OpenSSH authentication key
ssh-keygen generates, manages and converts
authentication keys for
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
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have been revoked by one.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
Normally each user wishing to use SSH with public key authentication runs this once to create the authentication key in ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk or ~/.ssh/id_rsa. Additionally, the system administrator may use this to generate host keys, as seen in /etc/rc.
Normally this program generates the key and asks for a file in
which to store the private key. The public key is stored in a file with the
same name but “.pub” appended. The program also asks for a
passphrase. The passphrase may be empty to indicate no passphrase (host keys
must have an empty passphrase), or it may be a string of arbitrary length. A
passphrase is similar to a password, except it can be a phrase with a series
of words, punctuation, numbers, whitespace, or any string of characters you
want. Good passphrases are 10-30 characters long, are not simple sentences
or otherwise easily guessable (English prose has only 1-2 bits of entropy
per character, and provides very bad passphrases), and contain a mix of
upper and lowercase letters, numbers, and non-alphanumeric characters. The
passphrase can be changed later by using the
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
It is still possible for
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)
After a key is generated,
ask where the keys should be placed to be activated.
The options are as follows:
- For each of the key types (rsa, dsa, ecdsa and ed25519) for which host
keys do not exist, generate the host keys with the default key file path,
an empty passphrase, default bits for the key type, and default comment.
-fhas 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.
- 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.
- Show the bubblebabble digest of specified private or public key file.
- 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
-bflag 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
-bflag will be ignored.
- Provides a new comment.
- 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.
- 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).
- Specifies the hash algorithm used when displaying key fingerprints. Valid options are: “md5” and “sha256”. The default is “sha256”.
- 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
-moption. The default export format is “RFC4716”. This option allows exporting OpenSSH keys for use by other programs, including several commercial SSH implementations.
-Fhostname | [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
-Hoption to print found keys in a hashed format.
- Specifies the filename of the key file.
- Use generic DNS format when printing fingerprint resource records using
- 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
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.
- When signing a key, create a host certificate instead of a user certificate. Please see the CERTIFICATES section for details.
- Specify the key identity when signing a public key. Please see the CERTIFICATES section for details.
- This option will read an unencrypted private (or public) key file in the
format specified by the
-moption 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”.
- 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.
- Generate a KRL file. In this mode,
ssh-keygenwill generate a KRL file at the location specified via the
-fflag 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.
- Prints the contents of one or more certificates.
- Show fingerprint of specified public key file. For RSA and DSA keys
ssh-keygentries 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.
- 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.
- Screen candidate parameters for Diffie-Hellman Group Exchange. This will accept a list of candidate numbers and test that they are safe (Sophie Germain) primes with acceptable group generators. The results of this operation may be added to the /etc/moduli file. See the MODULI GENERATION section for more information.
- Specify a key format for key generation, the
-e(export) conversion options, and the
-pchange 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.
- Provides the new passphrase.
- 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. Please see the CERTIFICATES section for details.
- Specify a key/value option. These are specific to the operation that
ssh-keygenhas 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 a key that will be hosted on a FIDO authenticator, this flag may be used to specify key-specific options. Those supported at present are:
- 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:”.
- Specifies a path to a challenge string that will be passed to the FIDO token 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).
- Explicitly specify a fido(4) device to use, rather than letting the token middleware select one.
- 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.
- Indicate that the key should be stored on the FIDO authenticator itself. Resident keys may be supported on FIDO2 tokens and typically require that a PIN be set on the token prior to generation. Resident keys may be loaded off the token using ssh-add(1).
- 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.
- Indicate that this private key should require user verification for each signature. Not all FIDO tokens support this option. Currently PIN authentication is the only supported verification method, but other methods may be supported in the future.
- May be used at key generation time to record the attestation data returned from FIDO tokens during key generation. Please note that this information is potentially sensitive. By default, this information is discarded.
-Ooption may be specified multiple times.
- Provides the (old) passphrase.
- 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.
- Test whether keys have been revoked in a KRL. If the
-loption is also specified then the contents of the KRL will be printed.
-Rhostname | [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
- Print the SSHFP fingerprint resource record named hostname for the specified public key file.
- Certify (sign) a public key using the specified CA key. Please see the
CERTIFICATES section for details.
When generating a KRL,
-sspecifies 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.
- Specifies the type of key to create. The possible values are
“dsa”, “ecdsa”, “ecdsa-sk”,
“ed25519”, “ed25519-sk”, or
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).
- When used in combination with
-s, this option indicates that a CA key resides in a ssh-agent(1). See the CERTIFICATES section for more information.
