Is the BB84 algorithm useful?

In the previous section, Alice and Bob formed a randomly generated secret. Both Alice and Bob know 10100 or whatever other sequence their BB84 algorithm generated. But what good is that? If Alice can’t decide exactly what she wants to say to Bob, why should she bother saying anything at all?

The answer lies in what Alice does with the randomly generated secret. By combining her randomly generated secret with some meaningful information, Alice can form a sequence of characters that’s meaningful to Bob but meaningless to an eavesdropper.

Here’s some terminology:

• The randomly generated secret is called a key.

For example, in the previous section, Alice and Bob created the key 10100.

• The meaningful information that must be kept from prying eyes is called plaintext.

Imagine that Alice wants to send the word Stop! to Bob. Alice doesn’t want anyone else to read this message. The word Stop! is an...