TL;DR: The S63 encryption scheme is old and can be broken pretty easily depending on the information you have. These are some notes on how with a simple Python script to show what I mean.
So this is an old standard that will soon be replaced by S100, but it’s still in place and used today. The encryption part of the scheme is mostly to allow data suppliers to license their data to users in a standardized way and collected payment for access to that data.
TL;DR: S-63 is a standard used to encrypt official Electronic Navigational Charts (ENCs). Encryption and signing allow clients to be sure that they have official data and that it has not been tampered with. It also make it possible for data providers to license sections of the charts (cells) and be sure that they are not being shared illegally / pirated. This post gives an overview of the process and shows how to implement the key steps.