An assertion is a verifiable statement or claim issued by one digital entity, typically an identity provider (IdP), about another entity such as a user or system. In identity and access management (IAM), assertions serve as structured data payloads that communicate critical information including the subject's unique identifier, evidence of successful authentication, and contextual attributes like group memberships, roles, or permissions. These claims form the foundation for establishing trust relationships between different systems and services.

To maintain strong security standards, assertions are cryptographically protected through digital signatures, ensuring their integrity, authenticity, and non-repudiation during transmission. This prevents unauthorized tampering and validates the origin of the claim. Service providers consume these validated assertions to make informed access control decisions, enabling seamless single sign-on (SSO) experiences while enforcing granular security policies. The accurate processing of assertions is essential for maintaining a robust security posture in modern enterprise architectures.