a. There are 26 letters in the English alphabet, with two cases for each letter; 10 numerical characters in the range 0-9; and 5 special characters; thus a total of 26•2 + 10 + 5 = 67 characters.
Any character can be used more than once, so there are
67⁶ + 67⁷ + 67⁸ + 67⁹
or 27,618,753,243,839,080 total possible passwords.
b. If we require at least 1 special character, then there are 62 choices for each ordinary character we use and 5 for each special character.
Suppose we use a password of length 6. Then there are
62⁵•5¹ + 62⁴•5² + 62³•5³ + 62²•5⁴ + 62¹•5⁵ + 62⁰•5⁶
or
[tex]\displaystyle \sum_{k=1}^6 62^{6-k}\cdot5^k[/tex]
We count the longer passwords in a similar fashion:
[tex]\displaystyle \sum_{m=6}^9 \left(\sum_{k=1}^m 62^{m-k}\cdot5^k\right)[/tex]
or 1,206,930,268,794,100 total passwords
c. 1 second (s) is equal to 10⁶ microseconds (μs). If it takes 1 μs to try 1 password, then it would take
(1,206,930,268,794,100 passwords) • (1 μs / password) • (1 s / 10⁶ μs)
= 1,206,930,268.7941 s
= (1,206,930,268.7941 s) • (1 min / 60 s)
≈ 20,115,504.4799 min
≈ (20,115,504.4799 min) • (1 h / 60 min)
≈ 335,258.408 h
≈ (335,258.408 h) • (1 day / 24 h)
≈ 13,969.1003 days
≈ (13,969.1003 days) • (1 year / 365 days)
≈ 38.2715 years
