What solution implements user identity assertions and transmits attestations between the principal, the relying party, and the identity provider?

The solution that implements user identity assertions and transmits attestations between the principal (the user), the relying party (the service provider), and the identity provider is Security Assertion Markup Language (SAML). SAML is a standard for exchanging authentication and authorization data across different domains, allowing for single sign-on (SSO) capabilities.

SAML works by allowing the identity provider to create security tokens that include assertions about the user's identity. These assertions are then sent to the relying party, enabling it to authenticate and authorize users based on the received assertions from the identity provider. This process establishes trust and verifies user identity without needing to share sensitive credentials directly between the user and the relying party.

While other options might involve aspects of authentication or identity management, they do not specifically focus on the standardized method of assertions and attestations that SAML provides. For instance, Shibboleth is an open-source implementation of SAML but does not define the standard itself. OpenID focuses more on user authentication without the breadth of assertions, and transitive trust describes a trust relationship concept that does not specifically address the transmission of identity assertions.