At work we have two competing theories for salts. The products I work on use something like a user name or phone number to salt the hash. Essentially something that is different for each user but is readily available to us. The other product randomly generates a salt for each user and changes each time the user changes the password. The salt is then encrypted in the database.
My question is if the second approach is really necessary? I can understand from a purely theoretical perspective that it is more secure than the first approach, but what about from a practicality point of view. Right now to authenticate a user, the salt must be unencrypted and applied to the login information.
After thinking about it, I just don't see a real security gain from this approach. Changing the salt from account to account, still makes it extremely difficult for someone to attempt to brute force the hashing algorithm even if the attacker was aware of how to quickly determine what it was for each account. This is going on the assumption that the passwords are sufficiently strong. (Obviously finding the correct hash for a set of passwords where they are all two digits is significantly easier than finding the correct hash of passwords which are 8 digits). Am I incorrect in my logic, or is there something that I am missing?
EDIT: Okay so here's the reason why I think it's really moot to encrypt the salt. (lemme know if I'm on the right track).
For the following explanation, we'll assume that the passwords are always 8 characters and the salt is 5 and all passwords are comprised of lowercase letters (it just makes the math easier).
Having a different salt for each entry means that I can't use the same rainbow table (actually technically I could if I had one of sufficient size, but let's ignore that for the moment). This is the real key to the salt from what I understand, because to crack every account I have to reinvent the wheel so to speak for each one. Now if I know how to apply the correct salt to a password to generate the hash, I'd do it because a salt really just extends the length/complexity of the hashed phrase. So I would be cutting the number of possible combinations I would need to generate to "know" I have the password + salt from 13^26 to 8^26 because I know what the salt is. Now that makes it easier, but still really hard.
So onto encrypting the salt. If I know the salt is encrypted, I wouldn't try and decrypt (assuming I know it has a sufficient level of encryption) it first. I would ignore it. Instead of trying to figure out how to decrypt it, going back to the previous example I would just generate a larger rainbow table containing all keys for the 13^26. Not knowing the salt would definitely slow me down, but I don't think it would add the monumental task of trying to crack the salt encryption first. That's why I don't think it's worth it. Thoughts?
Here is a link describing how long passwords will hold up under a brute force attack:
http://www.lockdown.co.uk/?pg=combi
Hiding a salt is unnecessary.
A different salt should be used for every hash. In practice, this is easy to achieve by getting 8 or more bytes from cryptographic quality random number generator.
From a previous answer of mine:
Salt helps to thwart pre-computed dictionary attacks.
Suppose an attacker has a list of likely passwords. He can hash each
and compare it to the hash of his victim's password, and see if it
matches. If the list is large, this could take a long time. He doesn't
want spend that much time on his next target, so he records the result
in a "dictionary" where a hash points to its corresponding input. If
the list of passwords is very, very long, he can use techniques like a
Rainbow Table to save some space.
However, suppose his next target salted their password. Even if the
attacker knows what the salt is, his precomputed table is
worthless—the salt changes the hash resulting from each password. He
has to re-hash all of the passwords in his list, affixing the target's
salt to the input. Every different salt requires a different
dictionary, and if enough salts are used, the attacker won't have room
to store dictionaries for them all. Trading space to save time is no
longer an option; the attacker must fall back to hashing each password
in his list for each target he wants to attack.
So, it's not necessary to keep the salt secret. Ensuring that the
attacker doesn't have a pre-computed dictionary corresponding to that
particular salt is sufficient.
After thinking about this a bit more, I've realized that fooling yourself into thinking the salt can be hidden is dangerous. It's much better to assume the salt cannot be hidden, and design the system to be safe in spite of that. I provide a more detailed explanation in another answer.
However, recent recommendations from NIST encourage the use of an additional, secret "salt" (I've seen others call this additional secret "pepper"). One additional iteration of the key derivation can be performed using this secret as a salt. Rather than increasing strength against a pre-computed lookup attack, this round protects against password guessing, much like the large number of iterations in a good key derivation function. This secret serves no purpose if stored with the hashed password; it must be managed as a secret, and that could be difficult in a large user database.
