Storing salt+password hash in DB and protecting against password attack

Question

Please help me with my understanding. Also I am not talking about SSL or DH key exchange. As the salt is stored in DB and is a secret to the attacker to just protect the user original password (Rainbow tables), in case attacker gets their hand on the actual DB itself. Then how will how you protect against brute/dictionary based attacks. Once again, logging the wrong requests and denying IP of many bad request is known, I am talking about cryptography here. As the password is same for user1, attacker got it from other websites, how does salt protects here. I guess not, then what are the best solutions available to stop such attacks. Assume data is really important like credit card numbers + CVV(I know don't store CVV, but that is not the question).

EDIT: By the way, I came up with some stupid idea, and it looks like a known method for stopping dictionary attacks. Read more this question: High cost encryption but less cost decryption

May be we can discuss some other methods here, to protect against brute/dictionary/social engineering password attack

Answer 1

It's a little unclear to me what your actual question is, but if it is "How does a salt help protect me against brute force attacks?" the answer is that technically it does not. There is nothing about a salt that makes brute force attacks more difficult, salts instead make it difficult to brute force multiple accounts simultaneously. Essentially salts artificially inflate search space required to do a brute force attack, making it computationally difficult to pre-calculate every possible password and then check them against the entire database. Salts can be stored in the clear, so long as they are unique to each password.

If you want to make brute forcing passwords more difficult, what you want is an adaptive hashing scheme. These schemes allow you to dictate how long hashing should take. Because an honest client should only have to authenticate on the order of tens of times, but an attacker will need to do it on the order of millions or billions of times, slower hashes make the task near impossible for the attacker while introducing little overhead in the system.

What this all boils down to is that you should use bcrypt if you are hashing passwords. It is designed to incorporate a salt and is an adaptive hashing system. For more info, see this article on security.stackexchange.com

Answer 2

The purpose of a salt is not to prevent dictionary attacks; it is to prevent precomputation attacks such as rainbow tables. Having a salt requires the attacker to attack each password individually, after they gain access to the database; they can't precompute hashes for passwords in the dictionary, or reuse this effort across users.

Password Stretching is a way to make dictionary attacks more difficult, by increasing the amount of work the attacker has to do to test each candidate password.

Answer 3

Without salt, an attacker can use an offline attack to precalculate the hash of common passwords: "secret" "qwerty" etc. No salt allows an attacker to tell when different users are using the same password, as they will have the same hashes. Salt prevents precalculation and avoids the matching hash problem.

An attacker with access to the datbase will also have access to the salts. She will need to attack each password separately, because of the different salts.

Using stretching (repeated hashing) can also slow down an attacker. Rather than storing hash(password + salt) you store hash^n(password + salt), where n is large enough for the overall calculation to take at least 0.1 second . That limits the attacker to around ten trials a second while having no discernible impact on the user.

Answer 4

About salt : If you search the "MD5" encrypted password using search engine like google, here you may find the original plain password. But if you mix the salt in your plain password and then apply "MD5" encryption, you wont be able to find it. If any hacker anyhow hacks your database and if you are using just MD5 encryption then he may use above method to hack passwords. For e.g. Search this string on google : 5f4dcc3b5aa765d61d8327deb882cf99, you'll get original password string. Salt is mainly added to protect against such attacks.

Check out here. Look at Just content and concept here to understand. This is from Spring security docs.