- 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.
- 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 in YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format, a relative time (to the current time) consisting of a minus sign followed by an interval in the format described in the TIME FORMATS section of sshd_config(5).
The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMM[SS] time, a relative time starting with a plus character or the string “forever” to indicate that the certificate has no expiry date.
For example: “+52w1d” (valid from now to 52 weeks and one day from now), “-4w:+4w” (valid from four weeks ago to four weeks from now), “20100101123000:20110101123000” (valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011), “-1d:20110101” (valid from yesterday to midnight, January 1st, 2011), “-1m:forever” (valid from one minute ago and never expiring).
- Verbose mode. Causes
ssh-keygento print debugging messages about its progress. This is helpful for debugging moduli generation. Multiple
-voptions increase the verbosity. The maximum is 3.
- 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.
- Find the principal(s) associated with the public key of a signature,
provided using the
-sflag in an authorized signers file provided using the
-fflag. 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.
- Checks that a signature generated using
signhas a valid structure. This does not validate if a signature comes from an authorized signer. When testing a signature,
ssh-keygenaccepts a message on standard input and a signature namespace using
-n. A file containing the corresponding signature must also be supplied using the
-sflag. Successful testing of the signature is signalled by
ssh-keygenreturning a zero exit status.
- Cryptographically sign a file or some data using a SSH key. When signing,
ssh-keygenaccepts zero or more files to sign on the command-line - if no files are specified then
ssh-keygenwill 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
-foption 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
-nflag. 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.
- Request to verify a signature generated using
signas described above. When verifying a signature,
ssh-keygenaccepts a message on standard input and a signature namespace using
-n. A file containing the corresponding signature must also be supplied using the
-sflag, along with the identity of the signer using
-Iand a list of allowed signers via the
-fflag. 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
-rflag. 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-keygenreturning a zero exit status.
- This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.
- 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”.
- 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
-zflag 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
generate option. The
desired length of the primes may be specified by the
bits option. For
# 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
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
screen option. In this
ssh-keygen will read candidates from standard
input (or a file specified using the
-f option). For
# 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
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
generator option. Valid
generator values are 2, 3, and 5.
Screened DH groups may be installed in /etc/moduli. It is important that this file contains moduli of a range of bit lengths and that both ends of a connection share common moduli.
A number of options are available for moduli generation and
screening via the
- Exit after screening the specified number of lines while performing DH candidate screening.
- Start screening at the specified line number while performing DH candidate screening.
- 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.
- Specify the amount of memory to use (in megabytes) when generating candidate moduli for DH-GEX.
- Specify start point (in hex) when generating candidate moduli for DH-GEX.
- 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
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
$ 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
$ 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
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 all enabled permissions. This is useful for clearing the default set of permissions so permissions may be added individually.
- Includes an arbitrary certificate critical option or extension. The specified name should include a domain suffix, e.g. “email@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.
- Forces the execution of command instead of any shell or command specified by the user when the certificate is used for authentication.
- Disable ssh-agent(1) forwarding (permitted by default).
- Disable port forwarding (permitted by default).
- Disable PTY allocation (permitted by default).
- Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).
- Disable X11 forwarding (permitted by default).
- Allows ssh-agent(1) forwarding.
- Allows port forwarding.
- Allows PTY allocation.
- Allows execution of ~/.ssh/rc by sshd(8).
- Allows X11 forwarding.
- 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
- 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.
- Require signatures made using this key indicate that the user was first
verified. This option only makes sense for the FIDO authenticator
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). Please refer to those manual pages for details.
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
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.
- 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-keygencommand line using the
- Revokes a certificate with the specified key ID string. The CA key must
have been specified on the
ssh-keygencommand line using the
- Revokes the specified key. If a certificate is listed, then it is revoked as a plain public key.
- Revokes the specified key by including its SHA1 hash in the KRL.
- 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.
- Revokes a key using a fingerprint hash, as obtained from a
sshd(8) authentication log message or the
-lflag. 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
-k. When this option is specified, keys
listed via the command line are merged into the KRL, adding to those already
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,
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
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):
- Indicates that this key is accepted as a certificate authority (CA) and that certificates signed by this CA may be accepted for verification.
- 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.
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 firstname.lastname@example.org,email@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. firstname.lastname@example.org namespaces="file" ssh-ed25519 AAA41...
- 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.
- 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-keygenbut it is offered as the default file for the private key. ssh(1) will read this file when a login attempt is made.
- Contains the 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.
- 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.