It's a secret (13)

So is the "Alice" cipher unbreakable?  Not entirely.  If you have reason to believe that a keyworm from some book or other publication is being used and that it is in English, then it is reasonable to assume that certain words and combinations of words will occur in it.  "And the" for example is pretty well bound to occur somewhere.  So you go through the ciphertext trying "and the" as a key over and over again to see if it produces anything sensible.  If it does then that may allow you to have a guess at what other parts of the message may be which in turn could give you more of the keystream, and so on.  And of course there are other commonly occurring combinations of words you can try.  So it's time-consuming and mind-numbing, but it can be done.  On the other hand, if the keyworm is just a random collection of letters, this won't work and this gives rise to what is known as a "one-time pad", of which more next time.