Cisco Systems An Extended Master Session Key (EMSK) is a cryptographic key generated from an Extensible Authentication Protocol (EAP) exchange reserved solely for the purpose of deriving master keys for one or more purposes identified as usage definitions. This document specifies a mechanism for deriving cryptographically separate root keys from the EMSK, called usage specific root Keys (USRK). The document provides a set of requirements for avoiding conflicts between usage definitions to ensure this cryptographic separation. The USRK is used according to the usage definition defined for a specific purpose.