the newer OpenSSH format, 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 For each of the key types (rsa, dsa, ecdsa and ed25519) for which host keys do not
exist, generate the host keys with the default key file path, an empty passphrase,
default bits for the key type, and default comment. If -f has also been specified,
its argument is used as a prefix to the default path for the resulting host key
files. This is used by system administration scripts to generate new host keys.
-a rounds
When saving a new-format private key (i.e. an ed25519 key or when the -o flag is
set), this option specifies the number of KDF (key derivation function) rounds used.
Higher numbers result in slower passphrase verification and increased resistance to
brute-force password cracking (should the keys be stolen).
When screening DH-GEX candidates (using the -T command). This option specifies the
number of primality tests to perform.
-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 1024 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. For
ECDSA keys, the -b flag determines the key length by selecting from one of three
elliptic curve sizes: 256, 384 or 521 bits. Attempting to use bit lengths other
than these three values for ECDSA keys will fail. Ed25519 keys have a fixed length
and the -b flag will be ignored.
-C comment
Provides a new comment.
-c Requests changing the comment in the private and public key files. This operation
is only supported for keys stored in the newer OpenSSH format. 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. When
used in combination with -s, this option indicates that a CA key resides in a
PKCS#11 token (see the CERTIFICATES section for details).
-E fingerprint_hash
Specifies the hash algorithm used when displaying key fingerprints. Valid options
are: “md5” and “sha256”. The default is “sha256”.
-e This option will read a private or public OpenSSH key file and print to stdout the
key in one of the formats specified by the -m option. The default export format is
“RFC4716”. This option allows exporting OpenSSH keys for use by other programs,
including 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 the format
specified by the -m option and print an OpenSSH compatible private (or public) key
to stdout. This option allows importing keys from other software, including several
commercial SSH implementations. The default import format is “RFC4716”.
-J num_lines
Exit after screening the specified number of lines while performing DH candidate
screening using the -T option.
-j start_line
Start screening at the specified line number while performing DH candidate screening
using the -T option.
-K checkpt
Write the last line processed to the file checkpt while performing DH candidate
screening using the -T option. This will be used to skip lines in the input file
that have already been processed if the job is restarted.
-k Generate a KRL file. In this mode, ssh-keygen will generate a KRL file at the
location specified via the -f flag that revokes every key or certificate presented
on the command line. Keys/certificates to be revoked may be specified by public key
file or using the format described in the KEY REVOCATION LISTS section.
-L Prints the contents of one or more certificates.
-l Show fingerprint of specified public key file. For RSA and DSA keys ssh-keygen
tries to find the matching public key file and prints its fingerprint. If combined
with -v, a visual 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.
-m key_format
Specify a key format for the -i (import) or -e (export) conversion options. The
supported key formats are: “RFC4716” (RFC 4716/SSH2 public or private key), “PKCS8”
(PEM PKCS8 public key) or “PEM” (PEM public key). The default conversion format is
“RFC4716”.
-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 option
Specify a certificate option when signing a key. This option may be specified
multiple times. See also the CERTIFICATES section for further details. The options
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.
critical:name[=contents]
extension:name[=contents]
Includes an arbitrary certificate critical option or extension. The
specified name should include a domain suffix, e.g. “name@example.com”. If
contents is specified then it is included as the contents of the
extension/option encoded as a string, otherwise the extension/option is
created with no contents (usually indicating a flag). Extensions may be
ignored by a client or server that does not recognise them, whereas unknown
critical options will cause the certificate to be refused.
At present, no standard options are valid for host keys.
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.
-o Causes ssh-keygen to save private keys using the new OpenSSH format rather than the
more compatible PEM format. The new format has increased resistance to brute-force
password cracking but is not supported by versions of OpenSSH prior to 6.5. Ed25519
keys always use the new private key format.
-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 Test whether keys have been revoked in a KRL.
-q Silence ssh-keygen.
-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.
When generating a KRL, -s specifies a path to a CA public key file used to revoke
certificates directly by key ID or serial number. See the KEY REVOCATION LISTS
section for details.
-T output_file
Test DH group exchange candidate primes (generated using the -G option) for safety.
-t dsa | ecdsa | ed25519 | rsa
Specifies the type of key to create. The possible values are “dsa”, “ecdsa”,
“ed25519”, or “rsa”.
-U When used in combination with -s, this option indicates that a CA key resides in a
ssh-agent(1). See the CERTIFICATES section for more information.
-u Update a KRL. When specified with -k, keys listed via the command line are added to
the existing KRL rather than a new KRL being created.
-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.
-z serial_number
Specifies a serial number to be embedded in the certificate to distinguish this
certificate from others from the same CA. The default serial number is zero.
When generating a KRL, the -z flag is used to specify a KRL version number.
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 screened 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 a set of options 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.
It is possible to sign using a CA key stored in a PKCS#11 token by providing the token
library using -D and identifying the CA key by providing its public half as an argument to
-s:
$ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub
Similarly, it is possible for the CA key to be hosted in a ssh-agent(1). This is indicated
by the -U flag and, again, the CA key must be identified by its public half.
$ ssh-keygen -Us ca_key.pub -I key_id user_key.pub
In all 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 host_key.pub
Additional limitations on the validity and use of user certificates may be specified through
certificate options. A certificate option 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 options, 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 are valid from
UNIX Epoch to the distant future.
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.
KEY REVOCATION LISTS
ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs). These binary files
specify keys or certificates to be revoked using a compact format, taking as little as one
bit per certificate if they are being revoked by serial number.
KRLs may be generated using the -k flag. This option reads one or more files from the
command line and generates a new KRL. The files may either contain a KRL specification (see
below) or public keys, listed one per line. Plain public keys are revoked by listing their
hash or contents in the KRL and certificates revoked by serial number or key ID (if the
serial is zero or not available).
Revoking keys using a KRL specification offers explicit control over the types of record
used to revoke keys and may be used to directly revoke certificates by serial number or key
ID without having the complete original certificate on hand. A KRL specification consists
of lines containing one of the following directives followed by a colon and some directive-
specific information.
serial: serial_number[-serial_number]
Revokes a certificate with the specified serial number. Serial numbers are 64-bit
values, not including zero and may be expressed in decimal, hex or octal. If two
serial numbers are specified separated by a hyphen, then the range of serial numbers
including and between each is revoked. The CA key must have been specified on the
ssh-keygen command line using the -s option.
id: key_id
Revokes a certificate with the specified key ID string. The CA key must have been
specified on the ssh-keygen command line using the -s option.
key: public_key
Revokes the specified key. If a certificate is listed, then it is revoked as a
plain public key.
sha1: public_key
Revokes the specified key by its SHA1 hash.
KRLs may be updated using the -u flag in addition to -k. When this option is specified,
keys listed via the command line are merged into the KRL, adding to those already there.
It is also possible, given a KRL, to test whether it revokes a particular key (or keys).
The -Q flag will query an existing KRL, testing each key specified on the command line. If
any key listed on the command line has been revoked (or an error encountered) then
ssh-keygen will exit with a non-zero exit status. A zero exit status will only be returned
if no key was revoked.
FILES
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
~/.ssh/id_rsa
Contains the DSA, ECDSA, Ed25519 or 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_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_rsa.pub
Contains the DSA, ECDSA, Ed25519 or 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), 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.