How would I securely generate a password that contains certain characters? - random

Question

When creating a password sometimes I need the password to contain uppercase, lowercase, number or a symbol. If I create the password like this it's not guaranteed that each is represented:
$input = 'a..zA..Z0..9$!_'; // generally the whole allowed alphabet
while (strlen($password) < 10) {
$password .= securelyPickRandomCharacterFromString($input);
}
In this case there is a chance that the random process (even though it's safe and completely random) will only select lowercase letters.
I can think of naive approaches like first selecting 1 from each group and then filling the rest randomly. But that would not be cryptographically safe as say random_bytes() is. Maybe shuffle after?
What would be a good and safe algorithm to generate the password containing each "group" of characters.
Note: The password requirements are usually for external system and are out of my control.

Answer 1

Here's a really naive way of doing it (pseudo code):
// Create your lists/arrays of groups
char listLowerCase[] = {'a','b',....};
char listUpperCase[] = {'A','B',....};
char listNums[] = {'1','2',....};
char listSpecialChars[] = {'#','#',...};
// Form a password from those groups (I'm just gonna use length 4 as example)
String pwd = listLowerCase[randomNumber] + listUpperCase[randomNumber]
+ listNums[randomNumber] + listSpecialChars[randomNumber];
// Shuffle the password
String shuffledPwd = shuffle(pwd);
In case you don't want to have the same number of characters from each group, you could try also setting random numbers that determine how many characters you will use in each group. Hope that helped you.

Answer 2

The simplest approach by far is to generate a random password using the chosen alphabet. Then check for constraints such as “must contain at least one of each character class”. If the password doesn't match the constraints, try again in a loop.
do {
$password = '';
while (strlen($password) < 10) {
$password .= securelyPickRandomCharacterFromString($input);
}
} while (!matches_constraints($password));
Within the given constraints on acceptable passwords, this has optimal security, since every acceptable password of the chosen length is equally likely.
In addition to simplicity and security, another benefit of this approach is that it accommodates arbitrary constraints, such as the common “must contain at least 3 of 4 character classes”, without having to write different, complex code for each different constraint.
With any sensible parameters, the number of redraws will be small. For example, a 10-character password generated from non-space printable ASCII characters (94-character alphabet) has a ((94-10)/94)^10 ≈ 0.32 chance of not containing a digit, the smallest of the four character classes. The chance of missing one of the four classes is a little under 0.42. So on average you'll need a little less than 2 iterations to get a suitable password with these particular parameters. Generating random letters is cheap, so performance is not a problem.
You should include protection against nonsensical parameters, for example attempting to generate a 3-character password that contains characters from all 4 classes (which would loop forever), or a 4-character password that contains characters from all 4 classes (which could take a very long time). So add a maximum iteration count.
$iterations = 0;
do {
if ($iterations >= 10) throw new NonsensicalPasswordConstraintException;
$password = '';
while (strlen($password) < 10) {
$password .= securelyPickRandomCharacterFromString($input);
}
++$iterations;
} while (!matches_constraints($password));