Ssh-keygen

ssh-keygen is a standard component of the Secure Shell (SSH) protocol suite found on Unix and Unix-like computer systems used to establish secure shell sessions between remote computers over insecure networks, through the use of various cryptographic techniques. The ssh-keygen utility is used to generate, manage, and convert authentication keys.

ssh-keygen is able to generate a key using one of three different digital signature algorithms.

With the help of the ssh-keygen tool, a user can create passphrase keys for any of these key types (to provide for unattended operation, the passphrase can be left empty, at increased risk).

These keys differ from keys used by the related tool GNU Privacy Guard.

Key formats supported

Originally, with SSH protocol version 1 (now deprecated) only the RSA algorithm was supported. As of 2016, RSA is still considered strong, but the recommended key length has increased over time.

The SSH protocol version 2 additionally introduced support for the DSA algorithm. As the DSA algorithm has ties to the American National Security Agency (NSA), aspects of its security have been called into question by Edward Snowden's global surveillance disclosures, particularly in light of the Bullrun decryption program.

Subsequently, OpenSSH added support for a third digital signature algorithm, ECDSA (this key format no longer uses the previous PEM file format for private keys, nor does it depend upon the OpenSSL library to provide the cryptographic implementation). A fourth format is supported using ed25519, originally developed by independent cryptography researcher Daniel J. Bernstein.

ssh-keygen command syntax

The syntax of the ssh-keygen file is as follows:

ssh-keygen [options]

Some of the important options of ssh-keygen command are as follows:

ssh-keygen command options	description
-b bits	Specifies the number of bits in the key to create. The minimum bit length is 768 bits and the default length is 2048 bits.
-C comment	Provides new comment.
-p	Requests changing the passphrase of a private key file instead of creating a new private key.
-t	Specifies the type of key to create.
-o	Use the new OpenSSH format.
-q	quiets ssh-keygen. It is used by the /etc/rc file while creating a new key.
-N	Provides a new Passphrase.
-F (or -B)	For ssh-keygen2, dumps the key's fingerprint in Bubble Babble format

Files used by the ssh-keygen utility

The ssh-keygen utility uses various files for storing public and private keys. The files used by ssh-keygen utility are as follows:

• $HOME/.ssh/identity: The $HOME/.ssh/identity file contains the RSA private key when using the SSH protocol version 1.
• $HOME/.ssh/identity.pub: The $HOME/.ssh/identity.pub file contains the RSA public key for authentication when you are using the SSH protocol version 1. A user should copy its contents in the $HOME/.ssh/authorized_keys file of the remote system where a user wants to log in using RSA authentication.
• $HOME/.ssh/id_dsa: The $HOME/.ssh/id_dsa file contains the protocol version 2 DSA authentication identity of the user.
• $HOME/.ssh/id_dsa.pub: The $HOME/.ssh/id_dsa.pub file contains the DSA public key for authentication when you are using the SSH protocol version 2. A user should copy its contents in the $HOME/.ssh/authorized_keys file of the remote system where a user wants to log in using DSA authentication.
• $HOME/.ssh/id_rsa: The $HOME/.ssh/id_rsa file contains the protocol version 2 RSA authentication identity of the user. This file should not be readable by anyone but the user.
• $HOME/.ssh/id_rsa.pub: The $HOME/.ssh/id_rsa.pub file contains the protocol version 2 RSA public key for authentication. The contents of this file should be added to $HOME/.ssh/authorized_keys on all computers where a user wishes to log in using public key authentication.

External links

• Generating an SSH key, a guide from GitHub
• ssh-keygen manual from the OpenBSD project
• Linux man page from die.net