As the example of COPACOBANA [105] shows, key-search machines need not be prohibitive from a monetary point of view. We...

As the example of COPACOBANA [105] shows, key-search machines need not be prohibitive from a monetary point of view. We now consider a simple brute- force attack on DES which runs on COPACOBANA.

1. Compute the runtime of an average exhaustive key-search on DES assuming the following implementational details:

 COPACOBANA platform with 20 FPGA modules  6 FPGAs per FPGA module  4 DES engines per FPGA  Each DES engine is fully pipelined and is capable of performing one encryp-

tion per clock cycle

 100 MHz clock frequency

2. How many COPACOBANA machines do we need in the case of an average search time of one hour?

3. Why does any design of a key-search machine constitute only an upper security threshold? By upper security threshold we mean a (complexity) measure which describes the maximum security that is provided by a given cryptographic algo- rithm.