MAN page from RedHat Other ssh-1.2.26-1i.i386.rpm

SSH-KEYGEN

Section: SSH (1)
Updated: November 8, 1995
Index

NAME

ssh-keygen - authentication key pair generation

SYNOPSIS

ssh-keygen[-b bits][-f file][-N new_passphrase][-C comment]

ssh-keygen -p[-P old_passphrase][-N new_passphrase]

ssh-keygen -c[-P passphrase][-C comment]

ssh-keygen -u[-f file][-P passphrase]

DESCRIPTION

Ssh-keygengenerates and manages authentication keys for ssh(1).Normally each user wishing to usesshwith RSA authentication runs this once to create the authenticationkey in$HOME/.ssh/identity.Additionally, the system administrator may use this to generate host keys.

Normally this program generates the key and asks for a file in whichto store the private key. The public key is stored in a file with thesame name but ".pub" appended. The program also asks for apassphrase. The passphrase may be empty to indicate no passphrase(host keys must have empty passphrase), or it may be a string ofarbitrary length. Good passphrases are 10-30 characters long and arenot simple sentences or otherwise easily guessable (Englishprose has only 1-2 bits of entropy per word, and provides very badpassphrases). The passphrase can be changed later by using the-poption.

There is no way to recover a lost passphrase. If the passphrase islost or forgotten, you will have to generate a new key and copy thecorresponding public key to other machines.

USING GOOD, UNGUESSABLE PASSPHRASES IS STRONGLY RECOMMENDED. EMPTYPASSPHRASES SHOULD NOT BE USED UNLESS YOU UNDERSTAND WHAT YOU ARE DOING.

There is also a comment field in the key file that is only forconvenience to the user to help identify the key. The comment cantell what the key is for, or whatever is useful. The comment isinitialized to userAATThost when the key is created, but can be changedusing the-coption.

The cipher to be used when encrypting keys with a passphrase isdefined in ssh.h. Using the-uoption, keys encrypted in any supported cipher can be updated touse this default cipher.

OPTIONS

-b bits: Specifies the number of bits in the key to create. Minimum is 512bits. Generally 1024 bits is considered sufficient, and key sizesabove that no longer improve security but make things slower. Thedefault is 1024 bits.
-c: Requests changing the comment in the private and public key files.The program will prompt for the file containing the private keys, forpassphrase if the key has one, and for the new comment.
-f: Specifies the file name in which to load/store the key.
-p: Requests changing the passphrase of a private key file instead ofcreating a new private key. The program will prompt for the filecontaining the private key, for the old passphrase, and twice for thenew passphrase.
-u: Requests that the key's cipher is changed to the current defaultcipher (determined at compile-time - currently 3DES).
-C: Provides the new comment.
-N: Provides the new passphrase.
-P: Provides the (old) passphrase.

FILES

$HOME/.ssh/random_seed: Used for seeding the random number generator. This file should not bereadable by anyone but the user. This file is created the first timethe program is run, and is updated every time.
$HOME/.ssh/identity: Contains the RSA authentication identity of the user. This fileshould not be readable by anyone but the user. It is possible tospecify a passphrase when generating the key; that passphrase will beused to encrypt the private part of this file using 3DES. This fileis not automatically accessed byssh-keygen,but it is offered as the default file for the private key.
$HOME/.ssh/identity.pub: Contains the public key for authentication. The contents of this fileshould be added to $HOME/.ssh/authorized_keys on all machineswhere you wish to log in using RSA authentication. There is noneed to keep the contents of this file secret.

AUTHOR

Tatu Ylonen <yloAATTssh.fi>

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