A dictionary attack is form of lookup attack used to crack passwords even when the passwords have been sent through a hash function. The attack is conducted by first obtaining hashed passwords, then comparing the hash against a dictionary (a table of popular passwords that have already been hashed using the same function). For example, in the table below, some commonly used passwords have been sent through an MD5 hash function. Even though you can't reverse the hash to derive the original password, you can compare the hash in the dictionary against the hash from the password file to determine the password.
Note that MD5 is not a cryptographically secure hash function and should never be used to store password.
Dictionary attacks are made more difficult to use if the hash function employs a salt value, but the if the salt value is discovered, the dictionary can be regenerated with the same salt.
- en.wikipedia.org/wiki/Dictionary_attack - Wikipedia.