A reparametrization-invariant approach to string field theory

Using reparametrization invariance as the only requirement, we show, by a series of algebraic steps, how the only reparametrization-invariant generalization of d'Alembert's operator for both open and closed bosonic strings, is in fact nilpotent and thus the BRST charge. The construction follows a proposal made many years ago by one of the authors. (orig.)