Linux: Added PAM Entry

wiki/linux/pam.md

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+# PAM
+
+PAM (Pluggable Authentication Modules) is the authentication framework used by most
+[Linux](/wiki/linux.md) distributions.
+It allows authentication methods to be configured independently of the applications using them.
+This makes it possible to integrate passwords,
+[fingerprint readers](/wiki/linux/fingerprint_reader.md), [FIDO2](/wiki/fido2.md) security keys and
+other authentication mechanisms into services such as `sudo`, `login` and
+[display managers](/wiki/linux/display_managers.md).
+
+## Authenticate `sudo` with a FIDO2 Security Key
+
+This section describes how to configure a FIDO2 security key for authentication when running `sudo`.
+The following guide is based on a
+[YouTube video by pixeledi](https://www.youtube.com/watch?v=e5k1ye-_tNs).
+
+First install the required package [PAM U2F](https://github.com/Yubico/pam-u2f) with a
+[package manager](/wiki/linux/package_manager.md) matching your
+[Linux distribution](/wiki/linux.md#distributions).
+It is often packaged under the name `pam-u2f`.
+
+Next create an UDEV rule that allows access to the FIDO2 device.
+
+```sh
+echo 'KERNEL=="hidraw*", SUBSYSTEM=="hidraw", MODE="0664", GROUP="plugdev"' | sudo tee /etc/udev/rules.d/fido2-u2f.rules
+```
+
+Then create the configuration directory and generate the authentication mapping file.
+
+```sh
+mkdir -p ~/.config/fido2-u2f
+cd ~/.config/fido2-u2f
+pamu2fcfg > keys.txt
+```
+
+Open the PAM configuration for `sudo`.
+
+```sh
+sudo vim /etc/pam.d/sudo
+```
+
+Add the following line before all existing `auth` entries.
+Replace `<username>` with the name of the current user.
+
+```txt
+auth            sufficient      pam_u2f.so      cue         authfile=/home/<username>/.config/fido2-u2f/keys.txt
+```
+
+The `cue` option causes PAM to prompt for user presence before requesting a touch on the security
+key.