I have been reading on correct procedures for storing and checking user passwords, and am puzzling a little with salts.
I get the idea that they are to prevent use of such tools as rainbow tables, but to me the idea of storing the salt along with the hash seems a potential security problem. Too much data in one place for my liking.
The idea I have that I would like to run by folks is to use a 'lucky number' to create a salt from a portion of the password hash.
Basically, along with choosing a password, a user would choose a 'lucky number' as well. This number would then be used as the starting index to retrieve a salt from the hashed pass.
So a very basic example would be something like this.
Pass = "Password"
Lucky Number = "4"
Pass Hash = "00003gebdksjh2h4"
Salt Length = "5"
Resulting Salt = "3gebd"
My thinking is that as the 'lucky number' would not need to be stored, the salt would then also take computational time to work out and so make any attacks even more difficult. Plus means storing slightly less data.
It wouldn't add more security. The goal of using rainbow table is to have a map of combinations of dictionary words and relevant hashes. By using a more-or-less distinct salt (which in general shouldn't be found in a dictionary) for each password hash you compute, you force the attacker to generate a new set of rainbow tables for each entry, which would be prohibitively computationally expensive. At no point (except for generating the hash table) the attacker calculates hashes, so your strategy fails here.
On the other hand when using a regular dictionary attack the attacker only needs a constant-time computation to calculate the salt, which he would have to compute only once for many millions or more combinations of hashes he would have to generate. It would only work if it was more computationally expensive than generating all that, but then you'd have to make the same computation each time a user wants to log in, which is not feasible.
Related
According to the mechanism of hash, which is a one-way calculation, can we calculate all the salted hash value to make the password useless?
For example, if we have something like
salt = [possible combinations];
password = [possible combinations]
Can we enumerate all the possibilities of salt as well as password and hash them to make a big table for reverse look-up?
Theoretically this is possible, and it has been done for very short salts, in this case the salt/password combination can be seen as just a longer password.
In reality you very soon reach a threshold, where todays and tomorrows computer won't be able calculate enough permutaions in reasonable time. In contrast to the passwords, salts should be random values and therefore one cannot take shortcuts with dictionaries.
So salts must have a minimum size, and if you choose a unique salt per password with 128-bit (BCrypt), then even for fast hash algorithms which can be calculated with 100Giga/sec one would expect about 5E19 years to find a match.
Let's say we don't use password_hash and use crypt() with sha512 instead to hash passwords. We need to add salt to the password, so an attacker couldn't use a rainbow table attack. Why the salt has to be good and very random as stated in many SO answers? Even if salt differs by a little or not very random, it will still give a totally different hash from others. So, an attacker won't know who uses the same passwords and he still won't be able to create just one rainbow table.
Computing and storing a strong salt requires minimal effort yet reduces the chances of a rainbow table having being pre-computed with the salt astronomically small.
If the salt was a 3 digit number it would be feasible that an attacker could have pre-computed rainbow tables for all possible salt combinations. If the salt is a random 24 character alpha-numeric string then the chances an attacker could pre-compute this for all possible salts are practically zero.
A salt is supposed to be unique, must be long enough, and should be unpredictable. Randomness is not necessary, but it is the easiest way for a computer to meet those requirements. And it is not the purpose of a salt to be secret, a salt fulfills its purpose even when known.
Uniqueness means that it should not only be unique in your database (otherwise you could use a userid), it should be unique worldwide. Somebody could create rainbowtables for salts like e.g. 1-1000 and would be able to retrieve passwords for all accounts with those userids (often admin accounts have low userids).
Long enough: If the salt is too short (too few possible combinations), it becomes profitable again to build rainbow-tables. Salt and password together can then be seen as just a longer password, and if you can build a rainbow-table for this longer passwords, you also get the shorter original passwords. For very strong and long passwords, salting would actually not be necessary at all, but most human generated passwords can be brute-forced because they are short (people have to remember them).
Also using salts derrived from other parameters can fall into this category. Only because you calculate a hash from the userid, this doesn't increase the possible combinations.
Unpredictability is a bit less important, but imagine once more the case that you use the userid as salt, an attacker can find out what the next few userids will be, and can therefore precalculate a narrow number of rainbow-tables. Depending of the used hash-algorithm this can be applicable or not. He has a time advantage then, can retrieve the password immediately. More of a problem will be, if the admin accounts used a predictable salt.
