ssh-keygen generates, manages and converts authentication keys for ssh(1). 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keys have been revoked by one.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 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 RSA1 keys and 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 (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 administration scripts to generate new
host keys.
-arounds
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 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.
-bbits
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.
-Ccomment
Provides a new comment.
-c Requests changing the comment in the private and public key files. This operation is only
supported for RSA1 keys and 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.
-Dpkcs11
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).
-Efingerprint_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.
-Fhostname
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.
-ffilename
Specifies the filename of the key file.
-Goutput_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.
-Icertificate_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”.
-Jnum_lines
Exit after screening the specified number of lines while performing DH candidate screening using
the -T option.
-jstart_line
Start screening at the specified line number while performing DH candidate screening using the -T
option.
-Kcheckpt
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. 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, a visual ASCII art representation of the key is supplied with the
fingerprint.
-Mmemory
Specify the amount of memory to use (in megabytes) when generating candidate moduli for DH-GEX.
-mkey_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”.
-Nnew_passphrase
Provides the new passphrase.
-nprincipals
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.
-Ooption
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 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 options are valid for host keys.
-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.
-Ppassphrase
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.
-Rhostname
Removes all keys belonging to hostname from a known_hosts file. This option is useful to delete
hashed hosts (see the -H option above).
-rhostname
Print the SSHFP fingerprint resource record named hostname for the specified public key file.
-Sstart
Specify start point (in hex) when generating candidate moduli for DH-GEX.
-sca_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.
-Toutput_file
Test DH group exchange candidate primes (generated using the -G option) for safety.
-tdsa | 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 command line are added to the existing
KRL rather than a new KRL being created.
-Vvalidity_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.
-Wgenerator
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.
-zserial_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.
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