Ask HN: How to communicate anonymously

What is the proper way for two parties, say A and B, to communicate securely where: - Communication between A and B must be anonymous. - A isn't necessarily endangered if it is known that they communicate anonymously with 3rd parties. - Knowledge that B is communicating anonymously with a 3rd party would endanger B. More specifically, lawyers needs to communicate with a client, the client cannot be known to be in communication with the lawyers.