We sign our official plugins yet this is not well-advertised and not at
all consumed during plugin installation. For plugins that are installed
over the intertubes, verifying that the downloaded artifact is signed by
our signing key would establish both integrity and validity of the
downloaded artifact. The chain of trust here is simple: our installable
artifacts (archive and package distributions) so that if a user trusts
our packages via their signatures, and our plugin installer (which would
be executing trusted code) verifies the downloaded plugin, then the user
can trust the downloaded plugin too. This commit adds verification of
official plugins downloaded during installation. We do not add
verification for offline plugin installs; a user can download our
signatures and verify the artifacts themselves.
This commit also needs to solve a few interesting challenges. One of
these is that we want the bouncy castle JARs on the classpath only for
the plugin installer, but not for the runtime
Elasticsearch. Additionally, we want these JARs to not be present for
the JAR hell checks. To address this, we shift these JARs into a
sub-directory of lib (lib/tools/plugin-cli) that is only loaded for the
plugin installer, and in the plugin installer we filter any JARs in this
directory from the JAR hell check.
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