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Simon Josefsson
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# Setting up your YubiKey for challenge response authentication (on Max OS X)
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Setting up your YubiKey for challenge response authentication on Max OS X
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-------------------------------------------------------------------------
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This article explains the process to get the challenge-response authentication possible with newer YubiKeys working on Mac OS X. Since Mac OS X uses PAM like most other Unix/POSIX systems do, most of this should apply to other operating systems, too.
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This article explains the process to get the challenge-response
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authentication possible with newer YubiKeys working on Mac OS X. Since
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Mac OS X uses PAM like most other Unix/POSIX systems do, most of this
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should apply to other operating systems, too.
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### Getting yubico-pam
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First you will have to install yubico-pam and its dependencies required for challenge-response authentication. Use your distribution's package manager to get it, or build from source. If you're on OS X you can use [MacPorts](http://www.macports.org/) to install yubico-pam:
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Getting yubico-pam
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------------------
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First you will have to install yubico-pam and its dependencies
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required for challenge-response authentication. Use your
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distribution's package manager to get it, or build from source. If
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you're on OS X you can use [MacPorts](http://www.macports.org/) to
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install yubico-pam:
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sudo port install yubico-pam
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**Note**: This will probably not work in non-superuser installations of MacPorts, because it needs to place the yubico PAM module into `/usr/lib/pam`.
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**Note**: This will probably not work in non-superuser installations
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of MacPorts, because it needs to place the yubico PAM module into
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`/usr/lib/pam`.
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### Configuring your YubiKey
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The next step would be to set up your YubiKey for challenge-response authentication, if you haven't done so already. Although this is possible with the command line `ykpersonalize` tool, the GUI "YubiKey Personalization Tool" is a more comfortable way to do this.
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Configuring your YubiKey
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------------------------
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The next step would be to set up your YubiKey for challenge-response
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authentication, if you haven't done so already. Although this is
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possible with the command line `ykpersonalize` tool, the GUI "YubiKey
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Personalization Tool" is a more comfortable way to do this.
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1. Plug in your YubiKey and start the YubiKey Personalization Tool
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**Note**: YubiKey Personalization Tool shows whether your YubiKey supports challenge-response in the lower right.
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@ -27,61 +44,140 @@ Apparently Yubico-OTP mode doesn't work with yubico-pam at the moment.
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You won't need this key again, it's sufficient to have it on your YubiKey. Note that the YubiKey Personalization Tool by default logs the key to configuration_log.csv in your home directory. Consider turning this off in the settings before writing or shredding the file after writing.
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8. Click "Write Configuration"
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### Configuring your user account to accept the YubiKey
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After setting up your YubiKey you need to configure your account to accept this YubiKey for authentication. To do this, open a terminal and run
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Configuring your user account to accept the YubiKey
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---------------------------------------------------
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After setting up your YubiKey you need to configure your account to
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accept this YubiKey for authentication. To do this, open a terminal
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and run
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# create the directory where ykpamcfg will store the initial challenge
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mkdir -m0700 -p ~/.yubico
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# get the initial challenge from the YubiKey
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ykpamcfg -2
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If you used slot 1 above, replace -2 with -1. If you configured your YubiKey to require a button press the LED on the YubiKey will start blinking; press the button to send a challenge-response response. `ykpamcfg` should finish successfully telling you that it stored the initial challenge somewhere inside your home directory:
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If you used slot 1 above, replace -2 with -1. If you configured your
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YubiKey to require a button press the LED on the YubiKey will start
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blinking; press the button to send a challenge-response
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response. `ykpamcfg` should finish successfully telling you that it
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stored the initial challenge somewhere inside your home directory:
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---
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> Stored initial challenge and expected response in '/path/to/your/home/.yubico/challenge-KEYID'.
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---
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This step will create a file with a challenge and the expected response (that can only be generated with the secret key[1]) in your home directory. The PAM module will later open this file, read the challenge, send it to the connected YubiKey and check whether its answer matches the one on file. If it does, it generates a new challenge, asks the YubiKey for the correct response for this challenge and writes both into the file. This also means that you need to keep this file secure from other users (which is why we created the .yubico directory in your home with mode 0700).
