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<?xml version="1.0"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
<chapter xml:id="sharing">
<title>Sharing PKCS#11 modules</title>
<section xml:id="sharing-problem">
<title>Multiple consumers of PKCS#11 in a process</title>
<para>As more and more applications and libraries use PKCS#11 we run
into a very basic problem. The PKCS#11 modules cannot be initialized and
finalized properly without coordination between the various consumers.
</para>
<para>An example: An application might use GnuTLS for
TLS connections, and use libgcr for display of certificates. Both of
these want to load (and initialze) the same PKCS#11 modules. There are
many places where this situation occurs, including large applications
like Evolution which due to their dependencies end up using both NSS and
GnuTLS.</para>
<para>Consumer A loads a PKCS#11 module and uses the module's
C_Initialize function to initialize it, which works as expected.
When consumer B initializes the module (also using C_Initialize),
the error code <literal>CKR_CRYPTOKI_ALREADY_INITIALIZED</literal>
is correctly returned. This is normal PKCS#11 specification
defined behavior for when a module is initalized twice in the
same process. If consumer B is aware of this situation they may
choose to ignore this error code.</para>
<para>However when the consumer A is done with its use of the
PKCS#11 module it finalizes the module using the module's
C_Finalize function. This is expected of a well behaved PKCS#11
consumer. This then causes errors and/or crashes for consumer B,
which cannot know that the module has now been finalized out
from underneath it.</para>
<para>It is necessary for the two consumers to coordinate their
initialization and finalization in some fashion. In
<literal>p11-kit</literal> we provide this coordination in a
loosely coupled, backwards compatible, and flexible way.</para>
</section>
<section xml:id="sharing-initialize">
<title>Solution: p11-kit</title>
<para><literal>p11-kit</literal> provides functions to
coordinate initialization and finalization of any PKCS#11
module. A module may be initialized any number of times using
the p11_kit_initialize_module() function. The first time that
p11_kit_initialize_module() is called for a module, that module's
C_Initialize function is used. Later invocations for the same
module cause p11-kit to increment an internal initialization
count, rather than calling C_Initialize again.</para>
<para>The p11_kit_finalize_module() is used to finalize a module.
Each time it is called it decrements the internal initialization
count for that module. When the internal initialization count
reaches zero, the module's C_Finalize function is called.</para>
<para>This is done in a thread-safe manner. These functions can
be used on modules that the consumer loads themselves.</para>
</section>
<section xml:id="sharing-module">
<title>Solution: proxy module</title>
<para>When an application is aware of the fact that coordination
is necessary between multiple consumers of a PKCS#11 module, it
can link to p11-kit and use the functions there to provide
this coordination.</para>
<para>However most current consumers of PKCS#11 are ignorant of
this problem, and do not link to p11-kit. In order to solve this
multiple initialization problem for all applications,
<literal>p11-kit</literal> provides a proxy compatibility
module.</para>
<para>This proxy module acts like a normal PKCS#11 module, but
internally loads a preconfigured set of PKCS#11 modules and
coordinates their initialization and finalization. Each slot
in the configured modules is exposed as a slot of the
<literal>p11-kit</literal> proxy module. The proxy module is
then used as a normal PKCS#11 module would be. It can be loaded by
crypto libraries like NSS and behaves as expected.</para>
<para>The proxy module bends the PKCS#11 rules slightly. It does
not return the <literal>CKR_CRYPTOKI_ALREADY_INITIALIZED</literal>
error code as specified in PKCS#11. However this is a small
price to pay for this compatibility.</para>
</section>
</chapter>
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