Clés SSH

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Page enfant de SSH

Qu'est-ce qu'une clé SSH?

SSH utilise la cryptographie à clé publique (CP) ou cryptographie asymétrique pour sécuriser les connexions.

Dans ce mode de cryptage, la clé qui est privée reste secrète et l'autre clé peut être divulguée à d'autres utilisateurs. Tous peuvent utiliser la clé publique pour encoder un message, mais seul le propriétaire de la clé privée peut utiliser sa clé privée pour le décodage.

La clé publique permet aussi de valider l'identité d'un utilisateur. Voyons un exemple : Robert veut communiquer avec Alice qui dit posséder une clé privée, mais il veut s'assurer qu'Alice est bien celle qui le prétend. Robert peut utiliser la clé publique d'Alice pour lui envoyer un message codé et si Alice peut prouver à Robert que son message est compris, nous pouvons au moins en conclure qu'Alice est effectivement propriétaire de la clé privée.

Les systèmes à CP sont au cœur des protocoles SSL et TLS qui protègent la plupart de communications sur l'internet, dont les sites HTTPS.

LA CP a plusieurs usages sur nos grappes :

Quand vous vous connectez à une grappe, votre client SSH utilise habituellement la clé publique de cette grappe pour vérifier que la connexion se fait au bon serveur.
Une session cryptée peut être établie pour prévenir l'interception des messages échangés.
Le serveur distant peut utiliser votre clé publique (qui se trouve dans le fichier .ssh/authorized_keys de votre répertoire /home) pour vérifier votre identité et ainsi vous permettre de vous connecter.

Nous vous recommandons fortement d'utiliser l'authentification par CP. Il vous faudra travailler un peu plus sur la configuration, mais le résultat sera plus sécuritaire et plus pratique.

Pour l'authentification à l'aide de ces clés :

Générez la paire de clés (la clé privée et la clé publique).
Copiez la clé publique de cette paire sur les serveurs auxquels vous voulez vous connecter et ajoutez la clé publique à votre fichier authorized_keys (voir Utiliser des clés SSH sous Linux).
Vérifiez les permissions (voir Utiliser des clés SSH sous Linux).
Testez.

You should ideally generate a key pair on your own system - a system that you believe to be secure. The reason for this is that the private part is security-sensitive, so you should always minimize its exposure.

When generating a key, you will be prompted you for a "passphrase". This is a string that is used to encrypt the private key. You should provide a strong passphrase that is memorable, and is not a password. This passphrase offers protection if the private key file is stolen.

The specific process to generate an SSH key pair depends on the operating system you use. For the Windows PuTTY or MobaXterm clients, see Generating SSH keys in Windows. For a Unix-like environment (Linux, Mac, Windows Subsystem for Linux or Cygwin), see Using SSH keys in Linux. In addition if you are using the cloud, OpenStack provides a method for creating keypairs: See the ssh key pair section on the Cloud Quick Start page.

Installing your key

Using CCDB

To install the key, you must make the target/destination system aware of the public part of your key. On Compute Canada, we have recently (March 2021) added a convenient way to do this. You should visit:

https://ccdb.computecanada.ca/ssh_authorized_keys

This page will allow you to paste in the public key. Since both the public and private keys are plain text, you can examine them - for instance,

cat .ssh/id_rsa.pub

which should show something like this:

ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGhczaUoV6SzR2VEf9Rp4/P9xHVU8S72CKHrwKU+Yntx user@machine

Once your public key is loaded into CCDB this way, you can use it to login to any of our clusters. At present, OpenStack (cloud systems) do not read your key from CCDB as shown in the link above.

Sometimes, you may encounter a key that is in an alternate format. For instance, this is a public key in PEM format:

Public keys in RFC4716 or PKCS8 formats will look similar to PEM, with small variations in the header and footer lines.

Using the authorized_keys file

The CCDB method described above makes your public key available on all Compute Canada HPC systems. This is convenient, and is often desired.

However, there may be circumstances in which you want to install a key only on a specific system. You can do this by adding the key to a file in your home directory on that system. For instance, to install a key that only works on Cedar, copy your public key into the file ~/.ssh/authorized_keys on Cedar. This will allow you to log in to any of Cedar's login nodes using PK. On our systems (or any other with OpenSSH) the ssh-copy-id command is the most convenient way to do this:

 ssh-copy-id -i computecanada-key username@cedar.computecanada.ca

The authorized_keys mechanism is standard, and almost universally used on the internet. It is however somewhat fragile: Specifically, SSH is quite picky about the permissions on the authorized_keys file, as well as your home directory and the .ssh subdirectory. This is described further in using SSH keys in Linux.

Advanced Key Usage

SSH Key Agent

Although it's important to secure your private key by encrypting it with the passphrase, it is inconvenient to have to enter your passphrase every time you use the key. Rather than leaving the private key unencrypted, we strongly suggest using an SSH key agent. You type the passphrase when starting up the agent, after which the agent supplies the private key for new connections. This avoids storing the unencrypted private key on permanent storage, where it is more vulnerable to being stolen or copied.

Advanced Key Generation

ssh-keygen shown above is using defaults, which are OK, but may not be ideal. for instance:

many people prefer a different key type (rather than the RSA default):

ssh-keygen -t ed25519

you can specify a comment for the key, which may be convenient for distinguishing among multiple keys:

ssh-keygen -C 'computecanada systems'

you can choose the name of the key file:

ssh-keygen -F computecanada-key

(this produces a file "computecanada-key" containing the private part, and "computecanada-key.pub" for the public part.

you can choose longer keys for some key types such as RSA:

ssh-keygen -t rsa -b 4096

SSH Key constraints =

The public key syntax permits you to provide a number of very useful constraints that limit what the key is allowed to do. By default, a public key installed without constraints can do anything. For instance, this public key:

 restrict,command="squeue --me" ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGhczaUoV6SzR2VEf9Rp4/P9xHVU8S72CKHrwKU+Yntx

can only perform one simple operation (showing whether you have any jobs in Slurm). An interesting example is:

 restrict,command="/usr/libexec/openssh/sftp-server" ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGhczaUoV6SzR2VEf9Rp4/P9xHVU8S72CKHrwKU+Yntx

which allows the key to be used only for SFTP, which is how sshfs works.

The key constraint can also limit which hosts can connect using the key:

 restrict,from="d24-141-114-17.home.cgocable.net" ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGhczaUoV6SzR2VEf9Rp4/P9xHVU8S72CKHrwKU+Yntx

Limiting by hosts is a powerful way to minimize the danger posed by a key being compromised. In this case, the "restrict" keyword turns off "pty allocation", which makes an interactive session behave peculiarly. For a source-constrained interactive session:

 restrict,from="d24-141-114-17.home.cgocable.net",pty ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGhczaUoV6SzR2VEf9Rp4/P9xHVU8S72CKHrwKU+Yntx

allows pty allocation.

There are a large number of these key constraints, which are documented in the sshd man page ("man sshd" on a linux system).

PK Best Practices

DO encrypt your private key.
DO avoid copying your private key. In particular, it should NOT appear on our clusters.
DO use ssh-agent to make encrypted keys convenient.
DO NOT use agent-forwarding if you can avoid it. With agent-forwarding, any intermediate system(s) become trusted.
DO apply constraints to your public key to make it less powerful (dangerous).

Voyez aussi ces courtes vidéos sur comment configurer les clés SSH.