Where she has gone to is beyond the wit of man to say.

The nymph grew pale, and in a mortal fright.

Ever and anon the guard's face could be dimly seen at his window, more like a ghost than a man; ….

The key that Alice and Bob are to share cannot be transmitted in the clear. Either it must be enciphered when sent, or Alice and Bob must derive it without an exhange of data from which the key can be derived.