Factoring reaches new heights.

A powerful computer takes just a few seconds to prove that a number consisting of 73 decimal digits is a prime, evenly divisible only by itself and one. In contrast, the same computer typically requires a much longer time to factor a 73-digit composite number into its prime-number components. Indeed, factoring numbers beyond about 130 digits has been impractical. This fortuitous circumstance underlies the widely used RSA method of encrypting digital information (SN: 5/7/94, p. 292).

Now, Herman te Riele and his coworkers at the National Research Institute for Mathematics and Computer Science (CWI) in Amsterdam report that they have factored a 186-digit number--the largest yet cracked--using a technique called the special number field sieve (SN: 5/31/97, p. 340). The number, [([2.sup.15]-135).sup.41]-1, turns out to have several factors, including a 73-digit prime and a 71-digit prime. The factorization required 88 computers running for 42 days.

The researchers soon expect to tackle the much tougher task of factoring a number consisting of a string of 512 ones and zeros. If they succeed, they would be able to decode secret messages created using one version of the RSA method.