Unilaterally-authenticated key exchange

Abstract

Key Exchange (KE), which enables two parties (e.g., a client and a server) to securely establish a common private key while communicating over an insecure channel, is one of the most fundamental cryptographic primitives. In this work, we address the setting of unilaterally-authenticated key exchange (UAKE), where an unauthenticated (unkeyed) client establishes a key with an authenticated (keyed) server. This setting is highly motivated by many practical uses of KE on the Internet, but received relatively little attention so far. Unlike the prior work, defining UAKE by downgrading a relatively complex definition of mutually authenticated key exchange (MAKE), our definition follows the opposite approach of upgrading existing definitions of public key encryption (PKE) and signatures towards UAKE. As a result, our new definition is short and easy to understand. Nevertheless, we show that it is equivalent to the UAKE definition of Bellare-Rogaway (when downgraded from MAKE), and thus captures a very strong and widely adopted security notion, while looking very similar to the simple “one-oracle” definition of traditional PKE/signature schemes. As a benefit of our intuitive framework, we show two exactly-as-you-expect (i.e., having no caveats so abundant in the KE literature!) UAKE protocols from (possibly interactive) signature and encryption. By plugging various one- or two-round signature and encryption schemes, we derive provably-secure variants of various well-known UAKE protocols (such as a unilateral variant of SKEME with and without perfect forward secrecy, and Shoup’s A-DHKE-1), as well as new protocols, such as the first 2-round UAKE protocol which is both (passively) forward deniable and forward-secure. To further clarify the intuitive connections between PKE/Signatures and UAKE, we define and construct stronger forms of (necessarily interactive) PKE/Signature schemes, called confirmed encryption and confidential authentication, which, respectively, allow the sender to obtain confirmation that the (keyed) receiver output the correct message, or to hide the content of the message being authenticated from anybody but the participating (unkeyed) receiver. Using confirmed PKE/confidential authentication, we obtain two concise UAKE protocols of the form: “send confirmed encryption/confidential authentication of a random key K.”