This lore can be found by a Trooper on Hoth.
Since the beginning of the Great War, one Imperial cipher has defied every attempt at decryption. Code-named “Umbra,” it has become a legend in the galactic cryptography community; the Republic even allowed scraps of Umbra transmissions to be released surreptitiously as “unbeatable puzzles” to university students in the hope of finding someone–anyone–who could crack the code. With theoretical efforts yielding no results, the Republic eventually took on a more direct approach, launching dozens of covert missions to capture one of the Empire’s Umbra encryption devices. None has succeeded.