The answer here is to ask yourself what you're really trying to protect from? If someone has access to your database, then they have access to the encrypted salts, and they probably have access to your code as well. With all that could they decrypt the encrypted salts? If so then the encryption is pretty much useless anyway. The salt really is there to make it so it isn't possible to form a rainbow table to crack your entire password database in one go if it gets broken into. From that point of view, so long as each salt is unique there is no difference, a brute force attack would be required with your salts or the encrypted salts for each password individually.
A hidden salt is no longer salt. It's pepper. It has its use. It's different from salt.
Pepper is a secret key added to the password + salt which makes the hash into an HMAC (Hash Based Message Authentication Code). A hacker with access to the hash output and the salt can theoretically brute force guess an input which will generate the hash (and therefore pass validation in the password textbox). By adding pepper you increase the problem space in a cryptographically random way, rendering the problem intractable without serious hardware.
For more information on pepper, check here.
See also hmac.
My understanding of "salt" is that it makes cracking more difficult, but it doesn't try to hide the extra data. If you are trying to get more security by making the salt "secret", then you really just want more bits in your encryption keys.
The second approach is only slightly more secure. Salts protect users from dictionary attacks and rainbow table attacks. They make it harder for an ambitious attacker to compromise your entire system, but are still vulnerable to attacks that are focused on one user of your system. If you use information that's publicly available, like a telephone number, and the attacker becomes aware of this, then you've saved them a step in their attack. Of course the question is moot if the attacker gets your whole database, salts and all.
EDIT: After re-reading over this answer and some of the comments, it occurs to me that some of the confusion may be due to the fact that I'm only comparing the two very specific cases presented in the question: random salt vs. non-random salt. The question of using a telephone number as a salt is moot if the attacker gets your whole database, not the question of using a salt at all.
... something like a user name or phone number to salt the hash. ...
My question is if the second approach is really necessary? I can understand from a purely theoretical perspective that it is more secure than the first approach, but what about from a practicality point of view?
From a practical point of view, a salt is an implementation detail. If you ever change how user info is collected or maintained – and both user names and phone numbers sometimes change, to use your exact examples – then you may have compromised your security. Do you want such an outward-facing change to have much deeper security concerns?
Does stopping the requirement that each account have a phone number need to involve a complete security review to make sure you haven't opened up those accounts to a security compromise?
Here is a simple example showing why it is bad to have the same salt for each hash
Consider the following table
UserId UserName, Password
1 Fred Hash1 = Sha(Salt1+Password1)
2 Ted Hash2 = Sha(Salt2+Password2)
Case 1 when salt 1 is the same as salt2
If Hash2 is replaced with Hash1 then user 2 could logon with user 1 password
Case 2 when salt 1 not the same salt2
If Hash2 is replaced with Hash1 then user2 can not logon with users 1 password.
There are two techniques, with different goals:
The "salt" is used to make two otherwise equal passwords encrypt differently. This way, an intruder can't efficiently use a dictionary attack against a whole list of encrypted passwords.
The (shared) "secret" is added before hashing a message, so that an intruder can't create his own messages and have them accepted.
I tend to hide the salt. I use 10 bits of salt by prepending a random number from 1 to 1024 to the beginning of the password before hashing it. When comparing the password the user entered with the hash, I loop from 1 to 1024 and try every possible value of salt until I find the match. This takes less than 1/10 of a second. I got the idea to do it this way from the PHP password_hash and password_verify. In my example, the "cost" is 10 for 10 bits of salt. Or from what another user said, hidden "salt" is called "pepper". The salt is not encrypted in the database. It's brute forced out. It would make the rainbow table necessary to reverse the hash 1000 times larger. I use sha256 because it's fast, but still considered secure.
