Are salts useless for security if the attacker kno

问题:

Let's say I have a table of users set up like this:

CREATE TABLE `users` (
    `id` INTEGER PRIMARY KEY,
    `name` TEXT,
    `hashed_password` TEXT,
    `salt` TEXT
)

When a user is created, a randomly-generated salt is produced and stored in the database alongside the results of something like get_hash(salt + plaintext_password).

I'm wondering that if a malicious user gets their hands on this data, would they be able to use it to crack users's passwords? If so, what's a way that it could be prevented?

回答1:

No, they're not useless.

So long as you use a unique salt for each row, then the salt will prevent slow down an attack. The attacker will need to mount a brute force attack, rather than using rainbow tables against the password hashes.

As mentioned in the comments, you should ensure that the salt is a sensible size.

回答2:

Salting was introduced (or at least made popular) in UNIX /etc/passwd file, which was world-readable. It is usually assumed that the salt as well as the encrypted password is known to the cracker. The purpose of the salt is the slow-down of the cracking process (since the same password won't map to the same encrypted string); it is not a secret in itself.

回答3:

Knowing the salt makes it possible to do a brute-force attack, but that doesn't make it useless. Salt prevents the attacker from using an already generated rainbow table (which you could find on the web).

The best way to prevent brute-forcing is simply to use long, complex passwords.

回答4:

If an attacker knows the salt, the hashed password and the hash algorithm, then they can mount a brute-force dictionary attack (or rainbow attack).

回答5:

This should give you an idea of how it works.

Lets say you want to encrypt a word "secret." After it is encrypted lets say it now looks like this 00110010.

If a hacker knows the encryption algorithm, they can create a table of words and their corresponding encrypted values. So they take the encrypted password "00110010" and find it in the table. Now they know that the password used to generate "00110010" was the word "secret." If you salt the word first, then a generic lookup table will be useless to the hacker. (A generic lookup table being a table of unsalted dictionary words and their encrypted values)

If you salt the word first ("saltsecret"), now the encrypted value will look different, and the hacker wont find it in the lookup table.

However, they can still start creating their own lookup table from scratch using your salt and eventually they will be able to reverse lookup passwords.

So to answer the question, if the passwords are sufficiently complex, it will take ages for the hacker to figure them out. You could change your salt every year and they would have to start creating a table all over again.

回答6:

No, it's not worthless.

To successfully attack an account, an attacker needs to know the salt for that account (and every account's salt should be different), the hashing algorightm used, and the final stored password hash.

Given all of that information, you can write a program that keeps trying to hash different potential passwords until it finds one that matches.

If it's a bad salt (too simple or short), this can be made much faster because the program can use rainbow lookup tables to match the final stored password hash to the string that was hashed, and then just subtract the salt. But they still need all the information.

If it's a shared salt, this is bad because an attacker and use the salt to generate a rainbow table in advance that's good for any account on your system.

回答7:

Assuming brute force attack of MD5,SHA1,SHA256 algorithms with GPU has a throughput greater than 1 billion of tries per second and SHA512 around 300M/s. If you use one of these algorithms, it will slow down hacker who used rainbow table (less likely), but it will not slow down hacker who used brute force attack (more likely). It will definitively not protect you, it just add a bit of security against outdated rainbow table (for these algo). A bit is better than nothing.

But if you use a strongest algorithm (eg. bcrypt), salt definitively worth it even if stored with hash because brut force is not feasible in term of time so rainbow make sense.

Have a look at this article and to summarize:

If you are a user:

Make sure all your passwords are 12 characters or more, ideally a lot more. I recommend adopting pass phrases, which are not only a lot easier to remember than passwords (if not type) but also ridiculously secure against brute forcing purely due to their length.

If you are a developer:

Use bcrypt or PBKDF2 exclusively to hash anything you need to be secure. These new hashes were specifically designed to be difficult to implement on GPUs. Do not use any other form of hash. Almost every other popular hashing scheme is vulnerable to brute forcing by arrays of commodity GPUs, which only get faster and more parallel and easier to program for every year.

Posted by Jeff Atwood