Ink

Ink is a fragrant blue-black musk that yinrih excrete from their writing claws. Before achieving sapience, this ink was used to mark territory. Over time, a written language evolved out of this scent-marking behavior.

To a human, a yinrih's ink smells strongly of petrichor (the earthy smell of rain). To other yinrih, however, ink presents a complex array of odors that identify the writer's age, gender, reproductive status, and even mood at the time of writing. The composition of one's ink is used as a bio-metric authentication factor. Combined with a distinct pattern drawn onto a special pad to act as a password, two-factor authentication can be achieved with a single action.