PKCS#11 wrapper library
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Latest commit 6f9baf4 Feb 14, 2017 @mtrojnar mtrojnar committed on GitHub Merge pull request #147 from Sp1l/master
Fix build issues with LibreSSL

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This code repository produces two libraries:

  • libp11 provides a higher-level (compared to the PKCS#11 library) interface to access PKCS#11 objects. It is designed to integrate with applications that use OpenSSL.
  • pkcs11 engine plugin for the OpenSSL library allows accessing PKCS#11 modules in a semi-transparent way.

The wiki page for this project is at and includes a bug tracker and source browser.


The PKCS#11 API is an abstract API to access operations on cryptographic objects such as private keys, without requiring access to the objects themselves. That is,i t provides a logical separation of the keys from the operations. The PKCS #11 API is mainly used to access objects in smart cards and Hardware or Software Security Modules (HSMs). That is because in these modules the cryptographic keys are isolated in hardware or software and are not made available to the applications using them.

PKCS#11 API is an OASIS standard and it is supported by various hardware and software vendors. Usually, hardware vendors provide a PKCS#11 module to access their devices. A prominent example is the OpenSC PKCS #11 module which provides access to a variety of smart cards. Other libraries like NSS or GnuTLS already take advantage of PKCS #11 to access cryptographic objects.

OpenSSL engines

OpenSSL implements various cipher, digest, and signing features and it can consume and produce keys. However plenty of people think that these features should be implemented in a separate hardware, like USB tokens, smart cards or hardware security modules. Therefore OpenSSL has an abstraction layer called engine which can delegate some of these features to different piece of software or hardware.

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 into the OpenSSL and one has to provide path to a PKCS#11 module which should be gatewayed to. This can be done by editing the OpenSSL configuration file (not recommended), by engine specific controls, or by using the p11-kit proxy module.

The p11-kit proxy module provides access to any configured PKCS #11 module in the system. See the p11-kit web pages for more information.

PKCS #11 module configuration

Copying the engine shared object to the proper location

OpenSSL has a location where engine shared objects can be placed and they will be automatically loaded when requested. It is recommended to copy engine_pkcs11 at that location as to ease usage. This is handle by 'make install' of engine_pkcs11.

Using in systems with p11-kit

In systems with p11-kit-proxy engine_pkcs11 has access to all the configured PKCS #11 modules and requires no further configuration.

Using in systems without p11-kit

In systems without p11-kit-proxy you need to configure OpenSSL to know about the engine and to use OpenSC PKCS#11 module by the engine_pkcs11. For that you add something like the following into your global OpenSSL configuration file (often in /etc/ssl/openssl.cnf).

pkcs11 = pkcs11_section

engine_id = pkcs11
dynamic_path =
MODULE_PATH = /usr/lib/
init = 0

The dynamic_path value is the engine_pkcs11 plug-in, the MODULE_PATH value is the OpenSC PKCS#11 plug-in. The engine_id value is an arbitrary identifier for OpenSSL applications to select the engine by the identifier. In systems with p11-kit-proxy installed and configured, you do not need to modify the OpenSSL configuration file; the configuration of p11-kit will be used.

Testing the engine operation

To verify that the engine is properly operating you can use the following example.

$ openssl engine pkcs11 -t
(pkcs11) pkcs11 engine
     [ available ]

Using the engine from the command line tool

This section demonstrates how to use the command line tool to create a self signed certificate for "Andreas Jellinghaus". The key of the certificate will be generated in the token and will not exportable.

For the examples that follow, we need to generate a private key in the token and obtain its private key URL. The following commands utilize p11tool for that.

$ p11tool --provider /usr/lib/ --login --generate-rsa --bits 1024 --label test-key
$ p11tool --provider /usr/lib/ --list-privkeys --login

Note the PKCS #11 URL shown above and use it in the commands below.

To generate a certificate with its key in the PKCS #11 module, the following commands commands can be used. The first command creates a self signed Certificate for "Andreas Jellinghaus". The signing is done using the key specified by the URL. The second command creates a self-signed certificate for the request, the private key used to sign the certificate is the same private key used to create the request. Note that in a PKCS #11 URL you can specify the PIN using the "pin-value" attribute.

$ openssl
OpenSSL> req -engine pkcs11 -new -key "pkcs11:object=test-key;type=private;pin-value=XXXX" \
         -keyform engine -out req.pem -text -x509 -subj "/CN=Andreas Jellinghaus"
OpenSSL> x509 -engine pkcs11 -signkey "pkcs11:object=test-key;type=private;pin-value=XXXX" \
         -keyform engine -in req.pem -out cert.pem

For the above commands to operate in systems without p11-kit you will need to provide the engine configuration explicitly. The following line loads engine_pkcs11 with the PKCS#11 module

OpenSSL> engine -t dynamic -pre SO_PATH:/usr/lib/engines/ \
         -pre ID:pkcs11 -pre LIST_ADD:1 -pre LOAD \
         -pre MODULE_PATH:/usr/lib/

Engine controls

The supported engine controls are the following.

  • SO_PATH: Specifies the path to the 'pkcs11-engine' shared library
  • MODULE_PATH: Specifies the path to the pkcs11 module shared library
  • PIN: Specifies the pin code
  • VERBOSE: Print additional details
  • QUIET: Do not print additional details
  • LOAD_CERT_CTRL: Load a certificate from token

An example code snippet setting specific module is shown below.

ENGINE_ctrl_cmd(engine, "MODULE_PATH",
        0, "/path/to/", NULL, 1);

In systems with p11-kit, if this engine control is not called engine_pkcs11 defaults to loading the p11-kit proxy module.

Developer information

Thread safety in libp11

Thread-safety requires dynamic callbacks to be registered by the calling application with the following OpenSSL functions:

  • CRYPTO_set_dynlock_create_callback
  • CRYPTO_set_dynlock_destroy_callback
  • CRYPTO_set_dynlock_lock_callback

Submitting pull requests

For adding new features or extending functionality in addition to the code, please also submit a test program which verifies the correctness of operation. See tests/ for the existing test suite.