secsh-keygen, ssh-keygen -- authentication key generation, management and conversion

SYNOPSIS

secsh-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]

secsh-keygen -p [-a rounds] [-f keyfile] [-m format] [-N new_passphrase] [-P old_passphrase] [-Z cipher]

secsh-keygen -i [-f input_keyfile] [-m key_format]

secsh-keygen -e [-f input_keyfile] [-m key_format]

secsh-keygen -y [-f input_keyfile]

secsh-keygen -c [-a rounds] [-C comment] [-f keyfile] [-P passphrase]

secsh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile]

secsh-keygen -B [-f input_keyfile]

secsh-keygen -D reader [-f input_keyfile]

secsh-keygen -G reader -t type [-qv] [-b bits] [-f input_keyfile]

secsh-keygen -F hostname [-lv] [-f known_hosts_file]

secsh-keygen -H [-f known_hosts_file]

secsh-keygen -K [-a rounds] [-w provider]

secsh-keygen -R hostname [-f known_hosts_file]

secsh-keygen -r hostname [-g] [-f input_keyfile]

secsh-keygen -M generate [-O option] output_file

secsh-keygen -M screen [-f input_file] [-O option] output_file

secsh-keygen -I certificate_identity -s ca_key [-hU] [-D reader] [-n principals] [-O option] [-V validity_interval] [-z serial_number] file ...

secsh-keygen -L [-f input_keyfile]

secsh-keygen -A [-a rounds] [-f prefix_path]

secsh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number] file ...

secsh-keygen -Q [-l] -f krl_file file ...

secsh-keygen -Y find [-O option] -s signature_file -f allowed_signers_file [-O option] [-V validity_interval] [-z serial_number] file ...

secsh-keygen -Y match -I signer_identity -f allowed_signers_file -f allowed_signers_file [-O option] [-V validity_interval] [-z serial_number] file ...

secsh-keygen -Y check [-O option] -n namespace -s signature_file -f allowed_signers_file [-O option] [-V validity_interval] [-z serial_number] file ...

secsh-keygen -Y sign [-O option] -f key_file -n namespace file ...

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 -D reader [-f input_keyfile]

ssh-keygen -G reader -t type [-qv] [-b bits] [-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 reader] [-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 [-O option] -s signature_file -f allowed_signers_file [-O option] [-V validity_interval] [-z serial_number] file ...

ssh-keygen -Y match -I signer_identity -f allowed_signers_file -f allowed_signers_file [-O option] [-V validity_interval] [-z serial_number] file ...

ssh-keygen -Y check [-O option] -n namespace -s signature_file -f allowed_signers_file [-O option] [-V validity_interval] [-z serial_number] file ...

ssh-keygen -Y sign [-O option] -f key_file -n namespace file ...

DESCRIPTION

secsh-keygen generates, manages and converts authentication keys for secsh. secsh-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, secsh-keygen will generate an ED25519 key.

Finally, secsh-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 RSA or DSA authentication runs this once to create the authentication key in $USERPROFILE/.ssh/id_dsa, $USERPROFILE/.ssh/id_ecdsa, $USERPROFILE/.ssh/id_ecdsa_sk, $USERPROFILE/.ssh/id_ed25519, $USERPROFILE/.ssh/id_ed25519_sk, or $USERPROFILE/.ssh/id_rsa. At the time of installation, the installer will also use this tool 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 with .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. Good passphrases are 10-30 characters long and are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per character, and provides very bad passphrases). 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 copied to the corresponding public key to other machines.

For keys stored in the newer OpenSSH format, there is also a comment field in the key file that is only for convenience to the user to help identify the key. The comment can tell what the key is for, or whatever is useful. The comment is initialized to user@host when the key is created, but can be changed using the -c option.

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

You can also call secsh-keygen as ssh-keygen.

Options

-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.

-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 reader

Download the public key stored in the smartcard in reader 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 reader

Uses the smartcard in the specified reader to generate a keypair. In this instance, secsh-keygen only creates a disk file for the public key in the keypair, as it is not possible to download private keys from a smartcard.

-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, secsh-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 secsh-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 secsh-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.

When generating SSHFP DNS records from public keys using the -r flag, the following options are accepted:

hashalg=algorithm: Selects a hash algorithm to use when printing SSHFP records using the -D flag. Valid algorithms are sha1 and sha256. The default is to print both.

The -O option may be specified multiple times.

-P passphrase

Provides the (old) passphrase.

