Experience Using a Low-Cost FPGA Design to Crack DES Keys

This paper describes the authors’ experiences attacking the IBM 4758 CCA, used in retail banking to protect the ATM infrastructure. One of the authors had previously proposed a theoretical attack to extract DES keys from the system, but it failed to take account of realworld banking security practice. We developed a practical scheme that collected the necessary data in a single 10-minute session. Risk of discovery by intrusion detection systems made it necessary to complete the key “cracking” part of the attack within a few days, so a hardware DES cracker was implemented on a US$995 off-the-shelf FPGA development board. This gave a 20-fold increase in key testing speed over the use of a standard 800 MHz PC. The attack was not only successful in its aims, but also shed new light on the protocol vulnerabilities being exploited. In addition, the FPGA development led to a fresh way of demonstrating the non-randomness of some of the DES S-boxes and indicated when pipelining can be a more effective technique than replication of processing blocks. The wide range of insights we obtained demonstrates that there can be significant value in implementing attacks “for real”.