Network Security Services (NSS) is a set of libraries designed to support cross-platform development of security-enabled client and server applications. NSS supports SSL v3-TLS 1.2 (experimental TLS 1.3), PKCS #5, PKCS #7, PKCS #11, PKCS #12, S/MIME, X.509 v3 certificates, and other security standards.
In order to get started create a new directory on that you will be uses as your local work area, and check out NSS and NSPR. (Note that there's no git mirror of NSPR and you require mercurial to get the latest NSPR source.)
git clone https://github.com/nss-dev/nss.git
hg clone https://hg.mozilla.org/projects/nspr