We propose a four-party password authenticated interdomain key exchange protocol which makes use of properties of identity-based cryptography and secret public keys. Being password-based and certificate-free, our protocol is lightweight and is suited to lightweight computing environments, such as pervasive computing. Apart from resistance against offline and active attacks, our protocol additionally provides perfect forward secrecy. We provide heuristic analysis of various security properties. Performance comparisons against other related protocols show that our protocol is efficient.