ssh-keygen(1) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | MODULI GENERATION | CERTIFICATES | FIDO AUTHENTICATOR | KEY REVOCATION LISTS | ALLOWED SIGNERS | ENVIRONMENT | FILES | SEE ALSO | AUTHORS | COLOPHON

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

NAME         top

       ssh-keygen — OpenSSH authentication key utility

SYNOPSIS         top

       ssh-keygen [-q] [-a rounds] [-b bits] [-C comment] [-f
       output_keyfile] [-m format] [-N new_passphrase] [-O option] [-t
       dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa] [-w
       provider] [-Z cipher] ssh-keygen -p [-a rounds] [-f keyfile] [-m
       format] [-N new_passphrase] [-P old_passphrase] [-Z cipher]
       ssh-keygen -i [-f input_keyfile] [-m key_format] ssh-keygen -e
       [-f input_keyfile] [-m key_format] ssh-keygen -y [-f
       input_keyfile] ssh-keygen -c [-a rounds] [-C comment] [-f
       keyfile] [-P passphrase] ssh-keygen -l [-v] [-E fingerprint_hash]
       [-f input_keyfile] ssh-keygen -B [-f input_keyfile] ssh-keygen -D
       pkcs11 ssh-keygen -F hostname [-lv] [-f known_hosts_file]
       ssh-keygen -H [-f known_hosts_file] ssh-keygen -K [-a rounds] [-w
       provider] ssh-keygen -R hostname [-f known_hosts_file] ssh-keygen
       -r hostname [-g] [-f input_keyfile] ssh-keygen -M generate [-O
       option] output_file ssh-keygen -M screen [-f input_file] [-O
       option] output_file ssh-keygen -I certificate_identity -s ca_key
       [-hU] [-D pkcs11_provider] [-n principals] [-O option] [-V
       validity_interval] [-z serial_number] file ... ssh-keygen -L [-f
       input_keyfile] ssh-keygen -A [-a rounds] [-f prefix_path]
       ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
       file ... ssh-keygen -Q [-l] -f krl_file file ... ssh-keygen -Y
       find-principals [-O option] -s signature_file -f
       allowed_signers_file ssh-keygen -Y match-principals -I
       signer_identity -f allowed_signers_file ssh-keygen -Y
       check-novalidate [-O option] -n namespace -s signature_file
       ssh-keygen -Y sign [-O option] -f key_file -n namespace file ...
       ssh-keygen -Y verify [-O option] -f allowed_signers_file -I
       signer_identity -n namespace -s signature_file [-r
       revocation_file]

DESCRIPTION         top

       generates, manages and converts authentication keys for ssh(1).
       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, will generate an Ed25519 key.

       is also used to generate groups for use in Diffie-Hellman group
       exchange (DH-GEX).  See the “MODULI GENERATION” section for
       details.

       Finally, 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_ecdsa_sk,
       ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk 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 the
       corresponding public key copied to other machines.

       will by default write keys in an OpenSSH-specific format.  This
       format is preferred as it offers better protection for keys at
       rest as well as allowing storage of key comments within the
       private key file itself.  The key comment may be useful to help
       identify the key.  The comment is initialized to “user@host” when
       the key is created, but can be changed using the -c option.

       It is still possible for to write the previously-used PEM format
       private keys using the -m flag.  This may be used when generating
       new keys, and existing new-format keys may be converted using
       this option in conjunction with the -p (change passphrase) flag.

       After a key is generated, will ask where the keys should be
       placed to be activated.

       The options are as follows:

       -A      Generate host keys of all default key types (rsa, ecdsa,
               and ed25519) if they do not already exist.  The host keys
               are generated 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 /etc/rc to generate new
               host keys.

       -a rounds
               When saving a private key, this option specifies the
               number of KDF (key derivation function, currently
               bcrypt_pbkdf(3)) rounds used.  Higher numbers result in
               slower passphrase verification and increased resistance
               to brute-force password cracking (should the keys be
               stolen).  The default is 16 rounds.

       -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 3072 bits.  Generally, 3072 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.  ECDSA-SK, Ed25519 and Ed25519-SK
               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.  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 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 a public 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 | [hostname]:port
               Search for the specified hostname (with optional port
               number) 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      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.  See the “CERTIFICATES” section for
               details.

       -I certificate_identity
               Specify the key identity when signing a public key.  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”.

