The researchers found that by varying the voltage on a device it was possible to get their hands on the 'private key' needed to beat the security feature. Using what they described as an inexpensive device specially-built for the experiment, the trio manipulated the voltage and caused the computer to make small mistakes in its communications with other clients. This ultimately revealed small pieces of the private key, which they eventually used to reconstruct the key offline.
From the paper:
"The paper makes three important contributions: first, we develop a systematic fault-based attack on the modular exponentiation algorithm for RSA. Second, we expose and exploit a severe flaw on the implementation of the RSA signature algorithm on OpenSSL, a widely used package for SSL encryption and authentication. Third, we report on the first physical demonstration of a fault-based security attack of a complete microprocessor system running unmodified production software: we attack the original OpenSSL authentication library running on a SPARC Linux system implemented on FPGA, and extract the system's 1024-bit RSA private key in approximately 100 hours."
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