Millions of high-security crypto keys crippled by newly discovered flaw (Ars Technica)

Ars Technica is reporting on a flaw in the RSA library developed by Infineon that drastically reduces the amount of work needed to discover a private key from its corresponding public key. This flaw, dubbed "ROCA", mainly affects key pairs that have been generated on keycards. "While all keys generated with the library are much weaker than they should be, it's not currently practical to factorize all of them. For example, 3072-bit and 4096-bit keys aren't practically factorable. But oddly enough, the theoretically stronger, longer 4096-bit key is much weaker than the 3072-bit key and may fall within the reach of a practical (although costly) factorization if the researchers' method improves.To spare time and cost, attackers can first test a public key to see if it's vulnerable to the attack. The test is inexpensive, requires less than 1 millisecond, and its creators believe it produces practically zero false positives and zero false negatives. The fingerprinting allows attackers to expend effort only on keys that are practically factorizable. The researchers have already used the method successfully to identify weak keys, and they have provided a tool here to test if a given key was generated using the faulty library. A blog post with more details is here."