J . Goubault − Larrecq and K . N . Verma Alternating Two − Way AC − Tree Automata Research

We explore the notion of alternating two-way tree automata modulo the theory of finitely many associative-commutative (AC) symbols, some of them with a unit (AC1). This was prompted by questions arising in cryptographic protocol verification, in particular in modeling group key agreement schemes based on Diffie-Hellman-like functions, where the emptiness question for intersections of such automata is fundamental. We show that the use of conditional push clauses, or of alternation, leads to undecidability, already in the case of one AC or AC1 symbol, with only functions of arity zero. On the other hand, emptiness is decidable in the general case of many function symbols, including many AC or AC1 symbols, provided push clauses are unconditional and intersection clauses are final. To this end, extensive use of refinements of resolution is made.