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Not Even Coppersmith's Attack

Earlier this year, in October, a new widespread cryptography vulnerability was announced. The initial announcement didn’t contain details about the vulnerability or much details on how to attack it (updated by now). It did state the affected systems though: RSA keys generated using smartcards and similar devices that use Infineon’s RSALib. The announcement came with obfuscated code that would check whether a public key is affected. Also, the name chosen by the researchers was a small hint on how to attack it: “Return of Coppersmith’s Attack”.

I decided to try and figure out the details before the conference paper describing them would be released. By the time the paper was released, I had reverse engineered the vulnerability and implemented my own attack, which did not use Coppersmith’s method at all. This post explains how I figured out what’s wrong with the affected RSA-keys and how I used that information to factor affected 512-bit RSA-keys.

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