Really, it depends on from what type of attack you're trying to protect your data.
The purpose of a unique salt for each password is to prevent a dictionary attack against the entire password database.
Encrypting the unique salt for each password would make it more difficult to crack an individual password, yes, but you must weigh whether there's really much of a benefit. If the attacker, by brute force, finds that this string:
Marianne2ae85fb5d
hashes to a hash stored in the DB, is it really that hard to figure out what which part is the pass and which part is the salt?
Related
Let's assume for each user, we store the user's password hash and a unique salt in the same table. For example, it looks like this:
user_email, sha256(raw_password+salt), salt
AFAIK, this is a conventional practice and considered safe because it prevents attacks using rainbow tables. The reasoning is since attackers don't have a precomputed list of sha256(raw_password+salt) they are forced to recompute this for every row, and this will take a lot of time.
But I don't understand the reasoning above. According to this old post, one core can run sha256 more than 20M times per second. Doesn't that make it trivial for attackers to just recompute sha256(raw_password+salt) for all rows if the entire users table is compromised?
Related:
Is It okay to save user's salt in the same table as password hash?
As John notes, your example is incorrect:
user_email, sha256(raw_password+salt), salt
This is not a good way to store passwords. You should replace sha256 here with a Key Derivation Function (KDF) such as PBKDF2 or scrypt. Then it would be fine. A properly tuned KDF can get the hashing rate down to dozens a second or fewer, even on good hardware (there are various competing factors here because the attacker doesn't have the same language and hardware restrictions you likely do, but even in the worst cases this value can be kept very low in cryptographic terms).
But even if you used sha256 here, it would be dramatically stronger than an unsalted hash. It makes every hash different. This means that if multiple people have the same password (very common), then breaking one doesn't break all users having the same password. This protects against rainbow tables, and particularly protects people who have very common passwords (password, dragon, mustang, etc.)
But it also protects against other password-collision attacks. For example, say I want to know Alice's password, and I can see it has the same hash as Bob. I now know that tricking either of them into revealing their password through some means will reveal both of their passwords.
Doesn't that make it trivial for attackers to just recompute sha256(raw_password+salt) for all rows if the entire users table is compromised?
This is thinking about the problem backwards. If the attacker knew raw_password, then yes, this would be trivial. But that's exactly the thing the attacker does not know (and if they did, they wouldn't need to do any hashing). So the attacker must make a full search of each row of the database, which even with just a single SHA-256 is quite slow.
There are approximately 96 characters you can easily type on most English keyboards. The complete space of those for an 8-character string is 96^8 or about 7x10^15. At 20M per second, that's about 360M seconds or roughly 11 CPU-years per row. That's not an impossible space to crack, but it's still not fast. (Obviously there are many thing pushing in both directions; users don't choose passwords randomly, but they also aren't limited to 8 characters. This computation is just for illustration.)
A key take-away is that knowing the salt gives you no information at all about sha256(salt+password) if you don't know the password, too. That's a key feature of all cryptographic hashes (including the SHA series). If knowing part of the data gave you any information about the hash of the entire data, then that would tell us that the hash isn't secure.
The title says everything. I don't understand: why you shouldn't keep your salt a secret like the password. Or did I misunderstand something?
The salt is treated as public primarily because keeping it secret isn't necessary.
The point of salt is primarily to make dictionary attacks more difficult/less practical. In a dictionary attack, the attacker hashes common words from a dictionary, and (if he's serious at all) supplements those with things like common names. Equipped with this, if he can get a hold of your list of hashed passwords, he can see if any of them matches a hash in his list. Assuming you have a significant number of users, he has a pretty good chance of finding at least one. When he does, he looks in his list to find what word produced that hash, and he can now use it to log in and impersonate that user.
Adding a salt means that instead of doing this once, he has to do it once for each possible salt value. For example, if you use a 24-bit salt, he has to hash each word in the dictionary ~16 million times, and store the results of all ~16 million hashes.
