ssh-keygen

authentication key generation, management and conversion

TLDR

Generate a key interactively

>_ ssh-keygen
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Specify file in which to save the key

>_ ssh-keygen -f ~/.ssh/[filename]
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Generate an ed25519 key with 100 key derivation function rounds

>_ ssh-keygen -t ed25519 -a 100
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Generate an RSA 4096 bit key with email as a comment

>_ ssh-keygen -t rsa -b 4096 -C "[email]"
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Retrieve the key fingerprint from a host (useful for confirming the authenticity of the host when first connecting to it via SSH)

>_ ssh-keygen -l -F [remote_host]
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Remove the keys of a host from the known_hosts file (useful when a known host has a new key)

>_ ssh-keygen -R [remote_host]
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Retrieve the fingerprint of a key in MD5 Hex

>_ ssh-keygen -l -E md5 -f ~/.ssh/[filename]
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Change the password of a key

>_ ssh-keygen -p -f ~/.ssh/[filename]
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SYNOPSIS

-words ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa] [-N new_passphrase] [-C comment] [-f output_keyfile] [-m format] ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile] [-m format] ssh-keygen -i [-m key_format] [-f input_keyfile] ssh-keygen -e [-m key_format] [-f input_keyfile] ssh-keygen -y [-f input_keyfile] ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile] ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile] ssh-keygen -B [-f input_keyfile] ssh-keygen -D pkcs11 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] [-g] ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point] ssh-keygen -T output_file -f input_file [-v] [-a rounds] [-J num_lines] [-j start_line] [-K checkpt] [-W generator] ssh-keygen -s ca_key -I certificate_identity [-h] [-U] [-D pkcs11_provider] [-n principals] [-O option] [-V validity_interval] [-z serial_number] file... ssh-keygen -L [-f input_keyfile] ssh-keygen -A [-f prefix_path] ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number] file... ssh-keygen -Q -f krl_file file...

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 RSA key.

ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange (DH-GEX). See the MODULI GENERATION section for details.

Finally, ssh-keygen can be used to generate and update Key Revocation Lists, and to test whether given keys have been revoked by one. See the KEY REVOCATION LISTS section for details.

Normally each user wishing to use SSH with public key authentication runs this once to create the authentication key in ~/.ssh/id_dsa , ~/.ssh/id_ecdsa , ~/.ssh/id_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.

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

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

After a key is generated, instructions below detail where the keys should be placed to be activated.

The options are as follows:

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 -G option. The desired length of the primes may be specified by the -b option. For example:

# ssh-keygen -G moduli-2048.candidates -b 2048

By default, the search for primes begins at a random point in the desired length range. This may be overridden using the -S 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 -T option. In this mode ssh-keygen will read candidates from standard input (or a file specified using the -f option). For example:

# ssh-keygen -T moduli-2048 -f moduli-2048.candidates

By default, each candidate will be subjected to 100 primality tests. This may be overridden using the -a 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 -W option. Valid generator values are 2, 3, and 5.

Screened DH groups may be installed in /etc/ssh/moduli . It is important that this file contains moduli of a range of bit lengths and that both ends of a connection share common moduli.

CERTIFICATES

ssh-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 user_key.pub

Similarly, it is possible for the CA key to be hosted in a ssh-agent(1). This is indicated by the -U flag and, again, the CA key must be identified by its public half.

$ ssh-keygen -Us ca_key.pub -I key_id user_key.pub

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

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

$ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub $ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub

Additional limitations on the validity and use of user certificates may be specified through certificate options. A certificate option may disable features of the SSH session, may be valid only when presented from particular source addresses or may force the use of a specific command. 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 are valid from UNIX Epoch to the distant future.

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

KEY REVOCATION LISTS

ssh-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.

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.

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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.

SEE ALSO

ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8) 2006, RFC 4716, The Secure Shell (SSH) Public Key File Format

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