NAME

ssh-keygen — Authentication key utility

SYNOPSIS

DESCRIPTION

ssh-keygen generates, manages and converts authentication keys for ssh(1). ssh-keygen can create keys for use by SSH protocol version 2.

The type of key to be generated is specified with the -t option. If invoked without any arguments, ssh-keygen will generate an Ed25519 key unless is run FIPS mode. In FIPS mode the default is RSA key.

By default key is stored in PKCS#8 PEM format. For compatibility with earlier versions programs read keys stored in traditional PEM format as well. Option -p could be used to convert private key between OpenSSH proprietary and more portable and widely used PKCS#8 PEM format depending from option arguments: -m PKCS8 or -m OpenSSH. Note if requested proprietary format but key contain X.509 certificate will be used PKCS#8 PEM format. Also in FIPS mode could be used only PKCS#8 PEM format.

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Finally, ssh-keygen can be used to generate and update custom Key Revocation Lists, and to test whether given keys have been revoked by one.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_ecdsa, ~/.ssh/id_ed25519 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.

For private keys stored in the proprietary 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, 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 in OpenSSH proprietary format, 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. 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 comment

Provides a new comment.

-c

Requests changing the comment in the private and public key files. This operation is only supported for keys stored in the proprietary OpenSSH format. 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 X.509 certificates or public keys provided by the PKCS#11 shared library pkcs11.

-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 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 keys for use by other programs, including several commercial SSH implementations.

-F hostname

Search for the specified hostname in a known_hosts file, listing any occurrences found. Specified hostname could be a hostname or an address optionally enclosed within ‘[’ and ‘]’ brackets then followed by ‘:’ and a port number. The format of known_hosts is documented in the SSH_KNOWN_HOSTS section of sshd(8). 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 custom certificate instead of a user custom 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 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”. Note PKCS8 format is usable only for public keys.

-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 custom certificate presented on the command line. Keys/custom 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 custom certificates.

-l

Show fingerprint of public key read from specified file. For all key types ssh-keygen will try to find file with the matching public key and prints its fingerprint. When identity contain X.509 certificate public key is extracted from it and fingerprint is printed for public key. 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 [APPDATA]/etc/moduli file. See the MODULI GENERATION section for more information.

-m 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 proprietary OpenSSH private key and portable PKCS#8 PEM private key formats. The supported formats are: “RFC4716” (RFC 4716/SSH2 public), “PKCS8” (PKCS#8 PEM public or private key), “PEM” (traditional PEM public or private key) or “OpenSSH” (proprietary OpenSSH public or private key). The default conversion format is “RFC4716” for export/import of public keys and “PKCS8” for private keys.

-N new_passphrase

Provides the new passphrase.

-n principals

Specify one or more principals (user or host names) to be included in a custom 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 custom 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 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. Valid algorithms are “sha1” and “sha256”. The default is to print both.

The -O option may be specified multiple times.

-o

Obsolete option replaced by -m OpenSSH. Causes ssh-keygen to save private keys using the proprietary OpenSSH format rather than the more compatible PKCS#8 PEM format. Note Ed25519 keys will use the proprietary key format if key is not supported by cryptographic library.

-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. Options -m PKCS8, -m PEM, or -m OpenSSH could by used to set explicitly format of stored key: either compatible PKCS#8 PEM, traditional PEM, or proprietary OpenSSH format. Note by default is used PKCS#8 PEM format.

-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

Removes all keys belonging to the specified hostname from a known_hosts file. Specified hostname could be a hostname or an address optionally enclosed within ‘[’ and ‘]’ brackets then followed by ‘:’ and a port number. The format of known_hosts is documented in the SSH_KNOWN_HOSTS section of sshd(8). This option is useful to delete hashed hosts (see the -H option above).

-r hostname

Print the CERT or SSHFP resource record named hostname for the specified public key file. Note CERT is printed for X.509 certificate based key.

-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 custom certificates directly by key ID or serial number. See the KEY REVOCATION LISTS section for details.

-t ecdsa | ed25519 | rsa

Specifies the type of key to create. The possible values are “ecdsa”, “ed25519”, or “rsa”. The default is “rsa” if FIPS mode is active otherwise “ed25519”.

This flag may also be used to specify the desired signature type when signing custom certificates using an RSA CA key. The available RSA signature variants are “ssh-rsa” (SHA1 signatures, not recommended), “rsa-sha2-256” (the default for RSA keys), and “rsa-sha2-512”.

-U

When used in combination with -s, this option indicates that a CA key resides in a ssh-agent(1). See the CERTIFICATES section for more information.

-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 custom certificate. A validity interval may consist of a single time, indicating that the custom 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 custom 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 custom 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.

-y

This option will read a private key file and print to stdout an public key in OpenSSH format. For this option keyfile name could use prefixes “engine:” or “store:” to load identifies using engine or store functionality provided by cryptographic library. For more details see configuration option IdentityFile in ssh_config(5).

-Z cipher

Specifies the cipher to use for encryption when saving a private key in OpenSSH proprietary format. 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 custom certificate to distinguish this custom 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 custom 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.

MODULI GENERATION

ssh-keygen may be used to generate groups for the Diffie-Hellman Group Exchange (DH-GEX) protocol. Generating these groups is a two-step process: first, candidate primes are generated using a fast, but memory intensive process. These candidate primes are then tested for suitability (a CPU-intensive process).

