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.
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.
As of this writing, the implementation supports only the RSA, SHA-1,
and SHA-2 algorithms, but the design is intended to be extensible.
Underlying cryptographic support is via Cryptlib, which may need
to change (more on this below).
The object store is currently implemented using SQLite3, which may
also need to change (more on this below too).
Testing to date has been done using the bin/pkcs11/ tools from the
BIND9 distribution and the hsmcheck tool from the OpenDNSSEC
distribution. 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, and generates what DNSPython thinks are
good signatures. Much more testing will be required, by a much wider
variety of test tools.
Two critical design choices in this software were made with full
knowledge that we might need to change them later:
-
The use of Cryptlib as a provider for the underlying cryptographic
operations, and
-
The use of SQLite3 as the object store.
Cryptlib is a very nice piece of software, but it's not really
designed for use under something like PKCS11. It seemed worth a
try anyway, because it would have been nice to be able to use
Cryptlib's RPC mechanism, take advantage of Cryptlib's rule-based data
protection system, and so forth, but implementing PKCS #11 requires
doing various things which Cryptlib quite correctly discourages. So,
not a perfect fit.
The current code works with Cryptlib's software RSA implementation,
primarily due to an oddity of RSA: once one has handled the PKCS #1.5
padding, the RSA signature and decryption (sic) operations are
mathematically identical. Fine so far as this goes, but:
-
This probably does not hold for other signature algorithms (well,
the math certainly does not, I haven't yet investigated what the
Cryptlib API does if one attempts to "decrypt" using an ECDSA key);
and
-
The Cryptlib manual says that there are some extra protections
around keys stored in hardware devices that would forbid using this
trick with an FPGA implementation of RSA.
The latter (extra protections) is probably something we could work
around if necessary, but the former may make this a moot point.
All of the above notwithstanding, Cryptlib was a reasonable choice for
the initial implementation, as we had no FPGA RSA to work with and
needed to develop with something. Surprisingly little effort has
gone into Cryptlib-specific code (probably less than would have been
required with, eg, OpenSSL, because the Cryptlib API is cleaner).
Bottom line: we haven't lost anything by this approach, we're just not
done yet.
There are a few other issues with using Cryptlib in this context which
I will detail if they become relevant, but I'll skip them for now
since I don't think they'll end up being relevant here.
The choice of SQLite3 for the data store was made 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: it fails the minimum complexity test,
and at least in the current implementation requires two separate kinds
of storage, one for keys (currently a PKCS #15 keyring) and one for
attributes (the SQLite3 database).
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.
