The failure of interfaces between bodies in frictional contact is central to a wide range of physical systems. Recent geophysical and laboratory observations indicate that interfacial motion can be mediated by slow rupture which is distinct from ordinary, earthquake-like, fast rupture. This slow rupture propagates at velocities much smaller than elastic wave-speeds and radiates away significantly reduced power. In spite of its prime importance, slow rupture is not yet fully understood. We develop a model for dry frictional interfaces, providing a natural theoretical framework for describing static and kinetic frictional states and the transition between them. The model predicts the existence of slow steady-state rupture fronts and the complete spectrum of fronts is explored. It is shown that the spectrum features a "forbidden gap" of velocities: rupture fronts cannot travel at a velocity smaller than a minimal one, which is a new velocity scale determined by the friction law. Above this minimum, the spectrum continuously spans the whole velocity range up to the elastic wave-speed. We suggest that these fronts are related to slow rupture fronts recently observed in laboratory experiments and possibly to slow/silent earthquakes.