D.J. Bernstein has proposed a circuit-based implementation of the matrix step of the number field sieve factorization algorithm (see "Circuits for integer factorization: a proposal", http://cr.yp.to/papers.html#nfscircuit, 2001). These circuits offer an asymptotic cost reduction under the measure "construction cost × run time". We evaluate the cost of these circuits, in agreement with Bernstein, but argue that, compared to previously known methods, these circuits can factor integers that are 1.17 times larger, rather than 3.01 as claimed (and even this is only under the non-standard cost measure). We also propose an improved circuit design based on a new mesh routing algorithm, and show that, for factorization of 1024-bit integers, the matrix step can, under an optimistic assumption about the matrix size, be completed within a day by a device that costs a few thousand dollars. We conclude that from a practical standpoint, the security of RSA relies exclusively on the hardness of the relation collection step of the number field sieve