  We present an algorithm to decide the intruder deduction problem (IDP) for a
class of locally stable theories enriched with normal forms. Our result relies
on a new and efficient algorithm to solve a restricted case of higher-order
associative-commutative matching, obtained by combining the Distinct
Occurrences of AC- matching algorithm and a standard algorithm to solve systems
of linear Diophantine equations. A translation between natural deduction and
sequent calculus allows us to use the same approach to decide the
\emphelementary deduction problem for locally stable theories. As an
application, we model the theory of blind signatures and derive an algorithm to
decide IDP in this context, extending previous decidability results.
