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ssh-keygen — authentication key generation, management and conversion
ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment] [-f output_keyfile] ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile] 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 [-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 num_trials] [-W generator] 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 generates, manages and converts authentication keys for ssh(1). ssh-keygen can create RSA keys for use by SSH protocol version 1 and DSA, ECDSA or RSA 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 for use in SSH protocol 2 connections. ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange (DH- GEX). See the MODULI GENERATION section for details. Normally each user wishing to use SSH with public key authentication runs this once to create the authentication key in ~/.ssh/identity, ~/.ssh/id_ecdsa, ~/.ssh/id_dsa or ~/.ssh/id_rsa. Additionally, the system administrator may use this 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 “.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 RSA1 keys, 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. The options are as follows: -A For each of the key types (rsa1, rsa, dsa and ecdsa) 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. This is used by system administration scripts to generate new host keys. -a trials Specifies the number of primality tests to perform when screening DH-GEX candidates using the -T command. -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 768 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 they 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. -C comment Provides a new comment. -c Requests changing the comment in the private and public key files. This operation is only supported for RSA1 keys. 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 RSA public keys provided by the PKCS#11 shared library pkcs11. When used in combination with -s, this option indicates that a CA key resides in a PKCS#11 token (see the CERTIFICATES section for details). -e This option will read 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 Search 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 filename of the key file. -G output_file Generate candidate primes for DH-GEX. These primes must be screened for safety (using the -T option) before use. -g Use generic DNS format when printing fingerprint resource records using the -r command. -H Hash a known_hosts file. This replaces all hostnames and addresses with hashed representations within the specified file; the original content is moved to a file with a .old suffix. These hashes may be used normally by ssh and sshd, but they do not reveal identifying information should the file's contents be disclosed. This option will not modify existing hashed hostnames and is therefore safe to use on files that mix hashed and non-hashed names. -h When signing a key, create a host certificate instead of a user certificate. Please see the CERTIFICATES section for details. -I certificate_identity Specify the key identity when signing a public key. 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”. -L Prints the contents of a certificate. -l Show fingerprint of specified public key file. Private RSA1 keys are also supported. For RSA and DSA keys ssh-keygen tries to find the matching public key file and prints its fingerprint. If combined with -v, an ASCII art representation of the key is supplied with the fingerprint. -M memory Specify the amount of memory to use (in megabytes) when generating candidate moduli for DH-GEX. -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”. -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. 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 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 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 options are valid for host keys. -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 Silence ssh-keygen. -R hostname Removes all keys belonging to hostname 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 start Specify start point (in hex) when generating candidate moduli for DH-GEX. -s ca_key Certify (sign) a public key using the specified CA key. Please see the CERTIFICATES section for details. -T output_file Test DH group exchange candidate primes (generated using the -G option) for safety. -t type Specifies the type of key to create. The possible values are “rsa1” for protocol version 1 and “dsa”, “ecdsa” or “rsa” for protocol version 2. -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 sshd_config(5). 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 Verbose mode. Causes ssh-keygen to print debugging messages about its progress. This is helpful for debugging moduli generation. Multiple -v options increase the verbosity. The maximum is 3. -W generator Specify desired generator when testing candidate moduli for DH-GEX. -y This option will read a private OpenSSH format file and print an OpenSSH public key to stdout. -z serial_number Specifies a serial number to be embedded in the certificate to distinguish this certificate from others from the same CA. The default serial number is zero.
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 tested 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.
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 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 user_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 sshd(8) or ssh(1). Please refer to those manual pages for details.
~/.ssh/identity Contains the protocol version 1 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 3DES. 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/identity.pub Contains the protocol version 1 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 RSA authentication. There is no need to keep the contents of this file secret. ~/.ssh/id_dsa ~/.ssh/id_ecdsa ~/.ssh/id_rsa Contains the protocol version 2 DSA, ECDSA 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. ssh(1) will read this file when a login attempt is made. ~/.ssh/id_dsa.pub ~/.ssh/id_ecdsa.pub ~/.ssh/id_rsa.pub Contains the protocol version 2 DSA, ECDSA 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. /etc/ssh/moduli Contains Diffie-Hellman groups used for DH-GEX. The file format is described in moduli(5).
ssh(1), ssh-add(1), ssh-agent(1), ssh-vulnkey(1), moduli(5), sshd(8) The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
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.