Sorting by Strip Swaps is NP-Hard

We show that Sorting by Strip Swaps (SbSS) is NP-hard by a polynomial reduction of Block Sorting. The key idea is a local gadget, a cage, that replaces every decreasing adjacency (ai,ai+1) by a guarded triple ai,mi,ai+1 enclosed by guards Li,Ui, so the only decreasing adjacencies are the two inside the cage. Small hinge gadgets couple adjacent cages that share an element and enforce that a strip swap that removes exactly two adjacencies corresponds bijectively to a block move that removes exactly one decreasing adjacency in the source permutation. This yields a clean equivalence between exact SbSS schedules and perfect block schedules, establishing NP-hardness.