ssh-keygen - Unix, Linux Command. ssh-keygen - authentication key generation, management and conversion. 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 RSA or DSA 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 RSA or DSA authentication runs this once to create the authentication key in. . 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 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 copied to the corresponding public key 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. Use generic DNS format when printing fingerprint resource records using the - r command. 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 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. This option will read an unencrypted private (or public) key file in SSH2-compatible format and print an OpenSSH compatible private (or public) key to stdout. ssh-keygen also reads the SECSH Public Key File Format . This option allows importing keys from several commercial SSH implementations. 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. This option will read a private OpenSSH format file and print an OpenSSH public key to stdout. 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 -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. 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/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. 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. Contains the protocol version 1 RSA public key for authentication. The contents of this file should be added to. 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. Contains the protocol version 2 DSA 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. Contains the protocol version 2 DSA public key for authentication. The contents of this file should be added to. 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. Contains the protocol version 2 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. Contains the protocol version 2 RSA public key for authentication. The contents of this file should be added to. 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. Contains Diffie-Hellman groups used for DH-GEX. The file format is described in moduli (5. 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.
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