ssh-keygen —
OpenSSH authentication key utility
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 |
-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] |
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 RSA 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update Key Revocation Lists, and to test whether given keys have been revoked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_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.
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 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 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,
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. For RSA and
DSA keys ssh-keygen tries to find the
matching public key file and prints its fingerprint. If combined with
-v, a visual ASCII art representation of the
key is supplied with the fingerprint.
-
-M
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/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.
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
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 a 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 [email protected] 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.
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
/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.
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. “[email protected]”. 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.
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.
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.
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
[email protected],[email protected] 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.
[email protected] namespaces="file" ssh-ed25519 AAA41...
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.
- ~/.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
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_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/ssh/moduli
- Contains Diffie-Hellman groups used for DH-GEX. The file
format is described in moduli(5).
ssh(1),
ssh-add(1),
ssh-agent(1),
moduli(5),
sshd(8)
The Secure Shell (SSH) Public Key
File Format, RFC 4716,
2006.
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu
Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt
and Dug Song removed many bugs, re-added newer features and created OpenSSH.
Markus Friedl contributed the support for SSH protocol versions 1.5 and
2.0.