SHELF = "Super Hard" ELF.

https:// www.hep.wisc.edu/~prepost/ELF.pdf

https:// fas.org/nuke/guide/usa/c3i/fs_clam_lake_elf2003.pdf

https:// www.naval-technology.com/features/featuredeep-secret-secure-submarine-communication-on-a-quantum-level/