Avant tout, il faut vérifier que le serveur SSH (CIBLE) autorise l'authentification par clé publique au niveau du fichier de configuration : sshd_config

Il faut décommenter les lignes PubkeyAuthentication et AuthorizedKeysFile :

$ vi /etc/ssh/sshd_config
_______________________________________________________________
...
# Authentication:
 
#LoginGraceTime 2m
#PermitRootLogin prohibit-password
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10
 
PubkeyAuthentication yes
 
# Expect .ssh/authorized_keys2 to be disregarded by default in future.
AuthorizedKeysFile      .ssh/authorized_keys .ssh/authorized_keys2
...

Nous devons générer une paire de clés publique/privée :

[user1@SOURCE ~]$ ssh-keygen -t rsa
		Generating public/private rsa key pair.
		Enter file in which to save the key (/root/.ssh/id_rsa):
		Enter passphrase (empty for no passphrase):
		Enter same passphrase again:
		Your identification has been saved in /root/.ssh/id_rsa.
		Your public key has been saved in /root/.ssh/id_rsa.pub.
		The key fingerprint is:
		b1:87:8d:3c:9e:1b:53:ca:f3:0a:fd:1f:37:f9:2d:c3 root@DEST
		The key's randomart image is:
		+--[ RSA 2048]----+
		|                 |
		|                 |
		|        .        |
		|       . *       |
		|        S +      |
		|       + *     . |
		|      . X   ..+  |
		|       . B   oEo.|
		|        o.o..  oo|
		+-----------------+

[user1@SOURCE ~]$ ssh-copy-id -i .ssh/id_rsa.pub user2@CIBLE
		user2@CIBLE's password:
		Now try logging into the machine, with 'ssh user2@CIBLE', and check in:

		  .ssh/authorized_keys

		to make sure we haven't added extra keys that you weren't expecting.

# On édite 'id_rsa.pub' et on copie juste la clé :
[user1@SOURCE ~]$ vi id_rsa.pub
_______________________________________________________________
---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-20220617"
AAAAB3NzaC1yc2EAAAADAQABAAABAQC6syxBU9C0S0xpWJQ1xIdrZ9sfsetMKrxd
A44P7Q/KTZ4YlBaeklFJD2Gjey0UCqK3ECixddeaqdoNg7IZSu/fn1yRq8A0HMQo
sOPy6k55/NJZ6KpWG9VwHFpLGA5lvpKhxbaEwCXCbJIaj8KT0XUAp9EzSe2n+ZV6
3/Ueb5fNtRyytVGOQVa/WYupIQGeJsREDJseWiq28b/33ufxWtkw+0+996NC+Lgn
f2jwVybyVSVVGQKXLEaJf5xyRGMsMvhsDc2Ngearex58ECPbrok6XPUXD3oDU1xz
1+r7cFONp80ty/XmaQemhroORdGr3ub0EMprImx/1HZrZfLjPdDL
---- END SSH2 PUBLIC KEY ----
 
# et on colle la clé après le mot-clé 'ssh-rsa' sur le serveur distant dans :
(on supprime les sauts de ligne)
[user2@CIBLE ~] vi ~/.ssh/authorized_keys
_______________________________________________________________
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC6syxBU9C0S0xpWJQ1xIdrZ9sfsetMKrxdA44P7Q/KTZ4YlBaeklFJD2Gjey0UCqK3ECixddeaqdoNg7IZSu/fn1yRq8A0HMQosOPy6k55/NJZ6KpWG9VwHFpLGA5lvpKhxbaEwCXCbJIaj8KT0XUAp9EzSe2n+ZV63/Ueb5fNtRyytVGOQVa/WYupIQGeJsREDJseWiq28b/33ufxWtkw+0+996NC+Lgnf2jwVybyVSVVGQKXLEaJf5xyRGMsMvhsDc2Ngearex58ECPbrok6XPUXD3oDU1xz1+r7cFONp80ty/XmaQemhroORdGr3ub0EMprImx/1HZrZfLjPdDL root@ma_machine
 
# on peut aussi limiter l'accès à une machine/@IP
[user2@CIBLE ~] vi ~/.ssh/authorized_keys
_______________________________________________________________
from="192.168.1.1,ma_machine.mon_domaine.com" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC6syxBU9C0S0xpWJQ1xIdrZ9sfsetMKrxdA44P7Q/KTZ4YlBaeklFJD2Gjey0UCqK3ECixddeaqdoNg7IZSu/fn1yRq8A0HMQosOPy6k55/NJZ6KpWG9VwHFpLGA5lvpKhxbaEwCXCbJIaj8KT0XUAp9EzSe2n+ZV63/Ueb5fNtRyytVGOQVa/WYupIQGeJsREDJseWiq28b/33ufxWtkw+0+996NC+Lgnf2jwVybyVSVVGQKXLEaJf5xyRGMsMvhsDc2Ngearex58ECPbrok6XPUXD3oDU1xz1+r7cFONp80ty/XmaQemhroORdGr3ub0EMprImx/1HZrZfLjPdDL root@ma_machine

Une fois que la clé publique a été copiée dans le authorized_keys du serveur CIBLE, on peut paramétrer PuTTY pour utiliser automatiquement la clé privée correspondante. Dans le paramétrage de la session correspondant au serveur CIBLE :

On lance ensuite la connexion :