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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.

BIND 9 is known to work with three HSMs: the AEP Keyper, which has been tested with Debian Linux, Solaris x86, and Windows Server 2003; the Thales nShield, tested with Debian Linux; and the Sun SCA 6000 cryptographic acceleration board, tested with Solaris x86. In addition, BIND can be used with all current versions of SoftHSM, a software-based HSM simulator library produced by the OpenDNSSEC project.

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.

There are two available mechanisms for PKCS#11 support in BIND 9: OpenSSL-based PKCS#11 and native PKCS#11. With OpenSSL-based PKCS#11, BIND uses a modified version of OpenSSL, which loads the provider library and operates the HSM indirectly; any cryptographic operations not supported by the HSM can be carried out by OpenSSL instead. Native PKCS#11 enables BIND to bypass OpenSSL completely; BIND loads the provider library itself, and uses the PKCS#11 API to drive the HSM directly.

Prerequisites

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

Native PKCS#11

Native PKCS#11 mode only works with an HSM capable of carrying out every cryptographic operation BIND 9 may need. The HSM's provider library must have a complete implementation of the PKCS#11 API, so that all these functions are accessible. As of this writing, only the Thales nShield HSM and SoftHSMv2 can be used in this fashion. For other HSMs, including the AEP Keyper, Sun SCA 6000, and older versions of SoftHSM, use OpenSSL-based PKCS#11. (Note: Eventually, when more HSMs become capable of supporting native PKCS#11, it is expected that OpenSSL-based PKCS#11 will be deprecated.)

To build BIND with native PKCS#11, configure it as follows:

$ cd bind9
$ ./configure --enable-native-pkcs11 \
    --with-pkcs11=provider-library-path

This causes all BIND tools, including named and the dnssec-* and pkcs11-* tools, to use the PKCS#11 provider library specified in provider-library-path for cryptography. (The provider library path can be overridden using the -E argument in named and the dnssec-* tools, or the -m argument in the pkcs11-* tools.)

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 (http://www.opendnssec.org) which provides a PKCS#11 interface to a virtual HSM, implemented in the form of a 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.

For more information, see https://gitlab.isc.org/isc-projects/bind9/-/wikis/BIND-9-PKCS11

PKCS#11 Tools

BIND 9 includes a minimal set of tools to operate the HSM, including pkcs11-keygen to generate a new key pair within the HSM, pkcs11-list to list objects currently available, pkcs11-destroy to remove objects, and pkcs11-tokens to list available tokens.

In UNIX/Linux builds, these tools are built only if BIND 9 is configured with the --with-pkcs11 option. (Note: If --with-pkcs11 is set to yes, rather than to the path of the PKCS#11 provider, the tools are built but the provider is left undefined. Use the -m option or the PKCS11_PROVIDER environment variable to specify the path to the provider.)

Using the HSM

For OpenSSL-based PKCS#11, the runtime environment must first be set up so the OpenSSL and PKCS#11 libraries can be loaded:

$ export LD_LIBRARY_PATH=/opt/pkcs11/usr/lib:${LD_LIBRARY_PATH}

This causes named and other binaries to load the OpenSSL library from /opt/pkcs11/usr/lib, rather than from the default location. This step is not necessary when using native PKCS#11.

Some HSMs require other environment variables to be set. For example, when operating an AEP Keyper, the location of the "machine" file, which stores information about the Keyper for use by the provider library, must be specified. If the machine file is in /opt/Keyper/PKCS11Provider/machine, use:

$ export KEYPER_LIBRARY_PATH=/opt/Keyper/PKCS11Provider

Such environment variables must be set when running any tool that uses the HSM, including pkcs11-keygen, pkcs11-list, pkcs11-destroy, dnssec-keyfromlabel, dnssec-signzone, dnssec-keygen, and named.

HSM keys can now be created and used. In this case, we will create a 2048-bit key and give it the label "sample-ksk":

$ pkcs11-keygen -b 2048 -l sample-ksk

To confirm that the key exists:

$ pkcs11-list
Enter PIN:
object[0]: handle 2147483658 class 3 label[8] 'sample-ksk' id[0]
object[1]: handle 2147483657 class 2 label[8] 'sample-ksk' id[0]

Before using this key to sign a zone, we must create a pair of BIND 9 key files. The dnssec-keyfromlabel utility does this. In this case, we are using the HSM key "sample-ksk" as the key-signing key for "example.net":

$ dnssec-keyfromlabel -l sample-ksk -f KSK example.net

The resulting K*.key and K*.private files can now be used to sign the zone. Unlike normal K* files, which contain both public and private key data, these files contain only the public key data, plus an identifier for the private key which remains stored within the HSM. Signing with the private key takes place inside the HSM.

To generate a second key in the HSM for use as a zone-signing key, follow the same procedure above, using a different keylabel, a smaller key size, and omitting -f KSK from the dnssec-keyfromlabel arguments:

$ pkcs11-keygen -b 1024 -l sample-zsk
$ dnssec-keyfromlabel -l sample-zsk example.net

Alternatively, a conventional on-disk key can be generated using dnssec-keygen:

$ dnssec-keygen example.net

This provides less security than an HSM key, but since HSMs can be slow or cumbersome to use for security reasons, it may be more efficient to reserve HSM keys for use in the less frequent key-signing operation. The zone-signing key can be rolled more frequently, if desired, to compensate for a reduction in key security. (Note: When using native PKCS#11, there is no speed advantage to using on-disk keys, as cryptographic operations are done by the HSM.)

Now the zone can be signed. Please note that, if the -S option is not used for dnssec-signzone, the contents of both K*.key files must be added to the zone master file before signing it.

$ dnssec-signzone -S example.net
Enter PIN:
Verifying the zone using the following algorithms:
NSEC3RSASHA1.
Zone signing complete:
Algorithm: NSEC3RSASHA1: ZSKs: 1, KSKs: 1 active, 0 revoked, 0 stand-by
example.net.signed

Specifying the Engine on the Command Line

When using OpenSSL-based PKCS#11, the "engine" to be used by OpenSSL can be specified in named and all of the BIND dnssec-* tools by using the -E <engine> command line option. If BIND 9 is built with the --with-pkcs11 option, this option defaults to "pkcs11". Specifying the engine is generally not necessary unless a different OpenSSL engine is used.

To disable use of the "pkcs11" engine - for troubleshooting purposes, or because the HSM is unavailable - set the engine to the empty string. For example:

$ dnssec-signzone -E '' -S example.net

This causes dnssec-signzone to run as if it were compiled without the --with-pkcs11 option.

When built with native PKCS#11 mode, the "engine" option has a different meaning: it specifies the path to the PKCS#11 provider library. This may be useful when testing a new provider library.

Running named With Automatic Zone Re-signing

For named to dynamically re-sign zones using HSM keys, and/or to sign new records inserted via nsupdate, 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 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.)

In native PKCS#11 mode, the PIN can be provided in a file specified as an attribute of the key's label. For example, if a key had the label pkcs11:object=local-zsk;pin-source=/etc/hsmpin, then the PIN would be read from the file /etc/hsmpin.

Warning

Placing the HSM's PIN in a text file in this manner may reduce the security advantage of using an HSM. Use caution when configuring the system in this way.