In 2012, the IEEE introduced IEEE Std 802.15.6 as the communication standard for Wireless Body Area Networks (WBANs). All key agreement protocols offered by this standard have been shown to exhibit grave security weaknesses. However, to date, no key agreement protocol has been proposed which fulfills all the requisite security and privacy objectives for deployment in a resource constrained WBAN environment. In this paper, based upon symmetric cryptographic primitives only, a key agreement protocol is presented which, in addition to good performance also offers the desirable privacy attributes of node anonymity and session unlinkability. The protocol is also suitable for post-quantum deployment scenarios as it is independent of any public key based operations.