Ok, I’m trying to understand the reason to use salt.
When a user registers I generate a unique salt for him/her that I store in DB. Then I hash it and the password with SHA1. And when he/she is logging in I re-hash it with sha1($salt.$password).
But if someone hacks my database he can see the hashed password AND the salt.
Is that harder to crack than just hashing the password with out salt? I don’t understand …
Sorry if I’m stupid …
If you don't use a salt then an attacker can precompute a password<->hash database offline even before they've broken into your server. Adding a salt massively increases the size of that database, making it harder to perform such an attack.
Also, once they've broken in they can guess a commonly used password, hash it, and then check all of the passwords in the database for a match. With a different salt for each user, they can only attack one password at a time.
There's an article at Wikipedia about salts in cryptography.
Another intention behind the use of a salt is to make sure two users with the same password won't end up having the same hash in the users table (assuming their salt are not the same). However, the combination of a salt and a password may lead to the same "string" or hash in the end and the hash will be exactly the same, so make sure to use a combination of salt and password where two different combination won't lead to the same hash.
If an attacker creates a giant table of hash values for plaintext passwords, using a salt prevents him from using the same table to crack more than one password. The attacker would have to generate a separate table for each salt. Note that for this to actually work propertly, your salt should be rather long. Otherwise the attacker's precomputed table is likely to contain the salt+password hash anyway.
Related
Is there really a point in salting a password?
if a program does all the processing of a SALT server side then does it really make it any more difficult for brute force or other attack. The code is only going to apply the salt to whatever is entered by a user.
Do I have this all wrong?
Yes, there is a point in salting a password.
The point is that each password has its own salt, so that an attacker can't make use of dictionaries and rainbow tables to brute force all passwords at once.
The salt doesn't make it harder to crack a single password¹, but it removes the benefit from attempting to crack multiple passwords at once. An attacker has to brute force one password at a time.
¹ At least not enough to be a good reason to use it. Using better passwords works much better.
In a word, yes.
Salting a password adds a level of complexity to the string and confuses humans, and makes dictionary attacks less likely to succeed.
Brute force can still crack this password however, hence the need for a randomly generated salt.
Salts are typically generated via byte-arrays, which is then fed into a function to combine the two strings into one at intervals. See my answer here.
The hashes may be leaked without the salt (common scenario: database gets dumped, but a salt i present in PHP source that does not leak).
You are right in a way but ... the most significant protection from SALT is that if the hashes ever do get released into the wild then reverse hash lookups are much much harder.
Hash a word and then put the hash result into your favourite search engine to see what I mean.
I understand that it's best practice to generate a long salt for each password you use. But does using a single salt provide any security benefit from not having a salt at all?
Having a shared salt makes you marginally more secure. It prevents an attacker from using a pre-computed rainbow table attack, but it does not prevent them from building a single, new rainbow table for your password database. Thus it is harder for an attacker to crack a single password with a given salt, but it is significantly easier for them to crack every password with that salt.
As an example, consider the following simplified set of passwords, salts and hashes:
Password Salt Hash
aaaaaz y03sar ze4lap
zzzzza y03sar enbe65
The attacker knows that your salt is y03sar and starts computing every hash with that start for every password from a to zzzzzz. Long before (in terms of iterations, the actual cracking would finish very quickly for passwords and salts of this complexity) they discover the password zzzzza, they will realize they have also discovered the password aaaaaz. In other words, brute forcing one password in your database is no harder in the worst case than brute forcing every password.
With different salts, each password must be attacked separately.
Password Salt Hash
aaaaaz bbq9f0 i2chf1
zzzzza y03sar enbe65
If the attacker again starts calculating hashes for every password from a to zzzzzz with the salt y03sar, then i2chf1 won't be in their output list (at least it's improbable with a reasonable length hash output; even if it is, the computed password for salt y03sar still won't work to gain access to the aaaaaz account because the hash would be different with salt bbq9f0).
To add some numbers to the mix, using a single salt lengthens the attack time from instantaneous (rainbow tables provide for a constant-time lookup on the hash digest) to linear time. As soon as the attacker has computed the hashes for every password allowed by your system, they have access to every account in your system. Even if you have 16 character passwords allowing [a-zA-Z0-9], then your whole system is compromised in days or weeks. (Linked question is just an example - GPU's can crack passwords even faster than the hardware in the answer.)
Now if you have distinct salts for every password, then in the same amount of time it took an attacker to crack your entire database, they have only cracked a single password.
That's a pretty big difference. Use unique salts. (And a good hashing algorithm, while we're on the topic.)
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've a legacy app where passwords are hashed using MD5 without salt. I'd like to switch to SHA1 with salt, but I'd like to keep current users' passwords.
My plan is to change hashing function to sha1(md5(password) + salt). I'll be able to batch process all existing hashes using sha1(<existing_pass> + salt).
Is it safe to keep md5 in this case?
Is it ok to have one single salt for all users?
It's not a good idea to keep md5, read this question: Use SHA-512 and salt to hash an MD5 hashed password?.
It's better to have one salt for each user. With the same salt, users with the same password will have the same hash, and a rainbow table can be created for all your passwords at the same time.
As for question 1 I'm not quite sure but it seems to be OK.
For question 2: It is never OK to have same salt for all users. Salt has two functions. To prevent using pre-generated hashes / rainbow tables to search leaked database, and to prevent generation of dictionary-based hashes and searching databases with them too. Common salt will work in first case making rainbow tables unusable, but won't prevent cracker from dictionary attack. If cracker knows the global salt, he can generate frequent passwords, hash them and grep entire database. If salt is generated per user this scenario isn't possible.
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