Just for the sake of argument, let's assume that without salt, it would take the attacker 8 hours to hash all the candidate words, and 16 megabytes to store the results (hashes and word that produced each). We'll further assume that the storage is equally divided between the hashes themselves and the list of words/names/whatever that produced them.
Using the same 24-bit salt, that means his time is multiplied by the same factor of ~16 million. His storage for the words that produced the hashes remains the same, but for the hashes themselves is (again) multiplied by the ~16 million. Working out the math, those come out to approximately 15,000 years of computation and 128 terabytes of storage.
In short, without salt, a dictionary attack is within easy reach of almost anybody. I could easily believe that (for example) somebody would let their computer run overnight to do the hashing just to pull a good April fools joke on a few of his co-workers (easy to believe, because I've seen it done).
When you get down to it, it's all a numbers game: a dictionary attack isn't betting that every user will have a password that's easy to guess, only that enough will for them to find at least a few open holes. Likewise, making the salt public does allow a somewhat simpler attack, by downloading the salt for each hash, and doing individual dictionary attacks on each, using the known salt for each one. Assuming a system has fewer users than possible hash values, this is a more practical attack. Nonetheless, he's now stuck with attacking each password individually, rather than using a single dictionary not only for an entire system, but in fact for all systems he might want to attack that use the same hash algorithm.
In summary: salt can do its job perfectly well even though it's made public. One of the aims of almost any security system is to minimize the amount of information that needs to be kept secret. Since salt can work even if it is public, it's generally assumed to be public knowledge. In a practical system, you certainly don't try to publish it to attackers, but you don't (shouldn't, anyway) rely on its remaining a secret either.
The purpose of salt is making an attack on several crypted passwords at the same time harder. It doesn't make an attack on a single crypted password harder.
With a salt, an attacker has to test each candidate plaintext password once for every different salt.
The reason as I found in this article is, that you actually need the salt to check an incoming password with the salted and hashed one in your database.
You should keep your salt a secret for the same reason that you salt in the first place.
Hackers can and have created Rainbow Tables whereby they hash using (md5, sha1, sha256, sha512, etc.) a list of the top 1,000 or so most common passwords.
If a hacker manages to get a hold of your database... its good that your passwords are hashed, but if they do a quick comparison and find a hash that matches one they have in their list, they know what the password is for that account.
The key to them doing the hack, is having that rainbow table handy. If you've added a salt, their rainbow table is useless... but if you make the salt east to find or you share it with others, then the hackers can re-build a new rainbow table using your salt.(*) e.g. you've made it easier for them to hack.
(*) Note this is a little harder than described, since the hacker may not know if you added the salt as a prefix, suffix, both, etc.
As said above, unique secret salt for each password will prevent anyone from pre-computing the hashes in a rainbow table; this is the sole purpose of unique salts.
As I understand it, the best practice for generating salts is to use some cryptic formula (or even magic constant) stored in your source code.
I'm working on a project that we plan on releasing as open source, but the problem is that with the source comes the secret formula for generating salts, and therefore the ability to run rainbow table attacks on our site.
I figure that lots of people have contemplated this problem before me, and I'm wondering what the best practice is. It seems to me that there is no point having a salt at all if the code is open source, because salts can be easily reverse-engineered.
Thoughts?
Since questions about salting hashes come along on a quite regular basis and there seems to be quite some confusion about the subject, I extended this answer.
What is a salt?
A salt is a random set of bytes of a fixed length that is added to the input of a hash algorithm.
Why is salting (or seeding) a hash useful?
Adding a random salt to a hash ensures that the same password will produce many different hashes. The salt is usually stored in the database, together with the result of the hash function.
Salting a hash is good for a number of reasons:
Salting greatly increases the difficulty/cost of precomputated attacks (including rainbow tables)
Salting makes sure that the same password does not result in the same hash.
