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Grade 9 Computer Science — How strong a password is

Work out how many different passwords a rule allows, and use that count to say which of two rules is harder to guess your way through.

Name: ________________________

  1. 1.A door keypad takes a code of exactly 4 characters, and each character is one of its 5 buttons. How many different codes are there for somebody to get through?

  2. 2.Put these rules in order of how many different passwords each one allows, fewest first.

    • 5 characters from 4
    • 3 characters from 9
    • 3 characters from 6
    • 2 characters from 25

  3. 3.One website's rule allows passwords 6 characters long, with 10 characters allowed in each position. Another website's rule allows passwords 4 characters long, with 30 characters allowed in each position. Which rule allows more passwords?

    • a) The rule with 4 characters and 30 choices
    • b) There is not enough here to tell
    • c) The rule with 6 characters and 10 choices
    • d) The two rules allow exactly the same number

  4. 4.A rule allows 3 characters taken from a set of 20, and a guessing machine gets through 1000 passwords a second. How many seconds does the machine need before it has tried the lot?

  5. 5.Sort each of these rules by how many different passwords it allows.

    Groups: Fewer than 1000 · More than 1000

    • 3 characters, each one of 11
    • 2 characters, each one of 30
    • 4 characters, each one of 7
    • 3 characters, each one of 9
    • 2 characters, each one of 45
    • 2 characters, each one of 12

  6. 6.Each of these rules allows some number of different passwords. Match each rule to that number.

    • 2 characters from 15
    • 3 characters from 5
    • 2 characters from 21
    • 4 characters from 4
    • 3 characters from 12
    • 256
    • 441
    • 125
    • 225
    • 1728

  7. 7.A student has 9 accounts and has used the same password on all but 3 of them. One of the websites is attacked and that password is taken. How many of the nine accounts can the attacker now open?

  8. 8.A badge code is exactly 2 characters long, and each character is one of the 5 shapes printed on the badge. How many different badge codes does that rule allow?

Answer key — Grade 9 Computer Science — How strong a password is

  1. 1. 625
  2. 2. 1. 3 characters from 6 2. 2 characters from 25 3. 3 characters from 9 4. 5 characters from 4
  3. 3. c) The rule with 6 characters and 10 choices
  4. 4. 8
  5. 5. 2 characters, each one of 12 → Fewer than 1000; 3 characters, each one of 9 → Fewer than 1000; 2 characters, each one of 30 → Fewer than 1000; 3 characters, each one of 11 → More than 1000; 4 characters, each one of 7 → More than 1000; 2 characters, each one of 45 → More than 1000
  6. 6. 2 characters from 15 → 225; 3 characters from 5 → 125; 2 characters from 21 → 441; 4 characters from 4 → 256; 3 characters from 12 → 1728
  7. 7. 6
  8. 8. 25