Secure Client Portal

I am working on building a website for a law firm with the client requesting that the website contain a secure portal for clients to access confidential information through the website.  I am trying to figure out how I can accomplish what the client desires through a Commerce website set up. I was initially thinking that this could be accomplished with customer accounts or perhaps the members area, but I am not sure if these would allow for secure communications which would provide the level of confidentiality the client is requesting.  So I have three questions for anyone who has mastered this challenge:

1. Do you have a code that would allow a page on the website to function as a secure client portal?

2. Do you know of a third party integration that can be integrated in order to function as a secure client portal?

3. Do you know a solution working within Squarespace which could accomplish the function of a secure client portal?

Hello does anybody know a solution for the secure client portal with a third-party integration?