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This step will create a file with a challenge and the expected
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response (that can only be generated with the secret key[1]) in your
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home directory. The PAM module will later open this file, read the
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challenge, send it to the connected YubiKey and check whether its
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answer matches the one on file. If it does, it generates a new
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challenge, asks the YubiKey for the correct response for this
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challenge and writes both into the file. This also means that you need
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to keep this file secure from other users (which is why we created the
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.yubico directory in your home with mode 0700).
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[1]: This is also the reason why you should avoid having copies of the key in other places than your YubiKey!
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[1]: This is also the reason why you should avoid having copies of the
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key in other places than your YubiKey!
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### Configuring your system to use Yubico PAM for authentication
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Linux, Solaris, OS X and most BSD variants use the [Pluggable Authentication Modules (PAM)](http://en.wikipedia.org/wiki/Pluggable_Authentication_Modules) framework to handle authentication. Using PAM you can specify which modules are used for authentication of users and which of them are required, optional and/or sufficient to authenticate a user. Using PAM you can for example set up multiple-factor authentication, by chaining multiple required modules.
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Configuring your system to use Yubico PAM for authentication
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------------------------------------------------------------
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PAM is configured through files in `/etc/pam.d` on most systems. Each file in this directory is used for a specific service, i.e. the file `/etc/pam.d/sudo` is used to authenticate users for the `sudo` program. Debian, for example, uses include directives in these files to have a central place to configure authentication; in this case we are not using this on purpose, because challenge-response authentication doesn't work remotely (e.g. via SSH), so we only want to configure it for services we use when on site.
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Linux, Solaris, OS X and most BSD variants use the [Pluggable
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Authentication Modules
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(PAM)](http://en.wikipedia.org/wiki/Pluggable_Authentication_Modules)
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framework to handle authentication. Using PAM you can specify which
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modules are used for authentication of users and which of them are
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required, optional and/or sufficient to authenticate a user. Using PAM
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you can for example set up multiple-factor authentication, by chaining
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multiple required modules.
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The file format in these files is documented in `man 5 pam.conf`; it basically looks like this:
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PAM is configured through files in `/etc/pam.d` on most systems. Each
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file in this directory is used for a specific service, i.e. the file
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`/etc/pam.d/sudo` is used to authenticate users for the `sudo`
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program. Debian, for example, uses include directives in these files
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to have a central place to configure authentication; in this case we
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are not using this on purpose, because challenge-response
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authentication doesn't work remotely (e.g. via SSH), so we only want
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to configure it for services we use when on site.
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The file format in these files is documented in `man 5 pam.conf`; it
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basically looks like this:
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function-class control-flag module-path arguments
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where
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* `function-class` is one of `auth`, `account`, `session`, and `password`. Since we only care about authentication with the YubiKey and yubico-pam only handles authentication, we will always be using `auth` here.
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* `control-flag` is one of `required`, `sufficient`, `optional` and some other values depending on your PAM implementation. If we want to make YubiKey challenge-response mandatory but combined with other methods (e.g. password), we can use `required`, if we want successful challenge-response to be enough to authenticate a user, we can use `sufficient`. `optional` is not really of any use for us in this case.
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* `module-path` selects the module to be used for this authentication step. This is used as filename in a directory where pam libraries are expected, on OS X e.g. `/usr/lib/pam`, `/usr/lib/security` on some other systems. We want `pam_yubico.so` in this case, which will load `/usr/lib/pam/pam_yubico.so`.
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* `arguments` are passed to the pam module and can be used to configure its behavior. See "Supported PAM module parameters" in [README](https://github.com/Yubico/yubico-pam/blob/master/README) for a list of possible values. Since we want to use challenge-response, we add `mode=challenge-response` and to debug the setup initially also `debug`, separated by spaces. `debug` can safely be removed later.
