An assertion is a statement of fact about a party, made by or on behalf of that party.
Any information presented by a party in an identity transaction may be considered to be an assertion. Assertions however have come to play a more complex role in digital identity services since the emergence of federated identity domains.
In federations the existence of multiple identity providers and cross-domain services complicates the task of tailoring entitlements and making authorisation decisions. This led to the need to provide more information about a requesting party during access requests.
The problem of how to produce, verify and process complex assertions has become a focus for the design of digital identity systems. For example, both the Microsoft and Internet2 architectures for federated identity include protocols and standards for the handling of complex assertions in identity transactions.
Assertions may be referred to as claims.
Claim is the term adopted by Microsoft for information asserted in an access transaction. Microsoft documentation uses the term claims-based identity management in its discussions of federated identity management.
The term assertion is more common in the realm of open source identity technologies and standards. For example, the Security Assertion Markup Language (SAML).