# Identity Federation Identity federation allows one identity system to supply authentication or identity assertions to another service. It can reduce repeated enrollment while making the upstream identity provider a high-centrality dependency. ## Notebook evidence [[Scanned_20260730-2016]] shows federation from the user side rather than as a protocol diagram. Google accounts anchor Gmail and Google Fi; Apple identities connect aliases, devices, and recovery; Facebook/Meta and telephone numbers connect social and payment experiments; and ID.me converts proofed identity into reusable authorization. PDF page 41 is the notebook’s clearest federation map. It places Facebook, WhatsApp, ID.me, LinkedIn, Tumblr, Namecheap, and a Pixel 6 in one authentication-recovery surface. ## Risk and continuity Federation simplifies access but can increase cascade risk: loss of the upstream email, telephone number, or authenticator device may affect several downstream services at once. ## Source - [[Scanned_20260730-2016]], PDF pages 16, 22, 25, and 41–42.