The foundation of Web security rests on the notion that two very large prime numbers, numbers divisible only by themselves and 1, once multiplied together are irreducibly difficult to tease back apart. Researchers have discovered, in some cases, that a lack of entropy—a lack of disorder in the selection of prime numbers—means by analogy that most buildings on the Web would stand in spite of gale winds and magnitude 10 earthquakes, while others can be pushed over with a finger or a breath. The weakness affects as many as 4 in 1,000 publicly available secured Web servers, but it appears in practice that few to no popular Web sites are at risk.