I am trying to understand how to calculate the average key search time given a specific scenario:
Suppose we have a program that uses standard DES with 56 key bits, and we can test 10^6 keys per second.
The key consists of 8 characters, a simple concatenation of the 8 ASCII characters yielding 64 = 8*8 key bits. With the permutation PC-1 in the key schedule, the least sig. bit of each 8-bit character is ignored, giving us 56 key bits.
What is the size of the key space if all 8 characters are randomly chosen 8-bit ASCII characters?
My thought is the key space is 2^56.
How long does an average key search take?
We can test 10^6 keys per second, and 2^56 is roughly 7.2058 x 10^16. I don't know if this is proper way to do it, I estimated ~3 seconds.
I am assuming there is much more accurate mathematical approach but I'm not familiar with it, could anyone shed some light on this for me?