Improved upper bounds for the expected circuit complexity of dense systems of linear equations over GF(2).

Minimizing the Boolean circuit implementation of a given cryptographic function is an important issue. A number of papers [1], [2], [3], [4] only consider cancellation-free straight-line programs for producing small circuits over GF(2). Cancellation is allowed by the Boyar-Peralta (BP ) heuristic [5, 6]. This yields a valuable tool for practical applications such as building fast software and low-power circuits for cryptographic applications, e.g. AES [5, 7], HMAC-SHA-1 [8], PRESENT [9], GOST [9], and so on. However, the BP heuristic does not take into account the matrix density. In a dense linear system the rows can be computed by adding or removing a few elements from a "common path" that is "close" to almost all rows. The new heuristic described in this paper will merge the idea of "cancellation" and "common path". An extensive testing activity has been performed. Experimental results of the new and the BP heuristic were compared. They show that the Boyar-Peralta results are not optimal on dense systems.