A mechanical probe station is a laboratory device used to physically acquire signals from the internal nodes of a chip. The station allows positioning of thin probing needles on the chip's surface, either using humanly operated manipulators or automatically. To protect a bit k against probing, one usually encodes k as bit-shares {s(j)}(0 <= j <= l-1) where {s(j)}(0 <= j <= l-2) are random and s(l-1) = k circle plus s(0)circle plus, ... circle plus s(l-2). If each s(i) is stored at a different RAM location, the opponent needs to probe all the l cells to learn k. Let k = {k(0), ..., k(n-1)} be an n-bit key recorded in an n x l matrix as {s(i, j)}(0 <= i <= n-1), (0 <= j <= l-1). To force the attacker to probe scattered data, we look for a function f mapping {i, j} to geometrical {x, y} coordinates such that, for u not equal v, the minimal distance between r(i,u) and r(i,v) is as large as possible. This paper solves this problemby exploiting the module structure of Z(2). Weinfer a theoretical lower bound on f's scattering capacity and compare the algorithm's performance with an approximate upper bound. The result is quite satisfying.
