ssh-keygen(1) General Commands Manual ssh-keygen(1)

NAME

 ssh-keygen - OpenSSH authentication key utility

SYNOPSIS

 ssh-keygen [-q] [-a rounds] [-b bits] [-C comment] [-f output_keyfile]
 [-m format] [-N new_passphrase] [-O option]
 [-t 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

 ssh-keygen generates, manages and converts authentication keys for
 ssh(1) . ssh-keygen can create keys for use by SSH protocol version 2.
 The type of key to be generated is specified with the -t option. If
 invoked without any arguments, ssh-keygen will generate an Ed25519 key.
 ssh-keygen is also used to generate groups for use in Diffie-Hellman
 group exchange (DH-GEX). See the MODULI GENERATION section for details.
 Finally, ssh-keygen can be used to generate and update Key Revocation
 Lists, and to test whether given keys have been revoked by one. See the
 KEY REVOCATION LISTS section for details.
 Normally each user wishing to use SSH with public key authentication runs
 this once to create the authentication key in ~/.ssh/id_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.
 ssh-keygen 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 ssh-keygen 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, ssh-keygen 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. 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, 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. ssh-keygen will
 try 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
 /opt/local/opt/local/etc/ssh/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 ssh-keygen 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 ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
 Specifies the type of key to create. The possible values are
 "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:
 o The string "always" to indicate the certificate has no
 specified start time.
 o A date or time in the system time zone formatted as YYYYMMDD
 or YYYYMMDDHHMM[SS].
 o A date or time in the UTC time zone as YYYYMMDDZ or
 YYYYMMDDHHMM[SS]Z.
 o 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) .
 o 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:
 o The string "forever" to indicate the certificate has no
 specified end time.
 o A date or time in the system time zone formatted as YYYYMMDD
 or YYYYMMDDHHMM[SS].
 o A date or time in the UTC time zone as YYYYMMDDZ or
 YYYYMMDDHHMM[SS]Z.
 o 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) .
 o 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 ssh-keygen to print debugging messages
 about its progress. This is helpful for debugging moduli
 generation. Multiple -v options increase the verbosity. The
 maximum is 3.
 -w 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 ssh-keygen -Y sign has a
 valid structure. This does not validate if a signature comes
 from an authorized signer. When testing a signature, ssh-keygen
 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 ssh-keygen returning a zero exit
 status.
 -Y sign
 Cryptographically sign a file or some data using an SSH key.
 When signing, ssh-keygen accepts zero or more files to sign on
 the command-line - if no files are specified then ssh-keygen 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 ssh-keygen -Y sign
 as described above. When verifying a signature, ssh-keygen
 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 ssh-keygen
 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

 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
 Exchange (DH-GEX) protocol. Generating these groups is a two-step
 process: first, candidate primes are generated using a fast, but memory
 intensive process. These candidate primes are then tested for
 suitability (a CPU-intensive process).
 Generation of primes is performed using the -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 ssh-keygen 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
 /opt/local/opt/local/etc/ssh/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

 ssh-keygen supports signing of keys to produce certificates that may be
 used for user or host authentication. Certificates consist of a public
 key, some identity information, zero or more principal (user or host)
 names and a set of options that are signed by a Certification Authority
 (CA) key. Clients or servers may then trust only the CA key and verify
 its signature on a certificate rather than trusting many user/host keys.
 Note that OpenSSH certificates are a different, and much simpler, format
 to the X.509 certificates used in ssl(8).
 ssh-keygen supports two types of certificates: user and host. User
 certificates authenticate users to servers, whereas host certificates
 authenticate server hosts to users. To generate a user certificate:
 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
 The resultant certificate will be placed in /path/to/user_key-cert.pub.
 A host certificate requires the -h option:
 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
 The host certificate will be output to /path/to/host_key-cert.pub.
 It is possible to sign using a CA key stored in a PKCS#11 token by
 providing the token library using -D and identifying the CA key by
 providing its public half as an argument to -s:
 $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub
 Similarly, it is possible for the CA key to be hosted in a ssh-agent(1) .
 This is indicated by the -U flag and, again, the CA key must be
 identified by its public half.
 $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub
 In all cases, key_id is a "key identifier" that is logged by the server
 when the certificate is used for authentication.
 Certificates may be limited to be valid for a set of principal
 (user/host) names. By default, generated certificates are valid for all
 users or hosts. To generate a certificate for a specified set of
 principals:
 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
 $ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub
 Additional limitations on the validity and use of user certificates may
 be specified through certificate options. A certificate option may
 disable features of the SSH session, may be valid only when presented
 from particular source addresses or may force the use of a specific
 command.
 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

 ssh-keygen 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

 ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs).
 These binary files specify keys or certificates to be revoked using a
 compact format, taking as little as one bit per certificate if they are
 being revoked by serial number.
 KRLs may be generated using the -k flag. This option reads one or more
 files from the command line and generates a new KRL. The files may
 either contain a KRL specification (see below) or public keys, listed one
 per line. Plain public keys are revoked by listing their hash or
 contents in the KRL and certificates revoked by serial number or key ID
 (if the serial is zero or not available).
 Revoking keys using a KRL specification offers explicit control over the
 types of record used to revoke keys and may be used to directly revoke
 certificates by serial number or key ID without having the complete
 original certificate on hand. A KRL specification consists of lines
 containing one of the following directives followed by a colon and some
 directive-specific information.
 serial: serial_number[-serial_number]
 Revokes a certificate with the specified serial number. Serial
 numbers are 64-bit values, not including zero and may be
 expressed in decimal, hex or octal. If two serial numbers are
 specified separated by a hyphen, then the range of serial numbers
 including and between each is revoked. The CA key must have been
 specified on the ssh-keygen command line using the -s option.
 id: key_id
 Revokes a certificate with the specified key ID string. The CA
 key must have been specified on the ssh-keygen command line using
 the -s option.
 key: public_key
 Revokes the specified key. If a certificate is listed, then it
 is revoked as a plain public key.
 sha1: public_key
 Revokes the specified key by 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 ssh-keygen -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 ssh-keygen will exit with a
 non-zero exit status. A zero exit status will only be returned if no key
 was revoked.

ALLOWED SIGNERS

 When verifying signatures, ssh-keygen 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

 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

 ~/.ssh/id_ecdsa
 ~/.ssh/id_ecdsa_sk
 ~/.ssh/id_ed25519
 ~/.ssh/id_ed25519_sk
 ~/.ssh/id_rsa
 Contains the 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 ssh-keygen but it is offered as the default file for
 the private key. ssh(1)  will read this file when a login attempt
 is made.
 ~/.ssh/id_ecdsa.pub
 ~/.ssh/id_ecdsa_sk.pub
 ~/.ssh/id_ed25519.pub
 ~/.ssh/id_ed25519_sk.pub
 ~/.ssh/id_rsa.pub
 Contains the 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.
 /opt/local/opt/local/etc/ssh/moduli
 Contains Diffie-Hellman groups used for DH-GEX. The file format
 is described in moduli(5) .

SEE ALSO

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

AUTHORS

 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
 de Raadt and Dug Song removed many bugs, re-added newer features and
 created OpenSSH. Markus Friedl contributed the support for SSH protocol
 versions 1.5 and 2.0.
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