A dictionary attack is a method of breaking into a password-protected computer or server by systematically entering every word in a dictionary as a password. A dictionary attack can also be used in an attempt to find the key necessary to decrypt an encrypted message or document.
Dictionary attacks work because many computer users and businesses insist on using ordinary words as passwords. Dictionary attacks are rarely successful against systems that employ multiple-word phrases, and unsuccessful against systems that employ random combinations of uppercase and lowercase letters mixed up with numerals. In those systems, the brute-force method of attack (in which every possible combination of characters and spaces is tried up to a certain maximum length) can sometimes be effective, although this approach can take a long time to produce results.
Vulnerability to password or decryption-key assaults can be reduced to near zero by limiting the number of attempts allowed within a given period of time, and by wisely choosing the password or key. For example, if only three attempts are allowed and then a period of 15 minutes must elapse before the next three attempts are allowed, and if the password or key is a long, meaningless jumble of letters and numerals, a system can be rendered immune to dictionary attacks and practically immune to brute-force attacks.
A form of dictionary attack is often used by spammers. A message is sent to e-mail addresses consisting of words or names, followed by the at symbol (@), followed by the name of a particular domain. Long lists of given names (such as frank, george, judith, or donna) and/or individual letters of the alphabet followed by surnames (such as csmith, jwilson, or pthomas) in combination with a domain name are usually effective.