PKCS #11


This is an implementation of the PKCS11 API for the Cryptech project. Like most PKCS #11 implementations, this one is incomplete and probably always will be: PKCS #11 is very open-ended, and the specification includes enough rope for an unwary developer to hang not only himself, but all of his friends, relations, and casual acquaintances.

Along with the PKCS #11 library itself, the package includes a companion Python interface ("py11"), which uses the ctypes module from the Python standard library to talk to the PKCS #11 implementation. The Python implementation is intended primarily to simplify testing the C code.

Novel design features

PKCS11's data model involves an n-level-deep hierarchy of object classes, which is somewhat tedious to implement correctly in C, particularly if one wants the correspondence between specification and code to be at all obvious. In order to automate much of the drudge work involved, this implementation uses an external representation of the object class hierarchy, which is processed at compile time by a Python script to generate tables which drive the C code which performs the necessary type checking.

Current status

As of this writing, the implementation supports only the RSA, ECDSA, SHA-1, and SHA-2 algorithms, but the design is intended to be extensible.

The underlying cryptographic support comes from the Cryptech libhal package.

The object store is currently implemented using SQLite3, which may also need to change (more on this below).

Testing to date has been done using the bin/pkcs11/ tools from the BIND9 distribution, the hsmcheck and ods-hsmutil tools from the OpenDNSSEC distribution, the hsmbully diagnostic tool, and a preliminary set of unit tests using Python's unittest library. Beyond the test results (such as they are) reported by these tools, the primary test of whether the PKCS #11 code is working as expected has been validation of the signed DNSSEC data generated by hsmcheck -s, via a script using DNSPython.

In a nutshell, the current state is that the code runs without throwing any obvious errors, generates what DNSPython thinks are good signatures, and passes some fairly basic tests. More testing would be a really good idea.

Open issue: SQLite3

The choice to use use of SQLite3 as the PKCS #11 object store was made with full knowledge that we might need to change it later. That said, we made the initial choice with several factors in mind:

  • Relative ease of development (it's all just SQL schemas and queries);

  • Relative ease of data normalization (foreign key constraints, etcetera) and debugging (command line tool available for arbitrary direct queries against stored data);

  • Licensing (SQLite3 is explictly public domain);

  • Support for embedded systems; and

  • Surprisingly small object code size (everything I found that was significantly smaller had license issues, eg, gdbm).

Overall, this has worked relatively well, but it's not necessarily what we want in the long run, if only because it fails the minimum complexity test.

The current implementation keeps much of the SQL data in an in-memory database: only "token objects" are stored in on disk. This matches the required PKCS #11 semantics, and using the same mechanism to handle both session objects and token objects simplifies the code considerably, but it does mean that much of the SQL code is really just dealing with a weird encoding of in-memory data structures.

At this point the schema may be stable enough that it would make sense to consider reimplementing without SQL. It's not urgent as long as we're just doing proof-of-concept work, but is something we should consider seriously before deciding that this is ready for "production" status.

The PKCS11 header files are "derived from the RSA Security Inc. PKCS #11 Cryptographic Token Interface (Cryptoki)". See the pkcs11*.h header files for details.

Code written for the Cryptech project is under the usual Cryptech BSD-style license.