Cryptography library that secures most of the internet's TLS. Foundational security; the AGL/Cloudflare BoringSSL fork remains influential too.
From Wikipedia
OpenSSL is an open-source software library and command-line toolkit that implements the Transport Layer Security (TLS) protocol and a range of cryptographic functions. It is used by Internet servers, operating systems, application frameworks, programming-language runtimes, and embedded software to provide encrypted communications and certificate-based authentication.