Every web site these days should support https:// on every page in order to be sufficiently secure and to provide privacy to its users. I'm writing this because I know how difficult it can be to convert an existing site to HTTPS but I still want everyone to do it. It will benefit everybody because your users will be more secure and your site will have a better reputation.
I won't try to convince you too much as to why you should use HTTPS on all your pages (Google has already done a great job: HTTPS Everywhere) but from my own testing I know just how easy it is for people to Man-in-the-Middle attack your users if you don't use HTTPS, or if you use it improperly. Encrypting passwords isn't enough if your cookies aren't secure and your cookies can't be secure if you're not using https everywhere. We don't want our faithful users to have their sessions hijacked while they're browsing our site at a coffee shop.
What I would like to talk about are the lessons that I learned from setting this up. I'll start by saying that you should not procrastinate on doing this. It is much easier to set up HTTPS everywhere if you start early but it is possible to do it on any website -- it will just take more work.
I recommend reading this entire article once through first, before trying any of it out. Use this information at your own risk since it is just from my experience and your mileage may vary.
The first thing you'll need is an SSL certificate. Find out if you will need a wildcard cert and so forth. Your site without a subdomain is called a "naked domain" (e.g. example.com). You may need to ensure your certificates include subdomains to avoid site visitors getting certificate errors if they go to the www. address for your site - or maybe you handle this by redirecting www to the naked domain in your web server config. And while not necessary, you should also decide if you want to use Extended Validation (EV) certificates. Anyway, do your homework before purchasing your certificate.
Once you have your certificates figured out, you start by making every page redirect from http to https. This is usually as simple as adding the appropriate redirect directives to wherever you terminate SSL (NGINX, Apache, etc). In NGINX this is usually as simple as adding return 301 https://$domain$request_uri; to your HTTP server block; then in your HTTPS server block you'll add directives to enable ssl, listen on port 443, link to your certificate and key and add any needed headers. Refer to your web server's documentation.
It is a myth that having https on every page is going to be too slow. So please do not try to only add https to a subset of pages, such as just the home page or login page. Doing so will only add complexity to your configurations without making your site secure enough.
In your web server config, you will also specify the protocols to support. You should disable support for SSL v2 and v3. With NGINX, just specify the protocols you want, and leave out what you don't want, otherwise it defaults to SSSv3. So do something like ssl_protocols TLSv1 TLSv1.1 TLSv1.2; This may seem confusing that we're using SSL and TLS interchangeably. Just note that you'll want to configure support for TLS encryption, not SSL. It helps to think about HTTPS as being either SSL or TLS - with TLS currently being the preferred encryption.
We're about half-way there. Unfortunately, simply having every web page be HTTPS isn't enough. You also need to have all your important resources that load on those pages (images and scripts) be https or you aren't really secure yet. Plus if you don't do this, you will see a lot of Mixed-Content warnings in the browser in the console and a warning on the lock icon. So having https:// in the address bar isn't enough. Don't try to cheat because you will not fool attackers nor your users.
If your site has hundreds of thousands of users, then running such a migration script could take hours or even days. So you will want to run a migration script first if needed. Once your migration finishes, you will also need to refactor your code to start saving your URLs as https so you don't run into the same problem again. Depending on when you deploy, you may need to run your migration script once more to catch up.
DO NOT skip this step: To prevent session hijacking, you'll want to enable the Secure and HttpOnly attributes on your session cookies. These are security flags that you add to your cookies by appending 'Secure; HttpOnly' to the cookie HTTP response header. The Secure flag makes the cookie only accessible over HTTPS so users can't unknowingly "leak" their cookies by typing http:// to get to your site before it redirects to https://. The HttpOnly flag makes cookies invisible to client-side scripts so cookies can't be stolen by XSS exploits.
To test that your cookies have the Secure and HttpOnly flags, simply check the Set-cookie value in the response from curl'ing your website. In a browser such as Google Chrome, you can also go into the developer tools and inspect your cookies and you should see checkmarks under the Secure and HttpOnly columns.
Your development sandboxes should use https:// since it's best that development machines closely match production. This will help prevent developers from hard-coding http:// url's but not realize it until there are https warnings on production. This means that your developer machines will need to be running NGINX or whatever web server you use in production. Creating a self-signed certificate will make this easy enough. There are additional benefits to this, such as using a hostname like https://yoursite.dev instead of http://localhost:3000.
Lastly, make sure that any middleware and front-end code that does redirects, does so as https. You can test this on the command-line by running:
$ curl -I -L https://yoursite/whatever
and checking if any Location: lines show http:// URLs. Assuming that /whatever is a controller that does redirects, you want to be sure that it doesn't do any redirecting back to http:// at any request. I've had to ensure https:// redirects at the WSGI-middleware and at the front-end controller code layers.
To further test that you set everything up correctly, run your domain through a scanner such as https://www.ssllabs.com/ssltest/. It will give you different grades and your goal is to get an 'A'. For a command-line tool, check out sslscan. Additionally you can check that your site uses strong certificates by using https://shaaaaaaaaaaaaa.com/. If you get bad scores, make sure you're using SHA-2 certificates and 256-bit TLS ciphers.
Further considerations are making sure you disable support for weak ciphers, using at least SHA-2 (even on your intermediate certs) and looking into SSLStrip and HSTS headers. Also, look into Perfect Forward Secrecy (PFS) so attackers can't use private key to decrypt archived sessions. Finally, make sure your supported browsers support everything you have set up.
You should also start linking to the https:// version of your site -- in emails and so on -- so your servers don't incur any unnecessary HTTP redirects and because you should be proud that you can provide https links to your web site.
So please get started on this as soon as possible. Don't put off the inevitable.