       -K      Download resident keys from a FIDO authenticator.  Public
               and private key files will be written to the current
               directory for each downloaded key.  If multiple FIDO
               authenticators are attached, keys will be downloaded from
               the first touched authenticator.  See the “FIDO
               AUTHENTICATOR” section for more information.

       -k      Generate a KRL file.  In this mode, 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 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 generate
               Generate candidate Diffie-Hellman Group Exchange (DH-GEX)
               parameters for eventual use by the
               ‘diffie-hellman-group-exchange-*’ key exchange methods.
               The numbers generated by this operation must be further
               screened before use.  See the “MODULI GENERATION” section
               for more information.

       -M screen
               Screen candidate parameters for Diffie-Hellman Group
               Exchange.  This will accept a list of candidate numbers
               and test that they are safe (Sophie Germain) primes with
               acceptable group generators.  The results of this
               operation may be added to the /etc/moduli file.  See the
               “MODULI GENERATION” section for more information.

       -m key_format
               Specify a key format for key generation, the -i (import),
               -e (export) conversion options, and the -p change
               passphrase operation.  The latter may be used to convert
               between OpenSSH private key and PEM private key formats.
               The supported key formats are: “RFC4716” (RFC 4716/SSH2
               public or private key), “PKCS8” (PKCS8 public or private
               key) or “PEM” (PEM public key).  By default OpenSSH will
               write newly-generated private keys in its own format, but
               when converting public keys for export the default format
               is “RFC4716”.  Setting a format of “PEM” when generating
               or updating a supported private key type will cause the
               key to be stored in the legacy PEM private key format.

       -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.  See
               the “CERTIFICATES” section for details.

       -O option
               Specify a key/value option.  These are specific to the
               operation that has been requested to perform.

               When signing certificates, one of the options listed in
               the “CERTIFICATES” section may be specified here.

               When performing moduli generation or screening, one of
               the options listed in the “MODULI GENERATION” section may
               be specified.

               When generating FIDO authenticator-backed keys, the
               options listed in the “FIDO AUTHENTICATOR” section may be
               specified.

               When performing signature-related options using the -Y
               flag, the following options are accepted:

               hashalg=algorithm
                       Selects the hash algorithm to use for hashing the
                       message to be signed.  Valid algorithms are
                       “sha256” and “sha512.” The default is “sha512.”

               print-pubkey
                       Print the full public key to standard output
                       after signature verification.

               verify-time=timestamp
                       Specifies a time to use when validating
                       signatures instead of the current time.  The time
                       may be specified as a date or time in the
                       YYYYMMDD[Z] or in YYYYMMDDHHMM[SS][Z] formats.
                       Dates and times will be interpreted in the
                       current system time zone unless suffixed with a Z
                       character, which causes them to be interpreted in
                       the UTC time zone.

               When generating SSHFP DNS records from public keys using
               the -r flag, the following options are accepted:

               hashalg=algorithm
                       Selects a hash algorithm to use when printing
                       SSHFP records using the -D flag.  Valid
                       algorithms are “sha1” and “sha256”.  The default
                       is to print both.

               The -O option may be specified multiple times.

       -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.  If the -l
               option is also specified then the contents of the KRL
               will be printed.

       -q      Silence ssh-keygen.

       -R hostname | [hostname]:port
               Removes all keys belonging to the specified hostname
               (with optional port number) 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 ca_key
               Certify (sign) a public key using the specified CA key.
               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 dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
               Specifies the type of key to create.  The possible values
               are “dsa”, “ecdsa”, “ecdsa-sk”, “ed25519”, “ed25519-sk”,
               or “rsa”.

               This flag may also be used to specify the desired
               signature type when signing certificates using an RSA CA
               key.  The available RSA signature variants are “ssh-rsa”
               (SHA1 signatures, not recommended), “rsa-sha2-256”, and
               “rsa-sha2-512” (the default).

       -U      When used in combination with -s or -Y sign, 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:
               The string “always” to indicate the certificate has
                   no specified start time.
               A date or time in the system time zone formatted as
                   YYYYMMDD or YYYYMMDDHHMM[SS].
               A date or time in the UTC time zone as YYYYMMDDZ or
                   YYYYMMDDHHMM[SS]Z.
               A relative time before the current system time
                   consisting of a minus sign followed by an interval in
                   the format described in the TIME FORMATS section of
                   sshd_config(5).
               A raw seconds since epoch (Jan 1 1970 00:00:00 UTC)
                   as a hexadecimal number beginning with “0x”.

