How to generate a new SSH Key with ssh-keygen ?

What Is ssh-keygen?

Ssh-keygen is a tool for creating new authentication key pairs for SSH. Such key pairs are used for automating logins, single sign-on, and for authenticating hosts.

SSH Keys and Public Key Authentication

The SSH protocol uses public key cryptography for authenticating hosts and users. The authentication keys, called SSH keys, are created using the keygen program.

SSH introduced public key authentication as a more secure alternative to the older .rhosts authentication. It improved security by avoiding the need to have password stored in files, and eliminated the possibility of a compromised server stealing the user’s password.

However, SSH keys are authentication credentials just like passwords. Thus, they must be managed somewhat analogously to user names and passwords. They should have a proper termination process so that keys are removed when no longer needed.

Creating an SSH Key Pair for User Authentication

The simplest way to generate a key pair is to run ssh-keygen without arguments. In this case, it will prompt for the file in which to store keys. Here’s an example:

klar (11:39) ~>ssh-keygen Generating public/private rsa key pair.

Enter file in which to save the key (/home/user/.ssh/id_rsa):

Enter passphrase (empty for no passphrase):  Enter same passphrase again:

Your identification has been saved in /home/user/.ssh/id_rsa.

Your public key has been saved in /home/user/.ssh/id_rsa.pub.

The key fingerprint is: SHA256:Up6KjbnEV4Hgfo75YM393QdQsK3Z0aTNBz0DoirrW+c

The key's randomart image is:

+---[RSA 2048]----+ |    .      ..oo..| |   . . .  . .o.X.| |    . . o.  ..+ B| 
|   .   o.o  .+ ..| |    ..o.S   o..  | |   . %o=      .  | |    @.B...     . |
|   o.=. o. . .  .| |    .oo  E. . .. | +----[SHA256]-----+ klar (11:40) ~>

First, the tool asked where to save the file. SSH keys for user authentication are usually stored in the user’s .ssh directory under the home directory. However, in enterprise environments, the location is often different. The default key file name depends on the algorithm, in this case id_rsa when using the default RSA algorithm. It could also be, for example, id_dsa or id_ecdsa.

Then it asks to enter a passphrase. The passphrase is used for encrypting the key, so that it cannot be used even if someone obtains the private key file. The passphrase should be cryptographically strong. Our online random password generator is one possible tool for generating strong passphrases.

Choosing an Algorithm and Key Size

SSH supports several public key algorithms for authentication keys. These include:

rsa – an old algorithm based on the difficulty of factoring large numbers. A key size of at least 2048 bits is recommended for RSA; 4096 bits is better. RSA is getting old and significant advances are being made in factoring. Choosing a different algorithm may be advisable. It is quite possible the RSA algorithm will become practically breakable in the foreseeable future. All SSH clients support this algorithm.
dsa – an old US government Digital Signature Algorithm. It is based on the difficulty of computing discrete logarithms. A key size of 1024 would normally be used with it. DSA in its original form is no longer recommended.
ecdsa – a new Digital Signature Algorithm standarized by the US government, using elliptic curves. This is probably a good algorithm for current applications. Only three key sizes are supported: 256, 384, and 521 (sic!) bits. We would recommend always using it with 521 bits, since the keys are still small and probably more secure than the smaller keys (even though they should be safe as well). Most SSH clients now support this algorithm.
ed25519 – this is a new algorithm added in OpenSSH. Support for it in clients is not yet universal. Thus its use in general purpose applications may not yet be advisable.

The algorithm is selected using the -t option and key size using the -b option. The following commands illustrate:

ssh-keygen -t rsa -b 4096
ssh-keygen -t dsa 
ssh-keygen -t ecdsa -b 521
ssh-keygen -t ed25519

Specifying the File Name

Normally, the tool prompts for the file in which to store the key. However, it can also be specified on the command line using the -f <filename> option.

ssh-keygen -f ~/name-key-ecdsa -t ecdsa -b 521

Copying the Public Key to the Server

To use public key authentication, the public key must be copied to a server and installed in an authorized_keys file. This can be conveniently done using the ssh-copy-id tool. Like this:

ssh-copy-id -i ~/.ssh/name-key-ecdsa user@host

Once the public key has been configured on the server, the server will allow any connecting user that has the private key to log in. During the login process, the client proves possession of the private key by digitally signing the key exchange.

Adding the Key to SSH Agent

ssh-agent is a program that can hold a user’s private key, so that the private key passphrase only needs to be supplied once. A connection to the agent can also be forwarded when logging into a server, allowing SSH commands on the server to use the agent running on the user’s desktop.

For more information on using and configuring the SSH agent, see the ssh-agent page.

Creating Host Keys

The tool is also used for creating host authentication keys. Host keys are stored in the /etc/ssh/ directory.

Host keys are just ordinary SSH key pairs. Each host can have one host key for each algorithm. The host keys are almost always stored in the following files:

/etc/ssh/ssh_host_dsa_key

/etc/ssh/ssh_host_ecdsa_key

/etc/ssh/ssh_host_ed25519_key

/etc/ssh/ssh_host_rsa_key

The host keys are usually automatically generated when an SSH server is installed. They can be regenerated at any time. However, if host keys are changed, clients may warn about changed keys. Changed keys are also reported when someone tries to perform a man-in-the-middle attack. Thus it is not advisable to train your users to blindly accept them. Changing the keys is thus either best done using an SSH key management tool that also changes them on clients, or using certificates.

Command and Option Summary

Here’s a summary of commonly used options to the keygen tool:

-b “Bits” This option specifies the number of bits in the key. The regulations that govern the use case for SSH may require a specific key length to be used. In general, 2048 bits is considered to be sufficient for RSA keys.

-e “Export” This option allows reformatting of existing keys between the OpenSSH key file format and the format documented in RFC 4716, “SSH Public Key File Format”.

-p “Change the passphrase” This option allows changing the passphrase of a private key file with [-P old_passphrase] and [-N new_passphrase], [-f keyfile].

-t “Type” This option specifies the type of key to be created. Commonly used values are: – rsa for RSA keys – dsa for DSA keys – ecdsa for elliptic curve DSA keys

-i “Input” When ssh-keygen is required to access an existing key, this option designates the file.

-f “File” Specifies name of the file in which to store the created key.

-N “New” Provides a new passphrase for the key.

-P “Passphrase” Provides the (old) passphrase when reading a key.

-c “Comment” Changes the comment for a keyfile.

-p Change the passphrase of a private key file.

-q Silence ssh-keygen.

-v Verbose mode.

-l “Fingerprint” Print the fingerprint of the specified public key.

-B “Bubble babble” Shows a “bubble babble” (Tectia format) fingerprint of a keyfile.

-F Search for a specified hostname in a known_hosts file.

-R Remove all keys belonging to a hostname from a known_hosts file.

-y Read a private OpenSSH format file and print an OpenSSH public key to stdout.

This only listed the most commonly used options. For full usage, including the more exotic and special-purpose options, use the man ssh-keygen command.

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