Privacy-enhancing Technologies

So now we know how Alice can hide the contents of her messages to Bob from eavesdroppers.

What if Eve can gain useful information simply by knowing that Alice and Bob are communicating, without knowing what they're saying?
They could be CEOs of companies considering a deal
Alice could be a whistleblower, and Bob the EPA

We need to hide not only the data, but the metadata, such as the addresses on the message, from Eve.

To do this, we use privacy-enhancing technologies.