Salt the passwords

This note is intended to shed light on the use of salt in password hashing. If you look at the comments on this topic habrahabr.ru/post/145642 , then there is a suspicion that some people misunderstand the purpose of salt. On examples I will try to show what this mechanism is used for. All interested in asking under the cat.

Imagine a simple authorization. A bunch of login / password values ​​come to us from the user, we get a password hash and compare this bunch with the data stored in the database. For simplicity, we will use MD5 and PHP code examples.

$password = md5($password); 

In this case, if the user has the qwerty password, we will get the following hash: d8578edf8458ce06fbc5bb76a58c5ca4 . If an attacker gains access to our database, he can use ready-made services ( http://wordd.org/D8578EDF8458CE06FBC5BB76A58C5CA4 ) for selecting passwords, which already have values ​​that give the hash, or you can download it yourself.
To protect the already existing hash tables with values, you can use a static salt:

  $password = md5($password . "MyUniqueSault"); 

Now with the same qwerty password we get a completely different hash bdadb0330124cda0e8499c9cd118f7bd . Ready-made tables will no longer help the attacker, he will have to use brute force. Here lies the minus of static salt: an attacker can generate his hash table with static salt and get the values ​​of most passwords from the database. To eliminate this disadvantage, a unique salt is used for each hash:

  $sault = GenerateRandomString(); $password = md5($password . $sault); 

Those. now, in addition to the login / password hash, the database will need to store the value of the generated salt for each user. Let us consider an example: we have two users: user1 and user2. Both use the qwerty password. But the first one generated salt zxcv and the second asdf . As a result, users with the same password will have different hashes: 1d8f3272b013387bbebcbedb4758586d and a192862aa3bf46dffb57b12bdcc4c199 . This gives: now you cannot generate a single hash table, to find the hash value with the dynamic salt you will need to generate All this is aimed at increasing the time of selection of values ​​in the case of a “drain” of the base, when using “good” hashing algorithms, at least a couple of passwords can take a significant amount of time to select. It is important to understand that the generated salt protects not one single user, but all of them from the mass brutus. That's all, I want to remind you that use crypto-resistant hashing algorithms SHA1, SHA512. The MD5 used above is not desirable because declared obsolete.

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Kolonist summarized well in his comment habrahabr.ru/post/145648/#comment_4894759 (for which he has a special thank you and a plus):
Again.

1. No salt - use ready-made rainbow tables.
2. There is one salt for all - we generate one rainbow table and “break” all users using it.
3. There is a separate salt for each user - separately brutforsim each user.