PyCryptodome is a self-contained Python package of low-level cryptographic primitives. It supports Python 2.6 and 2.7, Python 3.4 and newer, and PyPy. It assumes that you know what you're doing, so this is not a safe-to-use module. The Cryptographic Best Practices website should be open at the same time.
libSodium is an implementation of DJB's crypto library NaCl which is actually portable - it'll run on many different operating systems (not just a few Unices), it supports parallel processing, and includes all of the conservatively safe crypto primitives that you've come to expect from the original. Optimization and benchmarks are performed at compile time for your platform for maximum portability. Includes bindings for several different languages that NaCl doesn't.
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