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

SYNOPSIS

secsh-keygen [-qv] [-b bits] [-t type] [-N new_passphrase] [-C comment] [-f output_keyfile]

secsh-keygen -p [-v] [-P old_passphrase] [-N new_passphrase] [-f keyfile]

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

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

secsh-keygen -y [-v] [-f input_keyfile]

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

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

secsh-keygen -B [-v] [-f input_keyfile]

secsh-keygen -D reader [-v]

secsh-keygen -U reader [-v] [-f input_keyfile]

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

secsh-keygen -F hostname [-f known_hosts_file] {-l]

secsh-keygen -H [-f known_hosts_file]

secsh-keygen -R hostname [-f known_hosts_file]

secsh-keygen -r hostname [-f input_keyfile] {-q]

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

secsh-keygen -L [-f input_keyfile]

secsh-keygen -A

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

secsh-keygen -Q -f krl_file file ...

ssh-keygen [-qv] [-b bits] [-t type] [-N new_passphrase] [-C comment] [-f output_keyfile]

ssh-keygen -p [-v] [-P old_passphrase] [-N new_passphrase] [-f keyfile]

ssh-keygen -i [-v] [-m key_format] [-f input_keyfile]

ssh-keygen -e [-v] [-m key_format] [-f input_keyfile]

ssh-keygen -y [-v] [-f input_keyfile]

ssh-keygen -c [-v] [-P passphrase] [-C comment] [-f keyfile]

ssh-keygen -l [-v] [-f input_keyfile]

ssh-keygen -B [-v] [-f input_keyfile]

ssh-keygen -D reader [-v]

ssh-keygen -U reader [-v] [-f input_keyfile]

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

ssh-keygen -F hostname [-f known_hosts_file] {-l]

ssh-keygen -H [-f known_hosts_file]

ssh-keygen -R hostname [-f known_hosts_file]

ssh-keygen -r hostname [-f input_keyfile] {-q]

ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals] [-O option] [-V validity_interval] [-z serial_number] file ...

ssh-keygen -L [-f input_keyfile]

ssh-keygen -A

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

ssh-keygen -Q -f krl_file file ...

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 RSA 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_ed25519, 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, instructions below detail where the keys should be placed to be activated.

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

Options

-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.

-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).

-B

Shows 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 2048 bits. Generally, 2048 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. Ed25519 keys have a fixed length and the -b flag will be ignored.

-C

Provides the 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

Downloads the RSA 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

Reads a private or public OpenSSH key file and print to stdout the 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

Searches for the specified hostname 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 file name 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

Uses generic DNS format when printing fingerprint resource records using the -r command.

-H

Hashes 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 secsh and secshd, but they do not reveal identifying information should the file's contents be disclosed. This option does 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_authority

Specify the key identity when signing a public key. Please 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

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

Shows 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 key_format

Specify a key format for the -i (import) or -e (export) conversion options. The supported key formats are: RFC4716 (RFC 4716/SSH2 public or private key), PKCS8 (PEM PKCS8 public key) or PEM (PEM public key). The default conversion 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. Please see the CERTIFICATES section for details.

-O option

Specify a certificate option when signing a key. This option may be specified multiple times. Please see the CERTIFICATES section for details. 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.
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.

-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.

-q

Silences secsh-keygen.

-R hostname

Removes alll keys belonging to hostname from a known_hosts file. This option is useful to delete hashed hosts (see the -H above).

-r hostname

Prints 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. Please see the CERTIFICATES section for details.

-t type

Specifies the type of the key to create. The possible values are rsa, dsa, ecdsa, or ed25519.

-U reader

Upload an existing RSA private key into the smartcard in reader.

-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 a date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a relative time (to the current time) consisting of a minus sign followed by a relative time in the format described in the TIME FORMATS section of secshd. The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time starting with a plus character.

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).

-v

Causes secsh-keygen to display debugging messages about its progress. This is helpful in debugging smartcard and key generation problems. Specifying multiple -v options (to a maximum of three) increases the verbosity.

-w generator

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.

-y

Reads a private OpenSSH format file and prints an OpenSSH public key to stdout.

-z

Specifies a serial number to be embedded in the certificate to distinguish this certificate from others from the same CA. 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_ed25519
$USERPROFILE/.ssh/id_rsa: Contains the DSA, ECDSA, ED25519 or RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file using 128-bit AES. This file is not automatically accessed by ssh-keygen but it is offered as the default file for the private key. secsh will read this file when a login attempt is made.
$USERPROFILE/.ssh/id_dsa.pub
$USERPROFILE/.ssh/id_ecdsa.pub
$USERPROFILE/.ssh/id_ed25519.pub
$USERPROFILE/.ssh/id_rsa.pub: Contains the DSA, ECDSA, 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.

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.

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.

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 ssh-keygen command line using the -s option.
id: key_id: Revokes a certificate with the specified key ID string. The CA key must have been specified on the 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.

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 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.

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 8.1/2012R2/10/2016/2019/11/2022 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.

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