Question

Assume that we are planning to decrypt the cyphertext which was encrypted with DES encryption. We are using an ordinary household computer with 2GHz processor. Estimate the amount of time necessary to crack DES by testing all 56-bit possible keys. Also estimate the similar time for AES encryption with 128-bit key. (Assume that machine takes 100 cycles per brute force against a single key)

Answer 1

2 GHz = 2000000000hz

Since the DES has 56 possible keys, thus, 2^56 keys.

Assuming that the machin takes 100 cycles per brute force against a single key,

Amount of time necessary to crack DES : (2^56 key)*100 cycles/60sec/60min/24hour/365days/2000000000hz = 114.246566 years  

* Here the answer will be usually found in seconds thus we use 60 sec* 60 min* 24 hrs * 365 days to conver the answer into year.

Amount of time necessary to crack AES : (2^128 key)*100 cycles/60sec/60min/24hour/365days/2000000000hz = 5.39514154 × 1023 years