Secure historical communication
(I do some of my best thinking in the shower)
Let's go back into history for a moment, and take a couple of fictional countries.
Lets say... I, Brendicus, Supreme Potentate of Brendanistan, receive a diplomatic letter from Queen Juli of the Julites, basically saying
"Dear Esteemed and Supreme Potentate,
Couldn't help but notice my catapults are looking a little dusty lately, and I happened to hear that Lord Enemy, of EnemyBurg, was insulting our mutual god. What say you and I pool our forces together, take a run at the border, throw a few thousand troops at the problem, and see which ones don't die? Then we shall have dinner in the ruins of his palace.
Hope to see you on the border,
Queen Juli of the Julites, mistress of all she surveys, protector of the makeup mines of MACland, etc etc, flourish, titles, etc."
I take a look at the letter, delivered by her trusted vizier. It has her seal on it, unbroken.
How do I verify that its her seal, and not a forgery? Who is my guy, in a historical context, that can tell me "yep, that the Julite seal, alright. Looks valid to me, boss!" ... or the guy who can tell me what the Julite seal looks like in the first place.
I do wonder how secure communication (within the context and limits of the time, of course), was achieved.