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.
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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.