I've been contemplating this question since reading a wonderful short story, Bog Rhubard Shoots, by Yamashiro Tomoe (it's from the book, Reflections on the Way to the Gallows). It tells the tale of Mitsuko, a young Japanese woman, who, in the 1940s, was sent to prison for her involvement in the left-wing political movement (as was her husband, who was assigned to different prison).
Under the terms of her conviction, Mitsuko "could write only one letter of postcard length each month. Even though the frequency and length of the letters were limited, they still had to be checked over by four persons: the guard, the chief guard, the chaplain, and the supervisor of the guards. Under these circumstances, one hundred words had to do the job of ten thousand in the letters that Mitsuko and her husband exchanged. If a letter written under restricted conditions failed to pass the censors and was stamped 'To be handed over upon release,' the letter itself would probably shed tears."