Many systems will allow the use of weak passwords or cryptographic keys, and users will often choose easy to guess passwords, possibly found in a dictionary. Given this scenario, an attacker would cycle though the dictionary word by word, generating thousands or potentially millions of incorrect guesses searching for the valid password. When a guessed password allows access to the system, the brute force attack has been successful and the attacker is able access the account.

The same trial and error technique is also applicable to guessing encryption keys. When a web site uses a weak or small key size, its possible for an attacker to guess a correct key by testing all possible keys. Essentially there are two types of brute force attacks, (normal) brute force and reverse brute force. A normal brute force attack uses a single username against many passwords. A reverse brute force attack uses many usernames against one password. In systems with millions of user accounts, the odds of multiple users having the same password dramatically increases. While brute force techniques are highly popular and often successful, they can take hours, weeks or years to complete.