Identity Chaining Deep Dive: Connecting Identity Across Trust Domains with OAuth

Introduction I've been wondering about distributed systems lately—specifically, multiple microservices across different cloud providers. Say a request starts in Domain A, needs to access Domain B's services, and then Domain C. It seems simple at first glance. But what actually happens under the hood? Once you cross a trust domain boundary, your access token usually becomes worthless. Domain B won't accept Token A, and Domain C won't accept tokens from either. It makes you wonder: how exactly do you safely move identity across trust boundaries without leaking credentials or creating security holes? "Just pass Service A's access token directly to Service C"? Absolutely not. The audience is different. The signature issuer is different. The receiving end can't even verify it, and if it somehow accepted it, it would become a critical security vulnerability. Identity Chaining is the answer to this exact problem. What is Identity Chaining? The answer is Identity Chaining. Instead of inventing