A dictionary attack is a technique for defeating a cipher or authentication mechanism by trying to determine its decryption key or pass-phrase by trying hundreds or sometimes millions of likely possibilities, such as words in a dictionary.

A dictionary attack uses a targeted technique of successively trying all the words in an exhaustive list — called a dictionary. In contrast to a (pure) brute force attack — where a large proportion key space is searched systematically — a dictionary attack tries only those possibilities which are most likely to succeed, typically derived from a list of words for example a dictionary... hence the phrase dictionary attack.

It is common to use passwords from previous breaches as a basis for future dictionary attacks.