Botan is a BSD-licensed crypto library for C++. It provides
applications with the ability to use a number of cryptographic
algorithms, as well as SSL/TLS, X.509 certificates and CRLs, PKCS
#10 certificate requests, a filter/pipe message processing system,
and a wide variety of other features.

Optional Dependency

In case you'd like proper documentation to be build Sphinx needs to be
installed at build time.