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Duplicates

Reusing passwords is less safe than using unique passwords. If an attacker learns your password for one account (through hacking or by guessing), he/she is likely to use it to break into another account.

Common passwords

Some passwords such as "password" and "1234" are very popular and thus can be easily guessed by attackers. Avoid using passwords that are common dictionary words or sequences of letters or numbers.

Complexity

A common recommendation for creating a strong a password is to ensure that it is at least 8 characters long, and is combination of lowercase and uppercase letters, digits, and special characters. This add-on considers passwords with entropy lower than 52 to be "too simple" where entropy is calculated as
log₂(alphabet_size^{password_length})