.. Copyright (C) Internet Systems Consortium, Inc. ("ISC")
..
.. SPDX-License-Identifier: MPL-2.0
..
.. This Source Code Form is subject to the terms of the Mozilla Public
.. License, v. 2.0.  If a copy of the MPL was not distributed with this
.. file, you can obtain one at https://mozilla.org/MPL/2.0/.
..
.. See the COPYRIGHT file distributed with this work for additional
.. information regarding copyright ownership.

.. _pkcs11:

PKCS#11 (Cryptoki) Support
--------------------------

Public Key Cryptography Standard #11 (PKCS#11) defines a
platform-independent API for the control of hardware security modules
(HSMs) and other cryptographic support devices.

PKCS#11 uses a "provider library": a dynamically loadable
library which provides a low-level PKCS#11 interface to drive the HSM
hardware. The PKCS#11 provider library comes from the HSM vendor, and it
is specific to the HSM to be controlled.

BIND 9 uses engine_pkcs11 for PKCS#11. engine_pkcs11 is an OpenSSL
engine which is part of the `OpenSC`_ project. The engine is dynamically
loaded into OpenSSL and the HSM is operated indirectly; any
cryptographic operations not supported by the HSM can be carried out by
OpenSSL instead.

.. _OpenSC: https://github.com/OpenSC/libp11

Prerequisites
~~~~~~~~~~~~~

See the documentation provided by the HSM vendor for information about
installing, initializing, testing, and troubleshooting the HSM.

Building SoftHSMv2
^^^^^^^^^^^^^^^^^^

SoftHSMv2, the latest development version of SoftHSM, is available from
https://github.com/opendnssec/SoftHSMv2. It is a software library
developed by the OpenDNSSEC project (https://www.opendnssec.org) which
provides a PKCS#11 interface to a virtual HSM, implemented in the form
of an SQLite3 database on the local filesystem. It provides less security
than a true HSM, but it allows users to experiment with native PKCS#11
when an HSM is not available. SoftHSMv2 can be configured to use either
OpenSSL or the Botan library to perform cryptographic functions, but
when using it for native PKCS#11 in BIND, OpenSSL is required.

By default, the SoftHSMv2 configuration file is ``prefix/etc/softhsm2.conf``
(where ``prefix`` is configured at compile time). This location can be
overridden by the SOFTHSM2_CONF environment variable. The SoftHSMv2
cryptographic store must be installed and initialized before using it
with BIND.

::

   $  cd SoftHSMv2
   $  configure --with-crypto-backend=openssl --prefix=/opt/pkcs11/usr
   $  make
   $  make install
   $  /opt/pkcs11/usr/bin/softhsm-util --init-token 0 --slot 0 --label softhsmv2

OpenSSL-based PKCS#11
~~~~~~~~~~~~~~~~~~~~~

OpenSSL-based PKCS#11 uses engine_pkcs11 OpenSSL engine from libp11 project.

engine_pkcs11 tries to fit the PKCS#11 API within the engine API of OpenSSL.
That is, it provides a gateway between PKCS#11 modules and the OpenSSL engine
API.  One has to register the engine with OpenSSL and one has to provide the
path to the PKCS#11 module which should be gatewayed to. This can be done by
editing the OpenSSL configuration file, by engine specific controls, or by using
the p11-kit proxy module.

It is recommended, that libp11 >= 0.4.12 is used.

For more detailed howto including the examples, we recommend reading:

https://gitlab.isc.org/isc-projects/bind9/-/wikis/BIND-9-PKCS11

Using the HSM
~~~~~~~~~~~~~

The canonical documentation for configuring engine_pkcs11 is in the
`libp11/README.md`_, but here's copy of working configuration for
your convenience:

.. _`libp11/README.md`: https://github.com/OpenSC/libp11/blob/master/README.md#pkcs-11-module-configuration

We are going to use our own custom copy of OpenSSL configuration, again it's
driven by an environment variable, this time called OPENSSL_CONF.  We are
going to copy the global OpenSSL configuration (often found in
``etc/ssl/openssl.conf``) and customize it to use engines_pkcs11.

::

   cp /etc/ssl/openssl.cnf /opt/bind9/etc/openssl.cnf

and export the environment variable:

::

   export OPENSSL_CONF=/opt/bind9/etc/openssl.cnf

Now add following line at the top of file, before any sections (in square
brackets) are defined:

::

   openssl_conf = openssl_init

And make sure there are no other 'openssl_conf = ...' lines in the file.

Add following lines at the bottom of the file:

::

   [openssl_init]
   engines=engine_section

   [engine_section]
   pkcs11 = pkcs11_section

   [pkcs11_section]
   engine_id = pkcs11
   dynamic_path = <PATHTO>/pkcs11.so
   MODULE_PATH = <FULL_PATH_TO_HSM_MODULE>
   init = 0

Key Generation
~~~~~~~~~~~~~~

HSM keys can now be created and used.  We are going to assume that you already
have a BIND 9 installed, either from a package, or from the sources, and the
tools are readily available in the ``$PATH``.

