MIT computer scientist Ron Rivest, one of the co-creators, along with Len Adelman and Adi Shamir, of the RSA encryption algorithm, created a “timelock” puzzle that has gone unsolved since 1999.

The puzzle is based on finding the remainder when one large number, 7.2 quadrillion digits long, is divided by another, over-600-digit-long, number. The calculation was designed to take 35 years, using the fastest computer available each year.

There is a shortcut, however: if the 600-plus digit number can be factored into its two constituent prime numbers, the puzzle can be solved immediately. Factoring such a number is a difficult problem for today’s computers–a difficulty that underlies much of modern cryptography, including RSA.

Rivest said in New Scientist (subscription required) that computing speed has not increased as fast as he originally predicted, so the solution to the puzzle will likely take longer than 35 years to calculate, barring a breakthrough in factoring technique or the invention of a practical quantum computer. “Computing power isn’t increasing quite as fast as predicted, in terms of ability to do fast sequential computations,” he said.