Bilinear Hulls for Support Splitting: Characterization, Optimization, and Applications
Sendrier's Support Splitting Algorithm (SSA) is the standard solver for the Permutation Equivalence Problem (PEP); its cost is dominated by a term that grows exponentially in the hull dimension. We study, mathematically, how much this cost can be lowered by replacing the standard inner product, on which the hull is defined, with an alternative bilinear form. Two structural results give a complete answer: only a narrow two-parameter family of bilinear forms produces a hull on which SSA can still operate, namely those whose matrix in the standard basis is a linear combination of the identity and the all-ones matrix; and even within this family the hull dimension drops by at most one (with the reduction achieved on every self-dual code whose length is coprime to the field characteristic). Together, these results give a tight upper bound: since SSA on a code with hull dimension $h$ has cost dominated by $q^h$, replacing the standard hull with $\mathrm{Hull}_M$ lowers this dominant term from $q^h$ to at best $q^{h-1}$, so no bilinear-form generalization of the inner product can reduce the SSA cost by more than a factor of $q$, the field size. Going beyond this requires invariants outside the bilinear framework. As an application, we verify that PEP-based SPECK admits the factor-$q$ cost reduction, while LEP-based LESS and PKP-based PERK lie outside our framework. A SageMath implementation of the bilinear-hull preprocessing confirms feasibility at SPECK's recommended length $n = 252$.