For generating the keys, we are going to use ``pkcs11-tool`` available from the
OpenSC suite.  On both DEB-based and RPM-based distributions, the package is
called opensc.

We need to generate at least two RSA keys:

::

   pkcs11-tool --module <FULL_PATH_TO_HSM_MODULE> -l -k --key-type rsa:2048 --label example.net-ksk --pin <PIN>
   pkcs11-tool --module <FULL_PATH_TO_HSM_MODULE> -l -k --key-type rsa:2048 --label example.net-zsk --pin <PIN>

Remember that each key should have unique label and we are going to use that
label to reference the private key.

Convert the RSA keys stored in the HSM into a format that BIND 9 understands.
The :iscman:`dnssec-keyfromlabel` tool from BIND 9 can link the raw keys stored in the
HSM with the ``K<zone>+<alg>+<id>`` files.  You'll need to provide the OpenSSL
engine name (``pkcs11``), the algorithm (``RSASHA256``) and the PKCS#11 label
that specify the token (we asume that it has been initialized as bind9), the
name of the PKCS#11 object (called label when generating the keys using
``pkcs11-tool``) and the HSM PIN.

Convert the KSK:

::

   dnssec-keyfromlabel -E pkcs11 -a RSASHA256 -l "token=bind9;object=example.net-ksk;pin-value=0000" -f KSK example.net

and ZSK:

::

   dnssec-keyfromlabel -E pkcs11 -a RSASHA256 -l "token=bind9;object=example.net-zsk;pin-value=0000" example.net

NOTE: you can use PIN stored on disk, by specifying ``pin-source=<path_to>/<file>``, f.e.:

::

   (umask 0700 && echo -n 0000 > /opt/bind9/etc/pin.txt)

and then use in the label specification:

::

   pin-source=/opt/bind9/etc/pin.txt

Confirm that you have one KSK and one ZSK present in the current directory:

::

   ls -l K*

The output should look like this (the second number will be different):

::

   Kexample.net.+008+31729.key
   Kexample.net.+008+31729.private
   Kexample.net.+008+42231.key
   Kexample.net.+008+42231.private

A note on generating ECDSA keys: there is a bug in libp11 when looking up a key,
that function compares keys only on their ID, not the label. So when looking up
a key it returns the first key, rather than the matching key. The workaround for
this is when creating ECDSA keys, you should specify a unique ID:

::

   ksk=$(echo "example.net-ksk" | sha1sum - | awk '{print $1}')
   zsk=$(echo "example.net-zsk" | sha1sum - | awk '{print $1}')
   pkcs11-tool --module <FULL_PATH_TO_HSM_MODULE> -l -k --key-type EC:prime256v1 --id $ksk --label example.net-ksk --pin <PIN>
   pkcs11-tool --module <FULL_PATH_TO_HSM_MODULE> -l -k --key-type EC:prime256v1 --id $zsk --label example.net-zsk --pin <PIN>


Specifying the Engine on the Command Line
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When using OpenSSL-based PKCS#11, the "engine" to be used by OpenSSL can be
specified in :iscman:`named` and all of the BIND ``dnssec-*`` tools by using the ``-E
<engine>`` command line option. Specifying the engine is generally not necessary
unless a different OpenSSL engine is used.

The zone signing commences as usual, with only one small difference.  We need to
provide the name of the OpenSSL engine using the -E command line option.

::

   dnssec-signzone -E pkcs11 -S -o example.net example.net

Running :iscman:`named` With Automatic Zone Re-signing
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The zone can also be signed automatically by named. Again, we need to provide
the name of the OpenSSL engine using the :option:`-E <named -E>` command line option.

::

   named -E pkcs11 -c named.conf

and the logs should have lines like:

::

   Fetching example.net/RSASHA256/31729 (KSK) from key repository.
   DNSKEY example.net/RSASHA256/31729 (KSK) is now published
   DNSKEY example.net/RSA256SHA256/31729 (KSK) is now active
   Fetching example.net/RSASHA256/42231 (ZSK) from key repository.
   DNSKEY example.net/RSASHA256/42231 (ZSK) is now published
   DNSKEY example.net/RSA256SHA256/42231 (ZSK) is now active

For :iscman:`named` to dynamically re-sign zones using HSM keys,
and/or to sign new records inserted via nsupdate, :iscman:`named` must
have access to the HSM PIN. In OpenSSL-based PKCS#11, this is
accomplished by placing the PIN into the ``openssl.cnf`` file (in the above
examples, ``/opt/pkcs11/usr/ssl/openssl.cnf``).

The location of the openssl.cnf file can be overridden by setting the
``OPENSSL_CONF`` environment variable before running :iscman:`named`.

Here is a sample ``openssl.cnf``:

::

       openssl_conf = openssl_def
       [ openssl_def ]
       engines = engine_section
       [ engine_section ]
       pkcs11 = pkcs11_section
       [ pkcs11_section ]
       PIN = <PLACE PIN HERE>

This also allows the ``dnssec-\*`` tools to access the HSM without PIN
entry. (The ``pkcs11-\*`` tools access the HSM directly, not via OpenSSL, so
a PIN is still required to use them.)
