RFC 6750 specified the bearer token concept for securing access to protected resources. Bearer tokens need to be protected in transit as well as at rest. When a client requests access to a protected resource it hands-over the bearer token to the resource server. The OAuth 2.0 Proof-of-Possession security concept extends bearer token security and requires the client to demonstrate possession of a key when accessing a protected resource. This document describes how the client obtains this keying material from the authorization server.