4 ms·

Right next to the hashed passwords. The point of salting isn't to add an additional level of secrecy, it's just to prevent the reuse of hashing work for attack

by tomku 13y ago

Right next to the hashed passwords. The point of salting isn't to add an additional level of secrecy, it's just to prevent the reuse of hashing work for attacking other users.

chm 13y ago
But doesn't this render the process useless? If an attacker gets access to the hashes, he also gets access to the salts. If both hashes and salts were isolated, I suppose it would be much more secure, although maybe too slow.
- arjie 13y ago
  Without salts: Generate table of hashes vs. Plaintext for all plaintext up to certain length. Maybe use rainbow tables. Now, for each entry use this table. Time to get plaintext of n hashes is constant. With salt: Generate table of hashes vs. Plaintext by appending salt to each possible plaintext of given length. For first entry use first salt to generate this table. Table is now useless for second salt if it is different. Time to attack n hashes scales linearly.
- baddox 13y ago
  The salts provide their primary benefit (rendering rainbow tables useless) even when the attacker possesses the salts and the hashed passwords.
- ams6110 13y ago
  The point of the salt is to prevent precomputing the hashes (a rainbow table). Per-user salts take this a step farther, so that even if two users choose the same password, they will have different hashes. What the article really points out is what people have been saying for a while now, that if your passwords are exposed, salting really doesn't add more than a roadbump to a cracker if you are computing your hashes with fast message-digest algorithms like MD5 or SHA.
  - qu4z-2 13y ago
    I dunno... in the example there, I'd say that it costing 16000x as much to crack the passwords would be a little more than a "roadbump". (assuming people are renting {C,G}PUs in the cloud to perform the attacks)
- tomku 13y ago
  Absolutely not. Like I said, the point of the salt is not to provide a "second password" that needs to be independently stolen. It's to make the results from a cracking attempt on one account useless on the password of another. If you've read the article, you have a pretty good idea of how it works without salting. You have a list of N password hashes, and you come up with candidate passwords. You run each candidate password through the hash function, and compare the resulting hash to your list. If it matches any of them, you've got the plaintext for those user accounts. The key thing here is that you only have to hash each candidate password once, and you can compare that hash to as many accounts as you'd like. Now, let's add in a salt. It's stored next to the password, so after stealing the password db you've got a list of password hashes, each with its own salt. In order to check a candidate password against a particular user account, you have to append that user account's salt to the candidate, then hash it and compare against the stored hash. That part isn't any slower. The problem is when you try to take that hash and compare it to other accounts. Even if two users have the same plaintext password, their password hashes will be different because they have different salts. The end result is that you have to hash each candidate password fresh for every user.
  - chm 13y ago
    That's what I understood from the article, and why I suggested the salts and hashes should be handled separately. If you give the attacker the salts, difficulty scales linearly. For two identical plaintexts, all that differs is the salt, but it is given to him. If you store the hashes and salts separately (the technical details of which I know nothing about), then you augment the keyspace exponentially. For N byte salts, using P characters, you augment it by N^P. Furthermore, who forces you to append a salt? You could prepend. Or n-pend. I'm just throwing ideas around.
    - nknighthb 13y ago
      The server must have access to both the salt and the hash to verify a password. Therefore, upon compromise of the server, the attacker automatically has access to both the salt and the hash. There is no way around this problem that isn't simply obfuscation.
- deleted 13y ago
  [deleted]
- lambda 13y ago
  A salt is not for adding extra secrecy. It's for making it take more work to solve a large number of passwords at once. It doesn't need to be stored separately to do this. What you're thinking of is a secret-splitting scheme; where you split a secret among multiple parties, where you need the information from both parties to reconstruct the secret. That's a valid cryptographic technique, but that's not what's meant when you say "salted password". The problem is, for passwords, that approach isn't really workable. Whatever machine is verifying the passwords will need access to the full hash to do so, so if it's compromised, you could get access to the full hash. I suppose you could split the verification into two parts, on two separate machines. Each one would have it's own password database, with independent salts, and the machine trying to authenticate the user would request verification from each of the other machines before allowing you in. However, that would make your password system slower and more fragile; now if either of those machines goes offline, your login functionality breaks (and yes, you could do a "two out of three" system to avoid that, but now you're talking about tripling your hardware requirements just for your password verification system). Really the best bang for your buck will come from (a) securing your system so that it doesn't reveal your password database in the first place and (b) using bcrypt, scrypt, or PBKDF2 to make brute forcing a database that is disclosed much more difficult. It's far more likely that you will build a secure system by following basic security practices like isolating services and giving each the minimum privilege that it needs, doing code review, keeping inessential services off of important machines, using prepared statements rather than building SQL queries from user-supplied strings, and so on, as well as using off-the-shelf security components that have been well tested like the above hash algorithms, than you will by designing your own password system using some novel technique to spread the secret out among multiple machines.
  - PwdRsch 13y ago
    There actually has been research on how to split passwords across multiple servers (one example http://www.passwordresearch.com/papers/paper270.html http://www.passwordresearch.com/papers/paper270.html), and RSA is currently marketing a commercial product that does this. While I agree that most people probably aren't concerned enough about password exposure to do this, there are effective solutions to accommodate those that are.

