Using a Computer Safely
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.
A machine trying to break into an account does not guess the way a person guesses. It works through the passwords a rule allows, one after another, until one of them opens the account. So the honest question about a password rule is not whether the passwords look clever. It is how many passwords there are altogether, because that is the pile the machine has to get through.
Positions multiply
Count the characters allowed in a single position. Then multiply that number by itself once for every position the password has. Multiply, never add: two positions do not give you twice as many passwords, they give you one whole set of passwords for every character the first position could have been. That is also why one more position multiplies the entire count again, while allowing a few more characters only nudges the number being multiplied.
Worked example
A lock has three wheels, and each wheel carries the ten digits 0 to 9. How many settings does the lock have?
One wheel on its own can be in 10 settings.
Ten digits, one of them showing. Nothing has been multiplied yet.
Try it together
A website allows passwords of exactly two characters, and is deciding which characters to allow.
Two counts, and then the difference between them.
1.At first the website allows only the 26 small letters. How many two-character passwords does that allow?
Have a go
Have a go on your own. A padlock has three wheels and each wheel carries eight letters. How many settings does the padlock have?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.