Ring CT (Confidential Transactions)
Transaction quantities in Monero are concealed via RingCT, also known as Ring Confidential Transactions. In January 2017, Ring CT was implemented in block #1220516. This capability became necessary for all network transactions starting in September 2017.
RingCT introduces “A Multi-layered Linkable Spontaneous Anonymous Group signature”, an upgraded form of the ring signature that, with tolerable performance, enables hidden transaction amounts, origins, and destinations, as well as verifiable, trustless coin production.