So using a really random number, generated from the OS random source (dev/urandom), is the best you can do. Even when you ignore all reasons above, why should you use a derived salt when there is a better way, why not use the best way you know?
Hearing about all the recent hacks at big tech firms, it made me wonder their use of password storage.
I know salting + hashing is accepted as being generally secure but ever example I've seen of salting has the salt key hard-coded into the password script which is generally stored on the same server.
So is it a logical solution to hash the user's password initially, pass that hash to a "salting server" or some function stored off-site, then pass back the salted hash?
The way I I'm looking at it is, if an intruder gains access to the server or database containing the stored passwords, they won't immediately have access to the salt key.
No -- salt remains effective even if known to the attacker.
The idea of salt is that it makes a dictionary attack on a large number of users more difficult. Without salt, the attacker hashes all the words in a dictionary, and sees which match with your users' hashed paswords. With salt, he has to hash each word in the dictionary many times over (once for each possible hash value) to be certain of having one that fits each user.
This multiplication by several thousand (or possibly several million, depending on how large a salt you use) increases the time to hash all the values, and the storage need to store the results -- the point that (you hope) it's impractical.
I should add, however, that in many (most?) cases, a very large salt doesn't really add a lot of security. The problem is that if you use, say, a 24 bit salt (~16 million possible values) but have only, say, a few hundred users, the attacker can collect the salt values you're actually using ahead of time, then do his dictionary attack for only those values instead of the full ~16 million potential values. In short, your 24-bit salt adds only a tiny bit of difficulty beyond what a ~8 bit salt would have provided.
OTOH, for a large server (Google, Facebook, etc.) the story is entirely different -- a large salt becomes quite beneficial.
Salting is useful even if intruder knows the salt.
If passwords are NOT salted, it makes possible to use widely available precomputed rainbow tables to quickly attack your passwords.
If your password table was salted, it makes it very difficult to precompute rainbow tables - it is impractical to create rainbow table for every possible salt.
If you use random salt that is different for every password entry, and put it in plaintext right next to it, it makes very difficult for intruder to attack your passwords, short of brute force attack.
Salting passwords protects passwords against attacks where the attacker has a list of hashed passwords. There are some common hashing algorithms that hackers have tables for that allow them to look up a hash and retrieve the password. For this to work, the hacker has to have broken into the password storage and stolen the hashes.
If the passwords are salted, then the attacker must re-generate their hash tables, using the hashing algorithm and the salt. Depending on the hashing algorithm, this can take some time. To speed things up, hackers also use lists of the most common passwords and dictionary words. The idea of the salt is to slow an attacker down.
The best approach to use a different salt for each password, make it long and random, and it's ok to store the salt next to each password. This really slows an attacker down, because they would have to run their hash table generation for each individual password, for every combination of common passwords and dictionary words. This would make it implausible for an attacker to deduce strong passwords.
I had read a good article on this, which I can't find now. But Googling 'password salt' gives some good results. Have a look at this article.
I would like to point out, that the scheme you described with the hard-coded salt, is actually not a salt, instead it works like a key or a pepper. Salt and pepper solve different problems.
A salt should be generated randomly for every password, and can be stored together with the hashed password in the database. It can be stored plain text, and fullfills it's purpose even when known to the attacker.
A pepper is a secret key, that will be used for all passwords. It will not be stored in the database, instead it should be deposited in a safe place. If the pepper is known to the attacker, it becomes useless.
I tried to explain the differences in a small tutorial, maybe you want to have a look there.
Makes sense. Seems like more effort than worth (unless its a site of significant worth or importance) for an attacker.
all sites small or large, important or not, should take password hashing as high importance
as long as each hash has its own large random salt then yes it does become mostly impracticable, if each hash uses an static salt you can use Rainbow tables to weed out the users hashs who used password1 for example
using an good hashing algorithm is also important as well (using MD5 or SHA1 is nearly like using plaintext with the mutli gpu setups these days) use scrypt if not then bcrypt or if you have to use PBKDF2 then (you need the rounds to be very high)
I am having difficulty understanding how a salt which is appended to a hash helps improve the security when a database of passwords or other important information is compromised.