               The end time may be specified similarly to the start
               time:
               The string “forever” to indicate the certificate has
                   no specified end time.
               A date or time in the system time zone formatted as
                   YYYYMMDD or YYYYMMDDHHMM[SS].
               A date or time in the UTC time zone as YYYYMMDDZ or
                   YYYYMMDDHHMM[SS]Z.
               A relative time after the current system time
                   consisting of a plus sign followed by an interval in
                   the format described in the TIME FORMATS section of
                   sshd_config(5).
               A raw seconds since epoch (Jan 1 1970 00:00:00 UTC)
                   as a hexadecimal number beginning with “0x”.

               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.

               20100101123000Z:20110101123000Z
                       Similar, but interpreted in the UTC time zone
                       rather than the system time zone.

               -1d:20110101
                       Valid from yesterday to midnight, January 1st,
                       2011.

               0x1:0x2000000000
                       Valid from roughly early 1970 to May 2033.

               -1m:forever
                       Valid from one minute ago and never expiring.

       -v      Verbose mode.  Causes 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 provider
               Specifies a path to a library that will be used when
               creating FIDO authenticator-hosted keys, overriding the
               default of using the internal USB HID support.

       -Y find-principals
               Find the principal(s) associated with the public key of a
               signature, provided using the -s flag in an authorized
               signers file provided using the -f flag.  The format of
               the allowed signers file is documented in the “ALLOWED
               SIGNERS” section below.  If one or more matching
               principals are found, they are returned on standard
               output.

       -Y match-principals
               Find principal matching the principal name provided using
               the -I flag in the authorized signers file specified
               using the -f flag.  If one or more matching principals
               are found, they are returned on standard output.

       -Y check-novalidate
               Checks that a signature generated using -Y sign has a
               valid structure.  This does not validate if a signature
               comes from an authorized signer.  When testing a
               signature, accepts a message on standard input and a
               signature namespace using -n.  A file containing the
               corresponding signature must also be supplied using the
               -s flag.  Successful testing of the signature is
               signalled by returning a zero exit status.

       -Y sign
               Cryptographically sign a file or some data using an SSH
               key.  When signing, accepts zero or more files to sign on
               the command-line - if no files are specified then will
               sign data presented on standard input.  Signatures are
               written to the path of the input file with “.sig”
               appended, or to standard output if the message to be
               signed was read from standard input.

               The key used for signing is specified using the -f option
               and may refer to either a private key, or a public key
               with the private half available via ssh-agent(1).  An
               additional signature namespace, used to prevent signature
               confusion across different domains of use (e.g. file
               signing vs email signing) must be provided via the -n
               flag.  Namespaces are arbitrary strings, and may include:
               “file” for file signing, “email” for email signing.  For
               custom uses, it is recommended to use names following a
               NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous
               namespaces.

       -Y verify
               Request to verify a signature generated using -Y sign as
               described above.  When verifying a signature, accepts a
               message on standard input and a signature namespace using
               -n.  A file containing the corresponding signature must
               also be supplied using the -s flag, along with the
               identity of the signer using -I and a list of allowed
               signers via the -f flag.  The format of the allowed
               signers file is documented in the “ALLOWED SIGNERS”
               section below.  A file containing revoked keys can be
               passed using the -r flag.  The revocation file may be a
               KRL or a one-per-line list of public keys.  Successful
               verification by an authorized signer is signalled by
               returning a zero exit status.

       -y      This option will read a private OpenSSH format file and
               print an OpenSSH public key to stdout.

       -Z cipher
               Specifies the cipher to use for encryption when writing
               an OpenSSH-format private key file.  The list of
               available ciphers may be obtained using "ssh -Q cipher".
               The default is “aes256-ctr”.

       -z serial_number
               Specifies a serial number to be embedded in the
               certificate to distinguish this certificate from others
               from the same CA.  If the serial_number is prefixed with
               a ‘+’ character, then the serial number will be
               incremented for each certificate signed on a single
               command-line.  The default serial number is zero.

               When generating a KRL, the -z flag is used to specify a
               KRL version number.

