You shouldn't assume they're private just because they're encrypted. Employers can and will install SSL certs on your desktop machine so that they can decrypt and scan/archive everything at the gateway. This is standard practice in financial companies, and is easily done anywhere.

They can also install screen capture and key logging software if they want, but that's less common and without disclosure is a lot shadier (although certainly legal in the US). I wouldn't expect it most places; it's a more extreme step.

But never trust encryption at work unless you know your company's policies.