Encryption and Blood Empires

Spartan generals wrote their messages on a narrow strip of parchment
wrapped around a thin cylinder ("skytale"). When the parchment was
unwound, the message appeared as a nonsense sequence of letters and
could only be read by wrapping the parchment around another cylinder of
the same size.

Communication is arguably the most sensitive aspect of maintaining an
empire. What were/are some origins of the most successful forms of
encryptians from Alexander, The Mongols to modern times?

Edited by dirtnap

I think the Navajo code talkers were very clever and it was never
deciphered although that movie did little for me... At that same time, the
enigma code was also effective until British sailors(?) recovered one code
machine in a sinking u-boat in the Antarctic I believe... The intent to be
sure...

Interesting which topics get feedback and which ones do not, somehow
the worlds bloodiest battle or the most ruthless ruler always gets more
responses... I should word my questions to include biggest, best or
bloodiest for this forum...

Edited by dirtnap