libsodium is a portable repackaging of NaCl, a high-level crypto library.
RbNaCl is the Ruby wrapper:
rbnacl-libsodium is a gem which contains the complete libsodium sources and will compile them on installation much like other gems with native dependencies.
However, unlike other gems with native dependencies, RbNaCl is written with FFI and rbnacl-libsodium will install on JRuby and other non-CRuby Rubies!