Eight Hundred Leagues on the Amazon

"On what principle had these letters been arranged? He who held the paper was alone able to tell. With such cipher language it is as with the locks of some of our iron safes--in either case the protection is the same. The combinations which they lead to can be counted by millions, and no calculator's life would suffice to express them. Some particular "word" has to be known before the lock of the safe will act, and some "cipher" is necessary before that cryptogram can be read."