Sequential aggregate signature, first introduced and formalized by A. Lysyanskaya et al. (see EUROCRYPT 2004, p.74-90, 2004), is emerging as a useful tool to ensure routing security and at the same time to improve performance. We propose a new mechanism to construct sequential aggregate signatures based on the cipher block chaining (CBC) mode, which is different from previous known results. We then construct an efficient sequential aggregate signature scheme and show that our construction is provably secure in the random oracle paradigm, assuming that the RSA problem is hard. Finally, we propose an interesting aggregate routing protocol for wireless ad hoc networks as an immediate application of our protocol.