ssh-keygen Man page

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 dsa | ecdsa | ed25519 | rsa | rsa1] [-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] [-n principals] [-O option] [-V validity_interval] [-z serial_number] file …
ssh-keygen -L [-f input_keyfile] ssh-keygen -A
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. ssh-keygen can create keys for use by SSH protocol versions 1
and 2. Protocol 1 should not be used and is only offered to support
legacy devices. It suffers from a number of cryptographic weaknesses and
doesn’t support many of the advanced features available for protocol 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.

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Lists, and to test whether given keys have been revoked by one.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/identity,
~/.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 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 the corresponding public
key copied 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 For each of the key types (rsa1, 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. This is used by system adminis‐
tration scripts to generate new host keys.

-a rounds
When saving a new-format private key (i.e. an ed25519 key or any
SSH protocol 2 key when the -o flag is set), this option speci‐
fies 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. Gen‐
erally, 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 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 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 finger‐
prints. 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, includ‐
ing 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 perform‐
ing 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 perform‐
ing DH candidate screening using the -T option. This will be
used to skip lines in the input file that have already been pro‐
cessed 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. 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 sup‐
plied with the fingerprint.

-M memory
Specify the amount of memory to use (in megabytes) when generat‐
ing candidate moduli for DH-GEX.

-m key_format
Specify a key format for the -i (import) or -e (export) conver‐
sion 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 princi‐
pals 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. Please see the CERTIFICATES section
for details. The options that are valid for user certificates
are:

clear Clear all enabled permissions. This is useful for clear‐
ing 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 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 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-sepa‐
rated list of one or more address/netmask pairs in CIDR
format.

At present, no options are valid for host keys.

-o Causes ssh-keygen to save private keys using the new OpenSSH for‐
mat 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 | rsa1
Specifies the type of key to create. The possible values are
“rsa1” for protocol version 1 and “dsa”, “ecdsa”, “ed25519”, or
“rsa” for protocol version 2.

-u Update a KRL. When specified with -k, keys listed via the com‐
mand 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 valid‐
ity 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 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.

-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 ver‐
sion 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 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 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 cer‐
tificates 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 pro‐
viding 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

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 princi‐
pals:

$ 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 dis‐
able 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 cer‐
tificate that is presented at a time outside this range will not be con‐
sidered 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 pub‐
lic key must be trusted by sshd(8) or ssh. Please refer to those man‐
ual 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 con‐
tents 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 orig‐
inal certificate on hand. A KRL specification consists of lines contain‐
ing one of the following directives followed by a colon and some direc‐
tive-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/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 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
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
~/.ssh/id_rsa
Contains the protocol version 2 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 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 protocol version 2 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, ssh-add, ssh-agent, 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 cre‐
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.

BSD February 17, 2016 BSD