The world has known for decades that the RSA cryptosystem’s days are numbered. Once quantum computing becomes practical (estimates for that range from 3 to 20 or more years), the foundational security it provides will crumble. New research has revealed a novel method that uses classical computing to reduce the current RSA security level to an unacceptably low threshold.

The practical risk is limited, but still significant. Applying the attack against the deprecated use of 1024-bit keys took a handful of months on an academic CPU cluster, significantly less than the current estimates for 1024-bit factoring that would require resources that only nations or companies with massive resources could achieve. Widely used RSA implementations are also safe.

Nonetheless, the research has taken cryptographers by surprise because it introduces signature forgery, a new way to break RSA keys without factoring. Equally important, this novel method reduces the required computing resources by orders of magnitude.

Out of reach no more

“If this result holds up under peer review, it would indeed be a conceptual break-through,” Karsten Nohl, a cryptography expert and the head of innovation at Allurity, said in an interview. “RSA is as difficult to break as it is to factor large integers, at least so we thought. The researcher suggests that you can practically break RSA without cracking its key.”

Nadia Heninger, a University of California at San Diego professor and co-author, elaborated:

Cryptographers thought that the only way to compute valid RSA digital signatures was to first compute the private key by factoring, and then use the private key to compute the signatures. For 1024-bit RSA, this was thought to be very expensive, albeit probably doable if you have the computational resources of the large tech companies or the NSA—on the order of tens of millions of dollars of computation time for a single key. For 2048-bit RSA, it was thought to be totally out of reach.

The key forgery attack Heninger and the other researchers devised is fully practical now for 1024-bit RSA. Even for 2048- and 4096-bit keys, the method reduces the security of RSA to unacceptable levels. The National Security Agency, National Institute of Standards and Technology, and European Union Agency for Network and Information Security require that any cryptosystem should provide a level of no less than 128 or more bits, meaning the operations required must exceed 2128.