If the salt is, for example, "hello", and is appended to the password "password" then the salt and password are stored together, "hellopassword" and hashed to produce:
94e66f94517d606d5ad6d9191b980408952f2ed2 (sha1)
with the salt then appended:
hello$94e66f94517d606d5ad6d9191b980408952f2ed2
How is this more secure? The attacker knows the salt so can now compute the passwords with little extra difficulty... right? Or am I fundamentally misunderstanding something?
No, not with "little extra difficulty" - with potentially significantly more difficulty.
Imagine there are two billion common passwords. It's easy to hash all of those and store the results. Then if you have an unsalted password hash, you can just check which common passwords match the given hash.
Now compare that with a salted hash... now you have two billion common passwords, but also several billion possible salts. Computing all the possible salt/password combinations will take much, much longer - hopefully becoming infeasible.
Additionally, it means that even if two people have the same password, they are very likely to have different hashes - so carelessness of one user in revealing their password doesn't risk the security of the other.
See the Wikipedia entry (if you haven't already) for more on this.
salt helps in 2 ways:
1) When two (or more) people use the same password, without salt you can see who uses the same password (the hashes are all the same). So in theory, if that person knows one of those person's passwords he knows everyone's passwords with the same hash. This is a minor reason.
2) The main reason is to prevent attacks commonly called dictionary attacks or rainbow attacks. In these attacks someone uses a database of pre-calculated hashes for common passwords. Often times these databases are gigs in size. But it is very easy at that point to just do a lookup for the hashes you have (the hashed password) against the list of pre-calculated hashes and see what the associated password is.
By using a salt value (typically you want this to be a random number) the hash won't match the dictionary (the chance of them pre-calculating all passwords with all possible salt values is exponentially more difficult). So even if your user uses an easily attacked password, say "Password", which is pretty much guaranteed to be any in any password dictionary/rainbow table, by pre-pending your random salt value you make the hash pretty much guaranteed to be useless to the attacker. Meanwhile for you, since the salt is just stored in cleartext, it makes it very easy for you to add it to your cleartext for your comparison of the password the user entered.
The salt isn't appended to the hash, its appended to the password THEN hashed. This is more secure because hackers have to know the salt and the actual password, which you should both protect heavily. :D
I was wondering - is there any disadvantages in using the hash of something as a salt of itself?
E.g. hashAlgorithm(data + hashAlgorithm(data))
This prevents the usage of lookup tables, and does not require the storage of a salt in the database. If the attacker does not have access to the source code, he would not be able to obtain the algorithm, which would make brute-forcing significantly harder.
Thoughts? (I have a gut feeling that this is bad - but I wanted to check if it really is, and if so, why.)
If the attacker does not have access to the source code
This is called "security through obscurity", which is always considered bad. An inherently safe method is always better, even if the only difference lies in the fact that you don't feel save "because they don't know how". Someone can and will always find the algorithm -- through careful analysis, trial-and-error, or because they found the source by SSH-ing to your shared hosting service, or any of a hundred other methods.
Using a hash of the data as salt for the data is not secure.
The purpose of salt is to produce unpredictable results from inputs that are otherwise the same. For example, even if many users select the same input (as a password, for example), after applying a good salt, you (or an attacker) won't be able to tell.
When the salt is a function of the data, an attacker can pre-compute a lookup table, because the salt for every password is predictable.
The best salts are chosen from a cryptographic pseudo-random number generator initialized with a random seed. If you really cannot store an extra salt, consider using something that varies per user (like a user name), together with something application specific (like a domain name). This isn't as good as a random salt, but it isn't fatally flawed.
Remember, a salt doesn't need to be secret, but it cannot be a function of the data being salted.
This offers no improvement over just hashing. Use a randomly generated salt.
The point of salting is to make it so two chronologically distinct values' hashes differ, and by so doing breaks pre-calculated lookup tables.
Consider:
data = "test"
hash = hash("test"+hash("test"))
Hash will be constant whenever data = "test". Thus, if the attacker has the algorithm (and the attacker always has the algorithm) they can pre-calculate hash values for a dictionary of data entries.
This is not salt - you have just modified the hash function. Instead of using lookup table for the original hashAlgorithm, attacker can just get the table for your modified one; this does not prevent the usage of lookup tables.
It is always better to use true random data as salt. Imagine an implementation where the username ist taken as salt value. This would lead to reduced security for common names like "root" or "admin".
I you don't want to create and manage a salt value for each hash, you could use a strong application wide salt. In most cases this would be absolutely sufficient and many other things would be more vulnerable than the hashes.