The failure of interfaces between bodies in frictional contact is central to a wide range of engineering, physical and geophysical systems, ranging from micro-electro-mechanical systems to earthquake faults. 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 generic model for dry frictional interfaces, based on extending classical friction models, 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 furthur show that sloe rupture is significantly less spatially localized than fast rupture. We suggest that these fronts are related to slow rupture fronts recently observed in laboratory experiments and possibly to slow/silent earthquakes.