MODULI GENERATION         top

       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 -M generate option.
       The desired length of the primes may be specified by the -O bits
       option.  For example:

             # ssh-keygen -M generate -O bits=2048
             moduli-2048.candidates

       By default, the search for primes begins at a random point in the
       desired length range.  This may be overridden using the -O start
       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 -M
       screen option.  In this mode will read candidates from standard
       input (or a file specified using the -f option).  For example:

             # ssh-keygen -M screen -f moduli-2048.candidates
             moduli-2048

       By default, each candidate will be subjected to 100 primality
       tests.  This may be overridden using the -O prime-tests 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 -O generator 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.

       A number of options are available for moduli generation and
       screening via the -O flag:

       lines=number
               Exit after screening the specified number of lines while
               performing DH candidate screening.

       start-line=line-number
               Start screening at the specified line number while
               performing DH candidate screening.

       checkpoint=filename
               Write the last line processed to the specified file while
               performing DH candidate screening.  This will be used to
               skip lines in the input file that have already been
               processed if the job is restarted.

       memory=mbytes
               Specify the amount of memory to use (in megabytes) when
               generating candidate moduli for DH-GEX.

       start=hex-value
               Specify start point (in hex) when generating candidate
               moduli for DH-GEX.

       generator=value
               Specify desired generator (in decimal) when testing
               candidate moduli for DH-GEX.

CERTIFICATES         top

       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).

       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.

       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.

       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.

       no-touch-required
               Do not require signatures made using this key include
               demonstration of user presence (e.g. by having the user
               touch the authenticator).  This option only makes sense
               for the FIDO authenticator algorithms ecdsa-sk and
               ed25519-sk.

       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.

       verify-required
               Require signatures made using this key indicate that the
               user was first verified.  This option only makes sense
               for the FIDO authenticator algorithms ecdsa-sk and
               ed25519-sk.  Currently PIN authentication is the only
               supported verification method, but other methods may be
               supported in the future.

       At present, no standard options are valid for host keys.

       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 the 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).  Refer to
       those manual pages for details.

FIDO AUTHENTICATOR         top

       is able to generate FIDO authenticator-backed keys, after which
       they may be used much like any other key type supported by
       OpenSSH, so long as the hardware authenticator is attached when
       the keys are used.  FIDO authenticators generally require the
       user to explicitly authorise operations by touching or tapping
       them.  FIDO keys consist of two parts: a key handle part stored
       in the private key file on disk, and a per-device private key
       that is unique to each FIDO authenticator and that cannot be
       exported from the authenticator hardware.  These are combined by
       the hardware at authentication time to derive the real key that
       is used to sign authentication challenges.  Supported key types
       are ecdsa-sk and ed25519-sk.

       The options that are valid for FIDO keys are:

       application
               Override the default FIDO application/origin string of
               “ssh:”.  This may be useful when generating host or
               domain-specific resident keys.  The specified application
               string must begin with “ssh:”.

       challenge=path
               Specifies a path to a challenge string that will be
               passed to the FIDO authenticator during key generation.
               The challenge string may be used as part of an out-of-
               band protocol for key enrollment (a random challenge is
               used by default).

       device  Explicitly specify a fido(4) device to use, rather than
               letting the authenticator middleware select one.

       no-touch-required
               Indicate that the generated private key should not
               require touch events (user presence) when making
               signatures.  Note that sshd(8) will refuse such
               signatures by default, unless overridden via an
               authorized_keys option.

       resident
               Indicate that the key handle should be stored on the FIDO
               authenticator itself.  This makes it easier to use the
               authenticator on multiple computers.  Resident keys may
               be supported on FIDO2 authenticators and typically
               require that a PIN be set on the authenticator prior to
               generation.  Resident keys may be loaded off the
               authenticator using ssh-add(1).  Storing both parts of a
               key on a FIDO authenticator increases the likelihood of
               an attacker being able to use a stolen authenticator
               device.

       user    A username to be associated with a resident key,
               overriding the empty default username.  Specifying a
               username may be useful when generating multiple resident
               keys for the same application name.

       verify-required
               Indicate that this private key should require user
               verification for each signature.  Not all FIDO
               authenticators support this option.  Currently PIN
               authentication is the only supported verification method,
               but other methods may be supported in the future.

       write-attestation=path
               May be used at key generation time to record the
               attestation data returned from FIDO authenticators during
               key generation.  This information is potentially
               sensitive.  By default, this information is discarded.

