Symbolic Transducers

Symbolic Finite Transducers, or SFTs, is a representation of finite transducers that annotates transitionswith logical formulae to denote sets of concrete transitions. Th is representation has practical advantages in applications f or web security analysis, as it provides ways to succinctly represnt web sanitizers that operate on large alphabets. More important ly, the representation is also conducive for efficient analysis usi ng stateof-the-art theorem proving techniques. Besides introduci ng SFTs we provide algorithms for various closure properties including composition and domain restriction. A central result is that equivalence of SFTs is decidable when there is a fixed bound on how many different values that can be generated for arbitrary inputs. In practice, we use a semi-decision algorithm, enco ded axiomatically, for non-equivalence of arbitrary SFTs. We show that several of the main results lift to a more expressive ver sion of SFTs with Registers, SFTRs. They admit a fixed set of register s that can be referenced in the logical formulae, updated by input characters, or used to generate output.