This makes sure you cannot determine if two users have the same password. And, even more important, you cannot determine if the same person uses the same password across different systems.
Salting increases the complexity of passwords, thereby greatly decreasing the effectiveness of both Dictionary- and Birthday attacks. (This is only true if the salt is stored separate from the hash).
Proper salting greatly increases the storage need for precomputation attacks, up to the point where they are no longer practical. (8 character case-sensitive alpha-numeric passwords with 16 bit salt, hashed to a 128 bit value, would take up just under 200 exabytes without rainbow reduction).
There is no need for the salt to be secret.
A salt is not a secret key, instead a salt 'works' by making the hash function specific to each instance. With salted hash, there is not one hash function, but one for every possible salt value. This prevent the attacker from attacking N hashed passwords for less than N times the cost of attacking one password. This is the point of the salt.
A "secret salt" is not a salt, it is called a "key", and it means that you are no longer computing a hash, but a Message Authentication Code (MAC). Computing MAC is tricky business (much trickier than simply slapping together a key and a value into a hash function) and it is a very different subject altogether.
The salt must be random for every instance in which it is used. This ensures that an attacker has to attack every salted hash separately.
If you rely on your salt (or salting algorithm) being secret, you enter the realms of Security Through Obscurity (won't work). Most probably, you do not get additional security from the salt secrecy; you just get the warm fuzzy feeling of security. So instead of making your system more secure, it just distracts you from reality.
So, why does the salt have to be random?
Technically, the salt should be unique. The point of the salt is to be distinct for each hashed password. This is meant worldwide. Since there is no central organization which distributes unique salts on demand, we have to rely on the next best thing, which is random selection with an unpredictable random generator, preferably within a salt space large enough to make collisions improbable (two instances using the same salt value).
It is tempting to try to derive a salt from some data which is "presumably unique", such as the user ID, but such schemes often fail due to some nasty details:
If you use for example the user ID, some bad guys, attacking distinct systems, may just pool their resources and create precomputed tables for user IDs 1 to 50. A user ID is unique system-wide but not worldwide.
The same applies to the username: there is one "root" per Unix system, but there are many roots in the world. A rainbow table for "root" would be worth the effort, since it could be applied to millions of systems. Worse yet, there are also many "bob" out there, and many do not have sysadmin training: their passwords could be quite weak.
Uniqueness is also temporal. Sometimes, users change their password. For each new password, a new salt must be selected. Otherwise, an attacker obtained the hash of the old password and the hash of the new could try to attack both simultaneously.
Using a random salt obtained from a cryptographically secure, unpredictable PRNG may be some kind of overkill, but at least it provably protects you against all those hazards. It's not about preventing the attacker from knowing what an individual salt is, it's about not giving them the big, fat target that will be used on a substantial number of potential targets. Random selection makes the targets as thin as is practical.
In conclusion:
Use a random, evenly distributed, high entropy salt. Use a new salt whenever you create a new password or change a password. Store the salt along with the hashed password. Favor big salts (at least 10 bytes, preferably 16 or more).
A salt does not turn a bad password into a good password. It just makes sure that the attacker will at least pay the dictionary attack price for each bad password he breaks.
Usefull sources:
stackoverflow.com: Non-random salt for password hashes
Bruce Schneier: Practical Cryptography (book)
Matasano Security: Enough with the Rainbow Tables
usenix.org: Unix crypt used salt since 1976
owasp.org: Why add salt
openwall.com: Salts
Disclaimer:
I'm not a security expert. (Although this answer was reviewed by Thomas Pornin)
If any of the security professionals out there find something wrong, please do comment or edit this wiki answer.
Really salts just need to be unique for each entry. Even if the attacker can calculate what the salt is, it makes the rainbow table extremely difficult to create. This is because the salt is added to the password before it is hashed, so it effectively adds to the total number of entries the rainbow table must contain to have a list of all possible values for a password field.
