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copied!<p>The point of the salt is to prevent the attacker from performing parallel attacks. That parallelism must be understood both space- and time-wise; roughly speaking, this means sharing the attack cost between two or more attacks.</p> <p>For instance, consider a non-salted hashed password setting. The attacker can hash all words in a dictionary for a cost proportional to the size of the dictionary, and check those hashed words with regards to <em>several</em> hashed passwords. This can be simultaneous (the attacker has a list of hashed passwords and wants to crack one) or iterative (the attacker precomputes his hashed dictionary, then uses it as a tool against several passwords in distinct systems). Either way, this is cost sharing.</p> <p>The <em>salt</em> is some data which should be somewhat unique to each hashed password instance. Salting prevents such cost sharing, to the extent of the uniqueness of the salt.</p> <p>Using the user name (or hash thereof) as a salt leverages user name uniqueness: usually, <em>on a given system at a given time</em>, user names are unique. This prevents locally space-wise sharing: if the attacker gets a snapshot of all hashed passwords, he cannot attack them in parallel with cost sharing; he will have to incur the hashed dictionary cost for every attacked password. However, this does not prevent time-wise sharing (the attacker precomputes a hashed dictionary with the salt corresponding to user "bob" and will regularly try to guess Bob's password, assuming that Bob changes his password on a regular basis, e.g. because this is mandated by his system administrator). This does not prevent either some global sharing (there are several -- many -- systems out there, with a user going under the name of "bob").</p> <p>So using the user name as salt is not bad; this is better than using no salt at all. But a random salt is still better, because it will change even in situations where the the user name is kept unchanged (a user changing his password; two users on distinct systems with the same name).</p>

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