The traditional identity-based cryptography requires a user, who holds multiple identities, to hold multiple private keys, where each private key is associated with an identity. Managing multiple private/public keys is a heavy burden to a user due to key management and storage. The recent advancement of identity-based cryptography allow a single private key to map multiple public keys (identities); therefore the private key management is simplified. Unfortunately, the existing schemes capturing this feature do not allow dynamic changes of identities and have a large data size proportional to the number of the associated identities. To overcome these problems, in this paper, we present an efficient and dynamic identity-based key exchange protocol and prove its security under the Bilinear Diffie-Hellman assumption in the random oracle model. Our protocol requires a relatively small bandwidth for a key agreement communication, in comparison with other existing schemes. (C) 2012 Elsevier Inc. All rights reserved.