KEY REVOCATION LISTS         top

       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
               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 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 including its SHA1 hash in
               the KRL.

       sha256: public_key
               Revokes the specified key by including its SHA256 hash in
               the KRL.  KRLs that revoke keys by SHA256 hash are not
               supported by OpenSSH versions prior to 7.9.

       hash: fingerprint
               Revokes a key using a fingerprint hash, as obtained from
               a sshd(8) authentication log message or the -l flag.
               Only SHA256 fingerprints are supported here and resultant
               KRLs are not supported by OpenSSH versions prior to 7.9.

       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 will exit with a non-zero exit status.  A zero
       exit status will only be returned if no key was revoked.

ALLOWED SIGNERS         top

       When verifying signatures, uses a simple list of identities and
       keys to determine whether a signature comes from an authorized
       source.  This "allowed signers" file uses a format patterned
       after the AUTHORIZED_KEYS FILE FORMAT described in sshd(8).  Each
       line of the file contains the following space-separated fields:
       principals, options, keytype, base64-encoded key.  Empty lines
       and lines starting with a ‘#’ are ignored as comments.

       The principals field is a pattern-list (see PATTERNS in
       ssh_config(5)) consisting of one or more comma-separated
       USER@DOMAIN identity patterns that are accepted for signing.
       When verifying, the identity presented via the -I option must
       match a principals pattern in order for the corresponding key to
       be considered acceptable for verification.

       The options (if present) consist of comma-separated option
       specifications.  No spaces are permitted, except within double
       quotes.  The following option specifications are supported (note
       that option keywords are case-insensitive):

       cert-authority
               Indicates that this key is accepted as a certificate
               authority (CA) and that certificates signed by this CA
               may be accepted for verification.

       namespaces=namespace-list
               Specifies a pattern-list of namespaces that are accepted
               for this key.  If this option is present, the signature
               namespace embedded in the signature object and presented
               on the verification command-line must match the specified
               list before the key will be considered acceptable.

       valid-after=timestamp
               Indicates that the key is valid for use at or after the
               specified timestamp, which may be a date or time in the
               YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats.  Dates and
               times will be interpreted in the current system time zone
               unless suffixed with a Z character, which causes them to
               be interpreted in the UTC time zone.

       valid-before=timestamp
               Indicates that the key is valid for use at or before the
               specified timestamp.

       When verifying signatures made by certificates, the expected
       principal name must match both the principals pattern in the
       allowed signers file and the principals embedded in the
       certificate itself.

       An example allowed signers file:

          # Comments allowed at start of line
          user1@example.com,user2@example.com ssh-rsa AAAAX1...
          # A certificate authority, trusted for all principals in a domain.
          *@example.com cert-authority ssh-ed25519 AAAB4...
          # A key that is accepted only for file signing.
          user2@example.com namespaces="file" ssh-ed25519 AAA41...

ENVIRONMENT         top

       SSH_SK_PROVIDER
               Specifies a path to a library that will be used when
               loading any FIDO authenticator-hosted keys, overriding
               the default of using the built-in USB HID support.

FILES         top

       ~/.ssh/id_dsa
       ~/.ssh/id_ecdsa
       ~/.ssh/id_ecdsa_sk
       ~/.ssh/id_ed25519
       ~/.ssh/id_ed25519_sk
       ~/.ssh/id_rsa
               Contains the DSA, ECDSA, authenticator-hosted ECDSA,
               Ed25519, authenticator-hosted 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 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_ecdsa_sk.pub
       ~/.ssh/id_ed25519.pub
       ~/.ssh/id_ed25519_sk.pub
       ~/.ssh/id_rsa.pub
               Contains the DSA, ECDSA, authenticator-hosted ECDSA,
               Ed25519, authenticator-hosted 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/moduli
               Contains Diffie-Hellman groups used for DH-GEX.  The file
               format is described in moduli(5).

SEE ALSO         top

       ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8) The Secure
       Shell (SSH) Public Key File Format, RFC 4716, 2006.

AUTHORS         top

       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.

COLOPHON         top

       This page is part of the openssh (Portable OpenSSH) project.
       Information about the project can be found at
       http://www.openssh.com/portable.html.  If you have a bug report
       for this manual page, see ⟨http://www.openssh.com/report.html⟩.
       This page was obtained from the tarball openssh-9.6p1.tar.gz
       fetched from
       ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
       2023-12-22.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

GNU                         September 4, 2023              SSH-KEYGEN(1)