Why am I able to SSH a remote machine even with the wrong keys?

Description: I have created a ssh connection between my Windows Pc and Raspberry Pi. To so I followed the following steps:

Step 1: Somehow get the IP address of the Raspberry Pi. It should be something like this: 192.168.1.52

Step 2: Open a shell and access the Raspberry Pi via ssh:

ssh pi@192.168.1.52

You will need the password.

Step 3: In the home directory of the remote pc use these commands:

mkdir .ssh

Step 4: Secure the ssh connection via private/public key. In the local pc use this commands:

ssh-keygen -f .ssh/fede_windows -t rsa -b 4096

If your local machine is Linux based run this line:

chmod 600 .ssh/fede_windows # if linux

Finally:

scp .ssh/fede_windows.pub pi@192.168.1.52:.ssh

Step 5: In the remote pc use these commands:

sudo nano /etc/ssh/sshd_config

and modify the following lines of the config file:

ChallengeResponseAuthentication no
PasswordAuthentication no
UsePAM no

Finally:

sudo systemctl reload sshd

Step 6: In the remote computer use these commands:

cat ~/.ssh/fede_windows.pub >> ~/.ssh/authorized_keys
chmod 700 ~/.ssh/
chmod 600 ~/.ssh/*

Step 7: In the local computer run this command to log in to the remote one:

ssh -i .ssh/fede_windows pi@192.168.1.52

Problem: When I perform all these steps again in my Ubuntu Pc by generating this time a key named fede_ubuntu, it looks like that I am able to ssh my Raspberry Pi no matter what I insert in the command:

ssh -i .ssh/fede_xyz pi@192.168.1.52

It works all the time and this is not supposed to happen since it should be restricted only to the key I just created. If I switch to my Windows machine everything works as expected and only if I specify the right key works.

Question: Would you be able to suggest a possible reason of this issue and how to fix it please?

EDIT: By typing the following command ssh -i .ssh/key_that_does_not_exits -v pi@192.168.1.52 I get:

Warning: Identity file .ssh/key_that_does_not_exits not accessible: No such file or directory.
OpenSSH_8.2p1 Ubuntu-4ubuntu0.3, OpenSSL 1.1.1f 31 Mar 2020
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug1: Connecting to 192.168.1.52 [192.168.1.52] port 22.
debug1: Connection established.
debug1: identity file /home/federico/.ssh/id_rsa type -1
debug1: identity file /home/federico/.ssh/id_rsa-cert type -1
debug1: identity file /home/federico/.ssh/id_dsa type -1
debug1: identity file /home/federico/.ssh/id_dsa-cert type -1
debug1: identity file /home/federico/.ssh/id_ecdsa type -1
debug1: identity file /home/federico/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/federico/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/federico/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/federico/.ssh/id_ed25519 type -1
debug1: identity file /home/federico/.ssh/id_ed25519-cert type -1
debug1: identity file /home/federico/.ssh/id_ed25519_sk type -1
debug1: identity file /home/federico/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/federico/.ssh/id_xmss type -1
debug1: identity file /home/federico/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9p1 Raspbian-10+deb10u2+rpt1
debug1: match: OpenSSH_7.9p1 Raspbian-10+deb10u2+rpt1 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 192.168.1.52:22 as 'pi'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: MAC: <implicit> compression: none
debug1: kex: client->server cipher: MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:hC5w2kDxgHH5eFRY1vOJaS7ipPR+8OWX2tkkEZbF194
debug1: Host '192.168.1.52' is known and matches the ECDSA host key.
debug1: Found key in /home/federico/.ssh/known_hosts:1
debug1: rekey out after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 134217728 blocks
debug1: Will attempt key: federico@federico RSA SHA256:E96Hu2Ee+IyAuoZ06GxTvo+ZmAkzqfihbAKkFqxU1AU agent
debug1: Will attempt key: /home/federico/.ssh/id_rsa
debug1: Will attempt key: /home/federico/.ssh/id_dsa
debug1: Will attempt key: /home/federico/.ssh/id_ecdsa
debug1: Will attempt key: /home/federico/.ssh/id_ecdsa_sk
debug1: Will attempt key: /home/federico/.ssh/id_ed25519
debug1: Will attempt key: /home/federico/.ssh/id_ed25519_sk
debug1: Will attempt key: /home/federico/.ssh/id_xmss
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering public key: federico@federico RSA SHA256:E96Hu2Ee+IyAuoZ06GxTvo+ZmAkzqfihbAKkFqxU1AU agent
debug1: Server accepts key: federico@federico RSA SHA256:E96Hu2Ee+IyAuoZ06GxTvo+ZmAkzqfihbAKkFqxU1AU agent
debug1: Authentication succeeded (publickey).
Authenticated to 192.168.1.52 ([192.168.1.52]:22).
debug1: channel 0: new [client-session]
debug1: Requesting
debug1: Entering interactive session.
debug1: pledge: network
debug1: client_input_global_request: rtype want_reply 0
debug1: Remote: /home/pi/.ssh/authorized_keys:2: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug1: Remote: /home/pi/.ssh/authorized_keys:2: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug1: Sending environment.
debug1: Sending env LC_ADDRESS = it_IT.UTF-8
debug1: Sending env LC_NAME = it_IT.UTF-8
debug1: Sending env LC_MONETARY = it_IT.UTF-8
debug1: Sending env LC_PAPER = it_IT.UTF-8
debug1: Sending env LANG = en_US.UTF-8
debug1: Sending env LC_IDENTIFICATION = it_IT.UTF-8
debug1: Sending env LC_TELEPHONE = it_IT.UTF-8
debug1: Sending env LC_MEASUREMENT = it_IT.UTF-8
debug1: Sending env LC_TIME = it_IT.UTF-8
debug1: Sending env LC_NUMERIC = it_IT.UTF-8
Last login: Sun Aug 22 22:26:00 2021 from 192.168.1.197

1 Answer

The command cat ~/.ssh/fede_windows.pub >> ~/.ssh/authorized_keys appends the key to authorized_keys, it doesn't replace what's already there.

To replace all old keys, run cat foobar.pub > ~/.ssh/authorized_keys. This will truncate the file and then add the new information.

You can add as many keys as you want. To check, simply open the file with a text editor and have a look at the content.

The ability to have many keys is actually a security feature. It means you can use different keys from different computers, and if a computer is lost, you only have to remove the compromised keys, not all keys.