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In cryptanalysis and computer security, a dictionary attack is a technique for defeating a cipher or authentication mechanism by trying to determine its decryption key or passphrase by searching likely possibilities. In contrast with a 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 in a dictionary. Generally, dictionary attacks succeed because many people have a tendency to choose passwords which are short (7 characters or fewer), single words in a dictionary, or are simple variations that are easy to predict, such as appending a single digit to a word.
Dictionary attacks may be applied in two main situations:
- in cryptanalysis, in trying to determine the decryption key for a given piece of ciphertext;
- in computer security, in trying to circumvent an authentication mechanism for accessing a computer system by guessing passwords.
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 challenge-response authentication exchange between two parties. If an attacker can obtain the hashed password, they can test guessed passwords rapidly, often at a rate of tens or hundreds of millions of guesses per second.  The rate of guessing can be sharply reduced by using a key derivation function 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 email@example.com, firstname.lastname@example.org, email@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 tables." 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
From Wikipedia, the free encyclopedia
(Redirected from Mail server
A mail transfer agent (MTA) (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon)), is a computer program or software agent that transferselectronic mail messages from one computer to another.
The term mail server is also used to mean a computer acting as an MTA that is running the appropriate software. The term mail exchanger (MX), in the context of the Domain Name System formally refers to an IP address assigned to a device hosting a mail server, and by extension also indicates the server itself.
An MTA receives mail from another MTA (relaying) or from a mail user agent (MUA). The MTA works behind the scenes, while the user usually interacts with the MUA. Every time an MTA receives an e-mail, it will add a "Received:" trace header field to the top of the message. In this way, there is a record of which MTAs handled the e-mail and in which order. Upon final delivery, the "Return-Path:" header will also be added to record the return path.
The delivery of e-mail to a user's mailbox typically takes place via a mail delivery agent (MDA); many MTAs have basic MDA functionality built in, but a dedicated MDA like procmail can provide more sophisticated functionality.
According to one survey, sendmail, Microsoft Exchange Server, Postfix, and Exim together control over 85% of market share for SMTP service.
Another survey suggests a more balanced playing field, though it included hosted e-mail services such as Postini.