We present a formal theory for cryptographically-sound theorem proving. Our starting point is the Backes-Pfitzmann-Waidner (BPW) model, which is a symbolic protocol model that is cryptographically sound in the sense of blackbox reactive simulatability. To achieve cryptographic soundness, this model is substantially more complex than standard symbolic models and the main challenge in formalizing and using this model is overcoming this complexity. We present a series of cryptographically-sound abstractions of the original BPW model that bring it much closer to standard Dolev-Yao style models. We present a case study showing that our abstractions enable proofs of complexity comparable to those based on more standard models. Our entire development has been formalized in Isabelle/HOL.