This paper discusses public key-authenticated key agreement protocols. First, we critically analyze several authenticated key agreement protocols and uncover various theoretical and practical flaws. In particular, we present two new attacks on the Hashed Menezes-Qu-Vanstone (HMQV) protocol, which is currently being standardized by IEEE P1363. These attacks suggest the caution one should take when interpreting theoretical results from a formal model. We further point out that many of the protocol failures in the past are caused by sidestepping an important engineering principle, namely, “Do not assume that a message you receive has a particular form (such as gr for known r) unless you can check this.” Constructions in the past generally resisted this principle on the grounds of efficiency: checking the knowledge of the exponent is commonly seen as too expensive. In a concrete example, we demonstrate how to effectively integrate the zero-knowledge proof primitive into the protocol design and, meanwhile, achieve good efficiency. Our new key agreement protocol, YAK, has comparable computational efficiency to the MQV and HMQV protocols with clear advantages on security. Among all the related techniques, our protocol appears to be the simplest so far. We believe simplicity is also an important engineering principle. Copyright © 2012 John Wiley & Sons, Ltd.