Read the excerpt from The Code Book.The difference between ordinary and digital mail can be illustrated by imagining that Alice wants to send out invitations to her birthday party, and that Eve, who has not been invited, wants to know the time and place of the party. If Alice uses the traditional method of posting letters, then it is very difficult for Eve to intercept one of the invitations. . . .In comparison, Eve's task is made considerably easier if Alice sends her invitations by e-mail.