Since Unix became popular, the right way to store a password has been to append a random value (the salt) and hash it. Save the salt away where you can get to it later, but where you hope the bad guys won't get it.
This has some good effects. First, the bad guys can't just make a list of expected passwords like "Password1", hash them into a rainbow table, and go through your password file looking for matches. If you've got a good two-byte salt, they have to generate 65,536 values for each expected password, and that makes the rainbow table a lot less practical. Second, if you can keep the salt from the bad guys who are looking at your password file, you've made it much harder to calculate possible values. Third, you've made it impossible for the bad guys to determine if a given person uses the same password on different sites.
In order to do this, you generate a random salt. This should generate every number in the desired range with uniform probability. This isn't difficult; a simple linear congruential random number generator will do nicely.
If you've got complicated calculations to make the salt, you're doing it wrong. If you calculate it based on the password, you're doing it WAY wrong. In that case, all you're doing is complicating the hash, and not functionally adding any salt.
Nobody good at security would rely on concealing an algorithm. Modern cryptography is based on algorithms that have been extensively tested, and in order to be extensively tested they have to be well known. Generally, it's been found to be safer to use standard algorithms rather than rolling one's own and hoping it's good. It doesn't matter if the code is open source or not, it's still often possible for the bad guys to analyze what a program does.
You can just generate a random salt for each record at runtime. For example, say you're storing hashed user passwords in a database. You can generate an 8-character random string of lower- and uppercase alphanumeric characters at runtime, prepend that to the password, hash that string, and store it in the database. Since there are 628 possible salts, generating rainbow tables (for every possible salt) will be prohibitively expensive; and since you're using a unique salt for each password record, even if an attacker has generated a couple matching rainbow tables, he still won't be able to crack every password.
You can change the parameters of your salt generation based on your security needs; for example, you could use a longer salt, or you could generate a random string that also contains punctuation marks, to increase the number of possible salts.
Use a random function generator to generate the salt, and store it in the database, make salt one per row, and store it in the database.
I like how salt is generated in django-registration. Reference: http://bitbucket.org/ubernostrum/django-registration/src/tip/registration/models.py#cl-85
salt = sha_constructor(str(random.random())).hexdigest()[:5]
activation_key = sha_constructor(salt+user.username).hexdigest()
return self.create(user=user,
activation_key=activation_key)
He uses a combination of sha generated by a random number and the username to generate a hash.
Sha itself is well known for being strong and unbreakable. Add multiple dimensions to generate the salt itself, with random number, sha and the user specific component, you have unbreakable security!
In the case of a desktop application that encrypts data and send it on a remote server, how do you consider using a different salt each time?
Using PKCS#5 with the user's password, it needs a salt to generate an encryption key, to encrypt the data. I know that keep the salt hardcoded (obfuscated) in the desktop application is not a good idea.
If the remote server must NEVER know the user's password, is it possible to user different salt each time? If the user use the desktop application on another computer, how will it be able to decrypt the data on the remote server if he does not have the key (it is not hardcoded in the software) ?
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?
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.
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.
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.
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).
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.
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.
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
UPDATE: I recently learned from this question that in the entire discussion below, I (and I am sure others did too) was a bit confusing: What I keep calling a rainbow table, is in fact called a hash table. Rainbow tables are more complex creatures, and are actually a variant of Hellman Hash Chains. Though I believe the answer is still the same (since it doesn't come down to cryptanalysis), some of the discussion might be a bit skewed.
The question: "What are rainbow tables and how are they used?"
Typically, I always recommend using a cryptographically-strong random value as salt, to be used with hash functions (e.g. for passwords), such as to protect against Rainbow Table attacks.
But is it actually cryptographically necessary for the salt to be random? Would any unique value (unique per user, e.g. userId) suffice in this regard? It would in fact prevent using a single Rainbow Table to crack all (or most) passwords in the system...
But does lack of entropy really weaken the cryptographic strength of the hash functions?
