Key exchange protocols in the asymmetric-key setting are known to provide stronger security properties than protocols in symmetric-key cryptography. In particular, they can provide perfect forward secrecy, as illustrated by key exchange protocols based on the Diffie-Hellman scheme. However public-key algorithms are too heavy for low-resource devices, which can then not benefit from forward secrecy. In this paper, we describe a scheme that solves this issue. Using a nifty resynchronisation technique, we propose an authenticated key exchange protocol in the symmetric-key setting that guarantees perfect forward secrecy. We prove that the protocol is sound, and provide a formal security proof.