Manual Pages  — SSH-KEYGEN

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Finally, ssh-keygen can be used to generate and update Key Revocation Lists, and to test whether given keys have been revoked by one.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, as seen in /etc/rc.

Normally this program generates the key and asks for a file in which to store the private key. The public key is stored in a file with the same name but ".pub" appended. The program also asks for a passphrase. The passphrase may be empty to indicate no passphrase (host keys must have an empty passphrase), or it may be a string of arbitrary length. A passphrase is similar to a password, except it can be a phrase with a series of words, punctuation, numbers, whitespace, or any string of characters you want. Good passphrases are 10-30 characters long, are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per character, and provides very bad passphrases), and contain a mix of upper and lowercase letters, numbers, and non-alphanumeric characters. The passphrase can be changed later by using the -p option.

There is no way to recover a lost passphrase. If the passphrase is lost or forgotten, a new key must be generated and the corresponding public key copied to other machines.

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

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

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

The options are as follows:

-A
	Generate host keys of all default key types (rsa, ecdsa, and ed25519) if they do not already exist. The host keys are generated with the default key file path, an empty passphrase, default bits for the key type, and default comment. If -f has also been specified, its argument is used as a prefix to the default path for the resulting host key files. This is used by /etc/rc to generate new host keys.
-a rounds
	When saving a private key, this option specifies the number of KDF (key derivation function, currently bcrypt_pbkdf(3)) rounds used. Higher numbers result in slower passphrase verification and increased resistance to brute-force password cracking (should the keys be stolen). The default is 16 rounds.
-B
	Show the bubblebabble digest of specified private or public key file.
-b bits
	Specifies the number of bits in the key to create. For RSA keys, the minimum size is 1024 bits and the default is 3072 bits. Generally, 3072 bits is considered sufficient. 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/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 NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.
-Y verify
	Request to verify a signature generated using ssh-keygen -Y sign as described above. When verifying a signature, ssh-keygen accepts a message on standard input and a signature namespace using -n. A file containing the corresponding signature must also be supplied using the -s flag, along with the identity of the signer using -I and a list of allowed signers via the -f flag. The format of the allowed signers file is documented in the ALLOWED SIGNERS section below. A file containing revoked keys can be passed using the -r flag. The revocation file may be a KRL or a one-per-line list of public keys. Successful verification by an authorized signer is signalled by ssh-keygen returning a zero exit status.
-y
	This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.
-Z cipher
	Specifies the cipher to use for encryption when writing an OpenSSH-format private key file. The list of available ciphers may be obtained using "ssh -Q cipher". The default is "aes256-ctr".
-z serial_number
	Specifies a serial number to be embedded in the certificate to distinguish this certificate from others from the same CA. If the serial_number is prefixed with a '+' character, then the serial number will be incremented for each certificate signed on a single command-line. The default serial number is zero. When generating a KRL, the -z flag is used to specify a KRL version number.

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.amp; "name@example.com". If contents is specified then it is included as the contents of the extension/option encoded as a string, otherwise the extension/option is created with no contents (usually indicating a flag). Extensions may be ignored by a client or server that does not recognise them, whereas unknown critical options will cause the certificate to be refused.
force-command=command
	Forces the execution of command instead of any shell or command specified by the user when the certificate is used for authentication.
no-agent-forwarding
	Disable ssh-agent(1) forwarding (permitted by default).
no-port-forwarding
	Disable port forwarding (permitted by default).
no-pty
	Disable PTY allocation (permitted by default).
no-user-rc
	Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).
no-x11-forwarding
	Disable X11 forwarding (permitted by default).
permit-agent-forwarding
	Allows ssh-agent(1) forwarding.
permit-port-forwarding
	Allows port forwarding.
permit-pty
	Allows PTY allocation.
permit-user-rc
	Allows execution of ~/.ssh/rc by sshd(8).
permit-X11-forwarding
	Allows X11 forwarding.
no-touch-required
	Do not require signatures made using this key include demonstration of user presence (e.g. by having the user touch the authenticator). This option only makes sense for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.
source-address=address_list
	Restrict the source addresses from which the certificate is considered valid. The address_list is a comma-separated list of one or more address/netmask pairs in CIDR format.
verify-required
	Require signatures made using this key indicate that the user was first verified. This option only makes sense for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk. Currently PIN authentication is the only supported verification method, but other methods may be supported in the future.

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

Finally, certificates may be defined with a validity lifetime. The -V option allows specification of certificate start and end times. A certificate that is presented at a time outside this range will not be considered valid. By default, certificates are valid from the Unix Epoch to the distant future.

For certificates to be used for user or host authentication, the CA public key must be trusted by sshd(8) or ssh(1). Refer to those manual pages for details.

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

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

cert-authority
	Indicates that this key is accepted as a certificate authority (CA) and that certificates signed by this CA may be accepted for verification.
namespaces=namespace-list
	Specifies a pattern-list of namespaces that are accepted for this key. If this option is present, the signature namespace embedded in the signature object and presented on the verification command-line must match the specified list before the key will be considered acceptable.
valid-after=timestamp
	Indicates that the key is valid for use at or after the specified timestamp, which may be a date or time in the YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats. Dates and times will be interpreted in the current system time zone unless suffixed with a Z character, which causes them to be interpreted in the UTC time zone.
valid-before=timestamp
	Indicates that the key is valid for use at or before the specified timestamp.

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

An example allowed signers file:

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

RFC 4716, The Secure Shell (SSH) Public Key File Format, 2006.