Note, I am not asking about why to use salt, how to protect it (it doesn't need to be), using a single constant hash (don't), or what kind of hash function to use.
Just whether salt needs entropy or not.
Thanks all for the answers so far, but I'd like to focus on the areas I'm (a little) less familiar with. Mainly implications for cryptanalysis - I'd appreciate most if anyone has some input from the crypto-mathematical PoV.
Also, if there are additional vectors that hadn't been considered, that's great input too (see #Dave Sherohman point on multiple systems).
Beyond that, if you have any theory, idea or best practice - please back this up either with proof, attack scenario, or empirical evidence. Or even valid considerations for acceptable trade-offs... I'm familiar with Best Practice (capital B capital P) on the subject, I'd like to prove what value this actually provides.
EDIT: Some really good answers here, but I think as #Dave says, it comes down to Rainbow Tables for common user names... and possible less common names too. However, what if my usernames are globally unique? Not necessarily unique for my system, but per each user - e.g. email address.
There would be no incentive to build a RT for a single user (as #Dave emphasized, the salt is not kept secret), and this would still prevent clustering. Only issue would be that I might have the same email and password on a different site - but salt wouldnt prevent that anyway.
So, it comes back down to cryptanalysis - IS the entropy necessary, or not? (My current thinking is it's not necessary from a cryptanalysis point of view, but it is from other practical reasons.)
Salt is traditionally stored as a prefix to the hashed password. This already makes it known to any attacker with access to the password hash. Using the username as salt or not does not affect that knowledge and, therefore, it would have no effect on single-system security.
However, using the username or any other user-controlled value as salt would reduce cross-system security, as a user who had the same username and password on multiple systems which use the same password hashing algorithm would end up with the same password hash on each of those systems. I do not consider this a significant liability because I, as an attacker, would try passwords that a target account is known to have used on other systems first before attempting any other means of compromising the account. Identical hashes would only tell me in advance that the known password would work, they would not make the actual attack any easier. (Note, though, that a quick comparison of the account databases would provide a list of higher-priority targets, since it would tell me who is and who isn't reusing passwords.)
The greater danger from this idea is that usernames are commonly reused - just about any site you care to visit will have a user account named "Dave", for example, and "admin" or "root" are even more common - which would make construction of rainbow tables targeting users with those common names much easier and more effective.
Both of these flaws could be effectively addressed by adding a second salt value (either fixed and hidden or exposed like standard salt) to the password before hashing it, but, at that point, you may as well just be using standard entropic salt anyhow instead of working the username into it.
Edited to Add: A lot of people are talking about entropy and whether entropy in salt is important. It is, but not for the reason most of the comments on it seem to think.
The general thought seems to be that entropy is important so that the salt will be difficult for an attacker to guess. This is incorrect and, in fact, completely irrelevant. As has been pointed out a few times by various people, attacks which will be affected by salt can only be made by someone with the password database and someone with the password database can just look to see what each account's salt is. Whether it's guessable or not doesn't matter when you can trivially look it up.
The reason that entropy is important is to avoid clustering of salt values. If the salt is based on username and you know that most systems will have an account named either "root" or "admin", then you can make a rainbow table for those two salts and it will crack most systems. If, on the other hand, a random 16-bit salt is used and the random values have roughly even distribution, then you need a rainbow table for all 2^16 possible salts.
It's not about preventing the attacker from knowing what an individual account's salt is, it's about not giving them the big, fat target of a single salt that will be used on a substantial proportion of potential targets.
Using a high-entropy salt is absolutely necessary to store passwords securely.
Take my username 'gs' and add it to my password 'MyPassword' gives gsMyPassword. This is easily broken using a rainbow-table because if the username hasn't got enough entropy it could be that this value is already stored in the rainbow-table, especially if the username is short.
Another problem are attacks where you know that a user participates in two or more services. There are lots of common usernames, probably the most important ones are admin and root. If somebody created a rainbow-table that have salts with the most common usernames, he could use them to compromise accounts.