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* `function-class` is one of `auth`, `account`, `session`, and
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`password`. Since we only care about authentication with the YubiKey
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and yubico-pam only handles authentication, we will always be using
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`auth` here.
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**Warning**: If you misconfigure your PAM modules here you might lose your ability to sudo! Always keep a root shell open to be able to revert your changes in case something goes wrong!
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* `control-flag` is one of `required`, `sufficient`, `optional` and
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some other values depending on your PAM implementation. If we want
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to make YubiKey challenge-response mandatory but combined with other
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methods (e.g. password), we can use `required`, if we want
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successful challenge-response to be enough to authenticate a user,
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we can use `sufficient`. `optional` is not really of any use for us
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in this case.
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So, if we wanted to use the YubiKey to allow us to sudo without typing a password, we would add
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* `module-path` selects the module to be used for this authentication
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step. This is used as filename in a directory where pam libraries
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are expected, on OS X e.g. `/usr/lib/pam`, `/usr/lib/security` on
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some other systems. We want `pam_yubico.so` in this case, which will
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load `/usr/lib/pam/pam_yubico.so`.
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* `arguments` are passed to the pam module and can be used to
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configure its behavior. See "Supported PAM module parameters" in
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[README](https://github.com/Yubico/yubico-pam/blob/master/README)
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for a list of possible values. Since we want to use
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challenge-response, we add `mode=challenge-response` and to debug
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the setup initially also `debug`, separated by spaces. `debug` can
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safely be removed later.
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**Warning**: If you misconfigure your PAM modules here you might lose
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your ability to sudo! Always keep a root shell open to be able to
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revert your changes in case something goes wrong!
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So, if we wanted to use the YubiKey to allow us to sudo without typing
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a password, we would add
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---
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auth sufficient pam_yubico.so mode=challenge-response debug
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---
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To get this working on the loginwindow for local interactive login add the pam_yubico.so to the pam.d file authorization.
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as the first line. The whole file might look something like this (example taken from OS X):
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To get this working on the loginwindow for local interactive login add
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the pam_yubico.so to the pam.d file authorization as the first
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line. The whole file might look something like this (example taken
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from OS X):
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---
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# sudo: auth account password session
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auth sufficient pam_yubico.so mode=challenge-response debug
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auth required pam_opendirectory.so
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account required pam_permit.so
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password required pam_deny.so
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session required pam_permit.so
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---
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If we wanted to require successful challenge-response authentication in addition to the usual password, we can change the `sufficient` in the line we added to `required`.
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If we wanted to require successful challenge-response authentication
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in addition to the usual password, we can change the `sufficient` in
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the line we added to `required`.
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**Note**: In theory you can configure pretty much any service you use locally to use challenge-response authentication. In practice, I had problems configuring challenge-response into the login window of OS X. Keep a rescue disk or a remote root terminal available when attempting such configurations, just in case something goes wrong and you need to restore the PAM configuration to an old state.
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**Note**: In theory you can configure pretty much any service you use
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locally to use challenge-response authentication. In practice, I had
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problems configuring challenge-response into the login window of OS
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X. Keep a rescue disk or a remote root terminal available when
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attempting such configurations, just in case something goes wrong
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and you need to restore the PAM configuration to an old state.
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**Note #2**: On Debian it started working for me after accidentally getting the file-rights correctly. `755` for `~/.yubico` & `600` for the files therein. Otherwise the module can't find, read and/or write to the appropriate files. Your clue is the following debug messages.
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**Note #2**: On Debian it started working for me after accidentally
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getting the file-rights correctly. `755` for `~/.yubico` & `600` for
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the files therein. Otherwise the module can't find, read and/or
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write to the appropriate files. Your clue is the following debug
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messages.
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---
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[drop_privs.c:restore_privileges(128)] pam_modutil_drop_priv: -1
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[pam_yubico.c:do_challenge_response(542)] could not restore privileges
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[pam_yubico.c:do_challenge_response(664)] Challenge response failed: No such file or directory
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---
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