-p

Requests changing the passphrase of a private key file instead of creating a new private key. The program will prompt for the file containing the private key, for the old passphrase, and twice for the new passphrase.

-Q

Test whether keys have been revoked in a KRL. If the -l option is also specified then the contents of the KRL will be printed.

-q

Silence secsh-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.

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 indicates that a CA key resides in a secsh-agent. See the CERTIFICATES section for more information.

-u

Update a KRL. When specified with -k 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 secsh-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.

-v 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 secsh-keygen -Y sign has a valid structure. This does not validate if a signature comes from an authorized signer. When testing a signature, secsh-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 secsh-keygen returning a zero exit status.

-Y sign

Cryptographically sign a file or some data using an SSH key. When signing, secsh-keygen accepts zero or more files to sign on the command-line - if no files are specified then secsh-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 secsh-agent). 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 secsh-keygen -Y sign as described above. When verifying a signature, secsh-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 secsh-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.

FILES

$USERPROFILE/.ssh/id_dsa
$USERPROFILE/.ssh/id_ecdsa
$USERPROFILE/.ssh/id_ecdsa_sk
$USERPROFILE/.ssh/id_ed25519
$USERPROFILE/.ssh/id_ed25519_sk
$USERPROFILE/.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 secsh-keygen but it is offered as the default file for the private key. secsh will read this file when a login attempt is made.
$USERPROFILE/.ssh/id_dsa.pub
$USERPROFILE/.ssh/id_ecdsa.pub
$USERPROFILE/.ssh/id_ecdsa_sk.pub
$USERPROFILE/.ssh/id_ed25519.pub
$USERPROFILE/.ssh/id_ed25519_sk.pub
$USERPROFILE/.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.

ENVIRONMENT

SSH_SK_PROVIDER: Specifies a path to a library that will be used when loading any FIDO authenticator-hosted keys, overriding the default of using the built-in USB HID support.

MODULI GENERATION

secsh-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:

secsh-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 secsh-keygen will read candidates from standard input (or a file specified using the -f option). For example:

secsh-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 $ROOTDIR/etc/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.

CERTIFICATES

secsh-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 host_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. For a list of valid certificate options, see the documentation for the -O option above.

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. 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 secsh-agent 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 secshd (permitted by default).
no-x11-forwarding: Disable X11 forwarding (permitted by default).
permit-agent-forwarding: Allows secsh-agent forwarding.
permit-port-forwarding: Allows port forwarding.
permit-pty: Allows PTY allocation.
permit-user-rc: Allows execution of ~/.ssh/rc by secshd.
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 have a maximum validity interval.

For certificates to be used for user or host authentication, the CA public key must be trusted by secshd or secsh. Please refer to those manual pages for details.

FIDO AUTHENTICATOR

ssh-keygen is able to generate FIDO authenticator-backed keys, after which they may be used much like any other key type supported by OpenSSH, so long as the hardware authenticator is attached when the keys are used. FIDO 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 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 secshd 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 secsh-add. 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.

KEY REVOCATION LISTS

secsh-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 secsh-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 secsh-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 its SHA1 hash.

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 secsh-keygen will exit with a non-zero exit status. A zero exit status will only be returned if no key was revoked.

ALLOWED SIGNERS

When verifying signatures, secsh-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 secshd. 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
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...

AUTHORS

OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.

PORTABILITY

All UNIX systems. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022. Windows Server 2025.

NOTES

Typically, the $USERPROFILE directory in a Windows domain environment is local to the machine, but is automatically replicated to a server (typically to ~/username.profile) when the user logs in and out, and when roaming profiles are in use. This might not be a desirable location to store private keys from a key management and security point of view.

The secsh-keygen utility, by default, stores key files under the $USERPROFILE directory on 10/2016/2019/11/2022/2025 platforms instead of ~/. This was done because the ~/ directory might not be available for the case of a domain machine that cannot contact a domain controller. Note that the secure shell service continues to use the ~/.ssh directory to store configuration files.

Using SecurityKeyProvider(FIDO) requires the use of an external DLL that will act as an interface between OpenSSH and Windows Hello via a set of COM interfaces that Microsoft provides. This interface DLL is not currently provided by PTC, but there are 3rd parties that provide templates that can be used to develop your own.

AVAILABILITY

PTC MKS Toolkit for System Administrators
PTC MKS Toolkit for Developers
PTC MKS Toolkit for Interoperability
PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition

secsh-keygen, ssh-keygen
authentication key generation, management and conversion

Command