Generation of primes is performed using the -M generate option. The desired length of the primes may be specified by the -O bits option. For example:

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

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 [APPDATA]/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

ssh-keygen supports signing of keys to produce custom certificates that may be used for user or host authentication. Custom 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 custom Certification Authority (CA) key. Clients or servers may then trust only the CA key and verify its signature on a custom certificate rather than trusting many user/host keys. Note that OpenSSH custom certificates are a different, and much limited, format to the X.509 certificates used in TLS.

ssh-keygen supports two types of custom certificates: user and host. User custom certificates authenticate users to servers, whereas host custom certificates authenticate server hosts to users. To generate a user custom certificate:

The resultant custom certificate will be placed in /path/to/user_key-cert.pub. A host custom certificate requires the -h option:

The host custom certificate will be output to /path/to/host_key-cert.pub.

In all cases, key_id is a "key identifier" that is logged by the server when the custom certificate is used for authentication.

Custom certificates may be limited to be valid for a set of principal (user/host) names. By default, generated custom certificates are valid for all users or hosts. To generate a custom certificate for a specified set of principals:

Additional limitations on the validity and use of user custom certificates may be specified through custom certificate options. A custom 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 custom 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 custom 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 custom certificate to be refused.

force-command=command

Forces the execution of command instead of any shell or command specified by the user when the custom 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.

source-address=address_list

Restrict the source addresses from which the custom certificate is considered valid. The address_list is a comma-separated list of one or more address/netmask pairs in CIDR format.

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

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

For custom 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.

KEY REVOCATION LISTS

ssh-keygen is able to manage OpenSSH format custom Key Revocation Lists (KRLs). These binary files specify keys or custom certificates to be revoked using a compact format, taking as little as one bit per custom 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 custom 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 custom certificates by serial number or key ID without having the complete original custom 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 custom 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 custom certificate with the specified key ID string. The CA key must have been specified on the ssh-keygen command line using the -s option.

key: public_key

Revokes the specified key. If a custom certificate is listed, then it is revoked as a plain public key.

sha1: public_key

Revokes the specified key by including its SHA1 hash in the KRL.

sha256: public_key

Revokes the specified key by including its SHA256 hash in the KRL. KRLs that revoke keys by SHA256 hash are not supported by OpenSSH versions prior to 7.9.

hash: fingerprint

Revokes a key using a fingerprint hash, as obtained from a sshd(8) authentication log message or the ssh-keygen -l flag. Only SHA256 fingerprints are supported here and resultant KRLs are not supported by OpenSSH versions prior to 7.9.

KRLs may be updated using the -u flag in 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 ssh-keygen will exit with a non-zero exit status. A zero exit status will only be returned if no key was revoked.

ENVIRONMENT

SSH_ASKPASS

If ssh-keygen needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh-keygen does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS that may open a X11 window to read the passphrase. For keys from “engine” or “store” SSH_ASKPASS is preferred over associated terminal. Note if SSH_ASKPASS is not set will be executed program ssh-askpass located in “libexec” directory. (Note that on some machines it may be necessary to redirect the input from /dev/null to make this work.)

Remark: On Android DISPLAY is ignored and SSH_ASKPASS is preferred over associated terminal.

SSH_ENGINE_CONF

Overrides default location of engine configuration file.

FILES

~/.ssh/id_ecdsa

 

~/.ssh/id_ed25519

 

~/.ssh/id_rsa

Contains the ECDSA, Ed25519, or RSA authentication identity of the user.

This file should not be readable by anyone but the user.

For RSA, ECDSA, or Ed25519 identity file may contain X.509 certificate that match it. In addition file may contain extra X.509 certificates. Extra certificates along with certificates from X.509 store are used to build chain of certificates leading to a trusted certificate authority if required by public key algorithm format.

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 256-bit AES. This file is not automatically accessed by ssh-keygen but it is offered as the default file for the private key. ssh(1) will read this file when a login attempt is made.

~/.ssh/id_ecdsa.pub

 

~/.ssh/id_ed25519.pub

 

~/.ssh/id_rsa.pub

Contains the 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. If file ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, or ~/.ssh/id_ed25519 contain RSA/ECDSA/Ed25519 private key and X.509 certificates public key file must contain certificate that match private key! Use ssh-keygen with option -y to regenerate its content. Note in case with X.509 certificate you can append content to ~/.ssh/authorized_keys or to add certificate “Distinguished Name” / “Subject” in corresponding format to “authorized keys” file. See sshd(8). Using distinguished name is preferred as X.509 certificate could be renewed and after renewal there is no need to update “authorized keys” file.

[APPDATA]/etc/moduli

Contains Diffie-Hellman groups used for DH-GEX. The file format is described in moduli(5).

SEE ALSO

ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)

CONFORMING TO

1. D. Eastlake, Domain Name System Security Extensions, RFC 2535, March 1999.
2. D. Eastlake, RSA/MD5 KEYs and SIGs in the Domain Name System (DNS), RFC 2537, March 1999.
3. D. Eastlake and O. Gudmundsson, Storing Certificates in the Domain Name System (DNS), RFC 2538, March 1999.
4. D. Eastlake 3rd, RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS), RFC 3110, May 2001.
5. J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
6. J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.

AUTHORS

PKIX-SSH 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. Roumen Petrov contributed support for X.509 certificates.