ssh-keygen manpage

Search topic Section

SSH-KEYGEN(1)		  BSD General Commands Manual		 SSH-KEYGEN(1)

     ssh-keygen -- authentication key generation, management and conversion

     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 ...

     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     Lists, and to test whether given keys have been revoked by one.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, 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 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 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 /etc/rc to gener-
	     ate 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

     -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

     -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 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.

     -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

     -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.

     -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

	     clear   Clear all enabled permissions.  This is useful for clear-
		     ing the default set of permissions so permissions may be
		     added individually.

		     Forces the execution of command instead of any shell or
		     command specified by the user when the certificate is
		     used for authentication.

		     Disable ssh-agent(1) forwarding (permitted by default).

		     Disable port forwarding (permitted by default).

	     no-pty  Disable PTY allocation (permitted by default).

		     Disable execution of ~/.ssh/rc by sshd(8) (permitted by

		     Disable X11 forwarding (permitted by default).

		     Allows ssh-agent(1) forwarding.

		     Allows port forwarding.

		     Allows PTY allocation.

		     Allows execution of ~/.ssh/rc by sshd(8).

		     Allows X11 forwarding.

		     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

	     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

     -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

     -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,

     -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-

     -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.

     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.

     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-

	   $ 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(1).  Please refer to those man-
     ual pages for details.

     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.

	     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(1) will read this file when a login attempt is made.

	     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.

	     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(1) will
	     read this file when a login attempt is made.

	     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.

	     Contains Diffie-Hellman groups used for DH-GEX.  The file format
	     is described in moduli(5).

	     The reseeding of the OpenSSL random generator is usually done
	     from /dev/urandom.	 If the SSH_USE_STRONG_RNG environment vari-
	     able is set to value other than 0 the OpenSSL random generator is
	     reseeded from /dev/random.	 The number of bytes read is defined
	     by the SSH_USE_STRONG_RNG value.  Minimum is 14 bytes.  This set-
	     ting is not recommended on the computers without the hardware
	     random generator because insufficient entropy causes the connec-
	     tion to be blocked until enough entropy is available.

     ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)

     The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

     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				 June 15, 2024				   BSD