Manual: ssh-keygen
SSH-KEYGEN(1) General Commands Manual SSH-KEYGEN(1)
NAME
ssh-keygen â 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 âMODULI
GENERATIONâ 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Finally, ssh-keygen can be used to generate and update Key Revocation Lists, and to test whether given keys have been
revoked by one.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 âKEY REVOCATION LISTSâ 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.
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 â.pubâ 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, numâ
bers, 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 âuser@hostâ 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 system administration scripts 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 resisâ
tance 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 deâ
fault 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 conâ
taining 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 âCERTIFICATESâ section for details).
-E fingerprint_hash
Specifies the hash algorithm used when displaying key fingerprints. Valid options are: âmd5â and âsha256â.
The default is âsha256â.
-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 âRFC4716â. 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 norâ
mally by ssh and sshd, but they do not reveal identifying information should the file's contents be disâ
closed. 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 âCERTIFICATESâ section
for details.
-I certificate_identity
Specify the key identity when signing a public key. See the âCERTIFICATESâ 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 âRFC4716â.
-K Download resident keys from a FIDO authenticator. Public and private key files will be written to the curâ
rent directory for each downloaded key. If multiple FIDO authenticators are attached, keys will be downâ
loaded from the first touched authenticator. See the âFIDO AUTHENTICATORâ 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 âKEY REVOCATION LISTSâ 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
âdiffie-hellman-group-exchange-*â key exchange methods. The numbers generated by this operation must be furâ
ther screened before use. See the âMODULI GENERATIONâ 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/ssh/moduli file. See the âMODULI GENERATIONâ section for more informaâ
tion.
-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 forâ
mats. The supported key formats are: âRFC4716â (RFC 4716/SSH2 public or private key), âPKCS8â (PKCS8 public
or private key) or âPEMâ (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 âRFC4716â. Setting a format of
âPEMâ 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. Mulâ
tiple principals may be specified, separated by commas. See the âCERTIFICATESâ section for details.
-O option
Specify a key/value option. These are specific to the operation that ssh-keygen has been requested to perâ
form.
When signing certificates, one of the options listed in the âCERTIFICATESâ section may be specified here.
When performing moduli generation or screening, one of the options listed in the âMODULI GENERATIONâ section
may be specified.
When generating FIDO authenticator-backed keys, the options listed in the âFIDO AUTHENTICATORâ 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
âsha256â and âsha512.â The default is âsha512.â
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
âsha1â and âsha256â. 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 âCERTIFICATESâ 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 âKEY REVOCATION LISTSâ section for details.
-t ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
Specifies the type of key to create. The possible values are âecdsaâ, âecdsa-skâ, âed25519 (the default),â
âed25519-skâ, or ârsaâ.
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 âssh-rsaâ (SHA1 signatures, not recommended), ârsa-sha2-256â,
and ârsa-sha2-512â (the default for RSA keys).
-U When used in combination with -s or -Y sign, this option indicates that a CA key resides in an ssh-agent(1).
See the âCERTIFICATESâ 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 âalwaysâ 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 â0xâ.
The end time may be specified similarly to the start time:
⢠The string âforeverâ 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 â0xâ.
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 debugâ
ging 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 authoâ
rized signers file provided using the -f flag. The format of the allowed signers file is documented in the
âALLOWED SIGNERSâ 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 speciâ
fied 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 stanâ
dard 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 â.sigâ 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: âfileâ for file signing, âemailâ for email
signing. For custom uses, it is recommended to use names following a NAMESPACE@YOUR.DOMAIN pattern to generâ
ate unambiguous namespaces.
-Y verify
Request to verify a signature generated using ssh-keygen -Y sign as described above. When verifying a signaâ
ture, 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 âALLOWED SIGNERSâ 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 âaes256-ctrâ.
-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 â+â character, then the serial number will be increâ
mented 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 generâ
ator 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/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.
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. Cerâ
tificates 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 identifyâ
ing 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 an 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 authenticaâ
tion.
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. âname@example.comâ. If contents is specified then it is included as the contents of the extenâ
sion/option encoded as a string, otherwise the extension/option is created with no contents (usually indicatâ
ing 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, e.g. by PIN or on-token
biometrics. This option only makes sense for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.
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 authentiâ
cators 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 âssh:â. This may be useful when generating host or
domain-specific resident keys. The specified application string must begin with âssh:â.
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 signaâ
tures. 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 typiâ
cally 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 likeâ
lihood 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 userâ
name 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 authenticaâ
tors 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 certifiâ
cates 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 an 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 reâ
turned 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, opâ
tions, keytype, base64-encoded key. Empty lines and lines starting with a â#â 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 opâ
tion 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 signaâ
ture 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 usâ
ing 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.
/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.
Debian October 4, 2025 SSH-KEYGEN(1)