#sshacommand

What is ssh-keygen? Ssh meaning?

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 the password stored in files, and eliminated the possibility of a compromised server stealing the user's password.

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: ssh-keygen The result is: 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 user@host The key's randomart image is: +-------+ | . ..oo..| | . . . . .o.X.| | . . o. ..+ B| | . o.o .+ ..| | ..o.S o.. | | . %o= . | | @.B… . | | o.=. o. . . .| | .oo E. . .. | +---------+ 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. You can add this to your ssh agent. You can also use ssh port. You can follow manual pages to ssh a command to a remote server.

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 standardized 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

Copying the Public Key to the Server ( ssh tunnel )

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/tatu-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.

Command and Option Summary