Copyright (C) 2021, Axis Communications AB, Lund, Sweden. All Rights Reserved.
Signing using a private key is usually done in a secure part of a device. This usually requires device specific operations which cannot be generalized to an open source project. Therefore, there is a need to support signing through a concept of plugins.
The Signed Video Framework comes with implementations of two plugins; unthreaded-signing/plugin.c and threaded-signing/plugin.c. Both of them use OpenSSL APIs to generate a signature.
It is safe to use any of these plugins in a multi-threaded integration.
The unthreaded plugin blocks further operations until the signature has been generated. This is the default plugin. The check tests only work with this plugin. Further, the validation does not need a signing plugin, hence should preferably be built with the unthreaded one.
The threaded plugin calls the OpenSSL signing APIs from a separate thread. If the plugin is
initialized using sv_signing_plugin_init_new(user_data) a central thread is spawned. The
user_data is a pem_pkey_t struct (See
signed_video_openssl.h) and should include the signing key
to use. If the user runs multiple sessions they share the same input and output buffers.
If the plugin is not initialized signing is done from a local thread in each session. This can cause
quite some threads if multiple streams are signed. The implementation requires glib-2.0.
Through the meson option signingplugin, one of them can be selected and the source file is added
to the library sources. There is one extra option (threaded_unless_check_dep) which can be set if
the signing side should be built with a threaded plugin unless libcheck exists. The unthreaded
plugin is the library default.
It is feasible to build your own signing plugin. The only requirement is to implement the interfaces declared in signed_video_signing_plugin.h. For an example, see unthreaded-signing/plugin.c or threaded-signing/plugin.c.
The Signed Video Framework does currently not support arbitrary plugins, hence there are no means to automatically run the framework with your own signing plugin.