What is the oldest form of keeping messages secret, and how did it evolve into a subsequent art?

Compare and contrast the two forms of encryption: transposition and substitution. Imagine you have a secret message you wish to convey. In which situations would you use transposition or substitution? Why?

The author opens his analysis by retelling the story of the treason trial of Mary, Queen of Scots. What role has encryption played in politics over the centuries? How does the political use of encryption now differ from—or bear resemblance to—its political uses in the past?