-A | | 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. If -f has also been specified, its argument is used as a prefix to the de‐ fault 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) 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 RFC 4716. 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. Please see the CERTIFICATES section for details. |
-I | certificate_identity | Specify the key identity when signing a public key. |
-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. |
-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. |
-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. |
-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, 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 sup‐ ported 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. |
-O | option | Specify a key/value option. These are specific to the operation that ssh-keygen has been requested to perform. |
-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 | | Quiet operation |
-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. |
-t | <type> | 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, this option indicates that a CA key resides in a ssh-agent(1). |
-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. |
-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 | 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 corre‐ sponding 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 | 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. |
-O
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:
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 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).
device Explicitly specify a fido(4) device to use, rather than letting the token 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 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).
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 tokens 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 tokens during key generation. Please note that this information is potentially sensitive. By default, this information is discarded.
The -O option may be specified multiple times.
-V
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, Janu‐ ary 1st, 2011). “-1m:forever” (valid from one minute ago and never expiring).