Audit logs are an elemental part of computer systems due to their forensic value. Safeguarding audit logs on a physically unprotected machine are a challenging work, especially in the presence of active adversaries. Forward security is necessary for such a logging system. In this paper, we propose a new Forward secure sequential Aggregate Signature (FAS) scheme with optimal storage and communication for keys and signatures. To the best of our knowledge, our FAS scheme is the only scheme that has constant-sized public and secret keys as well as constant-sized aggregate signatures. Our proposed scheme supports aggregation of consecutive signatures by a third party. We prove the security of our proposed scheme under the hardness of factoring assumption, in the random oracle model.