Secure Shell
Table of Contents
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1. Overview
Secure communication between computers is enabled using secure shell (ssh). To access a remote computer via the command line, we can use a command such as
> ssh user@curie.physics.hmc.edu user password: xxxxxxx
Secure shell communication typically happens over port 22, which should be opened in the ufw.
For more secure and convenient access to a machine, you can generate a public/private key pair using ssh-keygen. Then you copy the public key to the computer you are trying to connect to and install it. After you do this, you won't be challenged to provide your password when you issue a command such as the one above and will be directly logged into the remote machine.
2. Setup
First check whether there is already a public/private key pair:
user$ ls -l ~/.ssh total 96 -rw------- 1 user staff 1679 Jul 9 2017 id_rsa -rw-r--r-- 1 user staff 405 Jul 9 2017 id_rsa.pub -rw-r--r--@ 1 user staff 15762 Jul 22 14:17 known_hosts
Note that the private key file, id_rsa, is not visible to those in the staff
group or others, only to user. You may also have public-private key pair in a
different format, such as the more recently developed ed25519 format.
If no key exists, you will see
ls: ~/.ssh: No such file or directory
In that case, run the command ssh-keygen -t ed25519. You will be prompted to
enter an optional passphrase. It is much handier not to enter a passphrase, so
simply press enter twice.
user$ ssh-keygen -t ed25519 Generating public/private ed25519 key pair. Enter file in which to save the key (/Users/saeta/.ssh/id_ed25519): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /Users/saeta/.ssh/id_ed25519. Your public key has been saved in /Users/saeta/.ssh/id_ed25519.pub. The key fingerprint is: SHA256:ZmI5KDaFht4/pmXqfemcdU89m2/NJMBaZHTjRKDHVTA saeta@Saeta-MBP19.local The key's randomart image is: +--[ED25519 256]--+ | .ooEo.| | . . oo= o | |. o . .+o . | |.... . . .+ | | .+.. = S o . | | . o.. = . o .| | * .. . . *.| | B ooo . o B| | .+ .o+ . +o| +----[SHA256]-----+
The ed25519 format was introduced fairly recently. You may need to generate an older style of pair. You can create an RSA key pair with 4096 bits with the following:
user$ ssh-keygen -b 4096 Generating public/private rsa key pair. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in id_rsa. Your public key has been saved in id_rsa.pub. The key fingerprint is: SHA256:rZcZBBirwfcgMoHnizMjy0IR5bdL8/0UnKox7c0Lcok user@machine The key's randomart image is: +---[RSA 4096]----+ | .o. .o. | |..oo .. . | | o+.+.+ . | | ..o.=.o o. . | | ....+ S o+ | |*.. . + +.o+. | |+= . E.B+. | |o. B.* | |. . . =. | +----[SHA256]-----+