I present a new method to compute a bit-parallel polynomial basis squarer for GF(2(m)) generated by an arbitrary irreducible polynomial using weakly dual basis. I apply the proposed method to irreducible pentanomial and derive the explicit formulae for squarer. It is the first time that gives the explicit formulae and an upper complexity bound of squarer for irreducible pentanomials. Moreover, such formulae permit one to choose pentanomial for any odd m is an element of [19,2000] whose multiplier, as well as squarer, can be performed more efficiently. (C) 2011 Elsevier B.V. All rights reserved.