Provably secure cryptography
This is a necessary property. A protocol without a proof of security is likely to be insecure and must not be trusted.
Software of excellent quality
Thoroughly tested code developed by experts. Minimal number of dependencies to potentially insecure software. Polished using multiple static analysis and test coverage tools. Zero reported vulnerabilities since 2010.
Easy to deploy and use
Portable on Linux and BSD, easy to install, and robust command line interface.
Comprehensive documentation
Mature research papers, whitepapers, overview descriptions, APIs, usage info, and documented code.
Free and open source MIT License
The MIT License is the most liberal among the widely adopted free and open source licenses .