In situations where the consumer doesn\'t trust/control the workstation and the

Question

In situations where the consumer doesn't trust/control the workstation and the network, can consumer safely do potentially sensitive activities (like making a payment or transferring confidential documents) over the internet when HTTPS is used?

For instance: when I use my employer's computer and am on the company network, or I am in an airport and use a public kiosk there, is there a way to keep them from spying on me?

Naturally, IT administrators in these circumstances are in full control of CA management, and could potentially use a proxy and load the proxy's certificate into the trusted root list, so my browser would show everything being fine.

My current understanding is that as long as I can verify that the cert isn't spoofed and is signed by a well known CA, I can be assured that there's no one eavesdropping on the traffic.

Now if my previous statement is correct what is the best way to verify that cert? I understand that I can just open it up and inspect the CA's name, but are there any extra steps I can take? For instance, could I somehow export the cert I'm receiving from the HTTPS site and submit it to some service for verification?

Explanation / Answer

On an untrusted computer, you cannot know that no one is eavesdropping. This is just a fact of security -- TLS can protect against a man-in-the-middle, but nothing whatsoever can protect you against someone with administrative access to your computer. Even if you have public key pinning for the site, which means that a rogue CA can't create a fake certificate for it, you have no way of knowing that there's not something installed on your computer that is recording the screen and logging keystrokes (such spying software isn't that uncommon). There are ways to mitigate an untrusted network. There are no ways, and can be no ways, to deal with an untrusted OS besides not using that OS.

TLS is designed to handle specific types of attackers -- specifically, active and passive attackers between you and the server, as well as people impersonating the server. It is not designed to protect against your situation; in general, implementations actually support the ability for an administrator to install their own root CA and MitM the traffic (Chrome will actually make an exception to public key pinning in this situation, because it requires admin access to the machine and there's no way of stopping an admin from doing what they want anyway).

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