Question :- a) List the requirements which should be met in the digital signature system? b) AES is symmetric cipher algorithm that most widely used at this moment in different fields. The design of AES with a key length 128-bit depends on long-term security regarding to exhaustive key-search attacks. suppose that a hacker has a special device which has power to checks 5x108 keys/sec ,the available budget for this issues is $1 million. each device costs $50, and we suppose 100% overhead for building the device (hardware design ,software, power, AC, etc.).within the available budget, how many number of devices that can operate in parallel? How long does it take in average to find the key? How long does it take relative to the age of the universe which is about 1010 years.?