The more you know: "Tessera"
I was just wondering why Latin programs use the word tessera, which means a small rectangular tile, to mean a password. Then I found this page, which says:

Why tessera also means “password” in Latin.

A watchword was used at night and it consisted of a word or phrase that someone must say to prove that he was a bona fide member of a Roman unit or, if the authorized password was not used, he was considered an enemy.

In the Roman army, the watchword for the night was not communicated verbally, but by means of a small rectangular tablet of wood upon which it was written. One man was chosen out of each of those maniples (common soldiers) and turmae (troop of cavalry containing thirty men, a squadron) that were quartered at that extremity of the lines most remote from the Principia.

Each of these individuals (tesserarius) went near sunset to the tent of the tribune, and received from him a tessera, on which the password and also a certain number or mark were inscribed.