They used to have a 12-bit salt. 12 bit are 4096 different combinations. That was not secure enough because that much information can be easily stored nowadays. The same applies for the 4096 most used usernames. It's likely that a few of your users will be choosing a username that belongs to the most common usernames.
I've found this password checker which works out the entropy of your password. Having smaller entropy in passwords (like by using usernames) makes it much easier for rainbowtables as they try to cover at least all passwords with low entropy, because they are more likely to occur.
It is true that the username alone may be problematic since people may share usernames among different website. But it should be rather unproblematic if the users had a different name on each website. So why not just make it unique on each website. Hash the password somewhat like this
hashfunction("www.yourpage.com/"+username+"/"+password)
This should solve the problem. I'm not a master of cryptanalysis, but I sure doubt that the fact that we don't use high entropy would make the hash any weaker.
I like to use both: a high-entropy random per-record salt, plus the unique ID of the record itself.
Though this doesn't add much to security against dictionary attacks, etc., it does remove the fringe case where someone copies their salt and hash to another record with the intention of replacing the password with their own.
(Admittedly it's hard to think of a circumstance where this applies, but I can see no harm in belts and braces when it comes to security.)
If the salt is known or easily guessable, you have not increased the difficulty of a dictionary attack. It even may be possible to create a modified rainbow table that takes a "constant" salt into account.
Using unique salts increases the difficulty of BULK dictionary attacks.
Having unique, cryptographically strong salt value would be ideal.
I would say that as long as the salt is different for each password, you will probably be ok. The point of the salt, is so that you can't use standard rainbow table to solve every password in the database. So if you apply a different salt to every password (even if it isn't random), the attacker would basically have to compute a new rainbow table for each password, since each password uses a different salt.
Using a salt with more entropy doesn't help a whole lot, because the attacker in this case is assumed to already have the database. Since you need to be able to recreate the hash, you have to already know what the salt is. So you have to store the salt, or the values that make up the salt in your file anyway. In systems like Linux, the method for getting the salt is known, so there is no use in having a secret salt. You have to assume that the attacker who has your hash values, probably knows your salt values as well.
The strength of a hash function is not determined by its input!
Using a salt that is known to the attacker obviously makes constructing a rainbow table (particularly for hard-coded usernames like root) more attractive, but it doesn't weaken the hash. Using a salt which is unknown to the attacker will make the system harder to attack.
The concatenation of a username and password might still provide an entry for an intelligent rainbow table, so using a salt of a series pseudo-random characters, stored with the hashed password is probably a better idea. As an illustration, if I had username "potato" and password "beer", the concatenated input for your hash is "potatobeer", which is a reasonable entry for a rainbow table.
Changing the salt each time the user changes their password might help to defeat prolonged attacks, as would the enforcement of a reasonable password policy, e.g. mixed case, punctuation, min length, change after n weeks.
However, I would say your choice of digest algorithm is more important. Use of SHA-512 is going to prove to be more of a pain for someone generating a rainbow table than MD5, for example.
Salt should have as much entropy as possible to ensure that should a given input value be hashed multiple times, the resulting hash value will be, as close as can be achieved, always different.
Using ever-changing salt values with as much entropy as possible in the salt will ensure that the likelihood of hashing (say, password + salt) will produce entirely different hash values.
The less entropy in the salt, the more chance you have of generating the same salt value, as thus the more chance you have of generating the same hash value.
It is the nature of the hash value being "constant" when the input is known and "constant" that allow dictionary attacks or rainbow tables to be so effective. By varying the resulting hash value as much as possible (by using high entropy salt values) ensures that hashing the same input+random-salt will produce many different hash value results, thereby defeating (or at least greatly reducing the effectiveness of) rainbow table attacks.
Entropy is the point of Salt value.
If there is some simple and reproducible "math" behind salt, than it's the same as the salt is not there. Just adding time value should be fine.