It was assumed that all correspondence to the west would be read by the security police and that any communication about the desire to emigrate would get them thrown in prison. So my mother, being an intelligent, imaginative woman and a writer by profession, came up with a code by means of which she could write letters to friends and relatives abroad filled with tame content: what plays she'd seen, what she had cooked for dinner, how I was doing in school, etc., which would be understood by her correspondents to relate to the details of our impending emigration. I never knew the exact code, but it worked very well, because we landed in Paris rather than a jail cell.
Nobody has mentioned this yet. You don't need encryption to keep secrets; you just need a willing and able partner. Perhaps this is why Plato thought artists dangerous.
Joanna