Nexus Authorization Logic (NAL) provides a principled basis for specifying and reasoning about credentials and authorization policies. It extends prior access control logics based on "says" and "speaksfor" operators, enabling within a single framework request authorization to depend on (i) the source or pedigree of the requester, (ii) the outcome of performing an analysis on the requester, or (iii) the use of trusted software to encapsulate or modify the requester. Prototype document-viewer applications that enforce integrity and confidentiality of document contents - all implemented on the Nexus operating system - illustrate the convenience and expressive power of this approach to authorization.