| Dictionary Attack |
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USE Dictionary attacks may be applied in two main situations:
In the latter case, the effect of a dictionary attack can be greatly reduced by limiting the number of authentication attempts that can be performed each minute, and even blocking further attempts after a threshold of failed authentication attempts is reached. Generally, 6 attempts is considered sufficient to cope with mistakes made by legitimate users; beyond that, one can safely assume that the user is a malicious attacker. However many systems store a hashed version of the password and make it available under certain circumstances, such as a that is computationally intensive, such as PBKDF2 . Since users often choose easily guessed passwords, this has historically succeeded more than 2 times out of 10 when a reasonably large list is used. Lists of commonly selected passwords are widely available on the Internet as are dictionaries for most human languages (even those no longer used), meaning even the use of foreign words has limited value in preventing dictionary attacks. Spammers often use a form of dictionary attack, sometimes known as a Directory Harvest Attack , for E-mail Address Harvesting . For example, a Spammer may try sending messages to adam@example.com, barbara@example.com, carl@example.com, etc. Any addresses to which messages are delivered, as opposed to being bounced back, can be added to the spammer's list of known-valid addresses. Clifford Stoll 's book, '' The Cuckoo's Egg '', contains an account of a dictionary attack against the encrypted passwords kept in the Passwd file on Unix systems, and of the reaction to the successful attack by the man ( Robert Morris ) who invented the one-way encryption system used for login passwords. PRE-COMPUTED DICTIONARY ATTACK It is possible to achieve a Time-space Tradeoff through precomputation by encrypting and storing a list of encrypted dictionary words, sorted by the encrypted value. This requires a considerable amount of preparation time, but makes the actual attack almost instantaneous. The storage requirements for the pre-computed tables were once a major cost, but are less of an issue today due to the rapid improvements in Hard Drive technology. Pre-computed dictionary attack are particularly effective when a large number of passwords are to be cracked at once. A more refined approach involves the use of " Rainbow Table s." Salting is a technique that forces the encrypted dictionary to be recomputed for each password sought, potentially making precomputation infeasable, provided the salt is large enough. SEE ALSO
EXAMPLES Well known examples of dictionary attack software tools:
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