by tomku 13y ago

Right next to the hashed passwords. The point of salting isn't to add an additional level of secrecy, it's just to prevent the reuse of hashing work for attacking other users.

chm 13y ago

But doesn't this render the process useless? If an attacker gets access to the hashes, he also gets access to the salts. If both hashes and salts were isolated, I suppose it would be much more secure, although maybe too slow.

arjie 13y ago
Without salts: Generate table of hashes vs. Plaintext for all plaintext up to certain length. Maybe use rainbow tables. Now, for each entry use this table. Time to get plaintext of n hashes is constant. With salt: Generate table of hashes vs. Plaintext by appending salt to each possible plaintext of given length. For first entry use first salt to generate this table. Table is now useless for second salt if it is different. Time to attack n hashes scales linearly.
baddox 13y ago
The salts provide their primary benefit (rendering rainbow tables useless) even when the attacker possesses the salts and the hashed passwords.
ams6110 13y ago
The point of the salt is to prevent precomputing the hashes (a rainbow table). Per-user salts take this a step farther, so that even if two users choose the same password, they will have different hashes. What the article really points out is what people have been saying for a while now, that if your passwords are exposed, salting really doesn't add more than a roadbump to a cracker if you are computing your hashes with fast message-digest algorithms like MD5 or SHA.
- qu4z-2 13y ago
  I dunno... in the example there, I'd say that it costing 16000x as much to crack the passwords would be a little more than a "roadbump". (assuming people are renting {C,G}PUs in the cloud to perform the attacks)
tomku 13y ago
Absolutely not. Like I said, the point of the salt is not to provide a "second password" that needs to be independently stolen. It's to make the results from a cracking attempt on one account useless on the password of another. If you've read the article, you have a pretty good idea of how it works without salting. You have a list of N password hashes, and you come up with candidate passwords. You run each candidate password through the hash function, and compare the resulting hash to your list. If it matches any of them, you've got the plaintext for those user accounts. The key thing here is that you only have to hash each candidate password once, and you can compare that hash to as many accounts as you'd like. Now, let's add in a salt. It's stored next to the password, so after stealing the password db you've got a list of password hashes, each with its own salt. In order to check a candidate password against a particular user account, you have to append that user account's salt to the candidate, then hash it and compare against the stored hash. That part isn't any slower. The problem is when you try to take that hash and compare it to other accounts. Even if two users have the same plaintext password, their password hashes will be different because they have different salts. The end result is that you have to hash each candidate password fresh for every user.
- chm 13y ago
  That's what I understood from the article, and why I suggested the salts and hashes should be handled separately. If you give the attacker the salts, difficulty scales linearly. For two identical plaintexts, all that differs is the salt, but it is given to him. If you store the hashes and salts separately (the technical details of which I know nothing about), then you augment the keyspace exponentially. For N byte salts, using P characters, you augment it by N^P. Furthermore, who forces you to append a salt? You could prepend. Or n-pend. I'm just throwing ideas around.
  - nknighthb 13y ago
    The server must have access to both the salt and the hash to verify a password. Therefore, upon compromise of the server, the attacker automatically has access to both the salt and the hash. There is no way around this problem that isn't simply obfuscation.
deleted 13y ago
[deleted]
lambda 13y ago
A salt is not for adding extra secrecy. It's for making it take more work to solve a large number of passwords at once. It doesn't need to be stored separately to do this. What you're thinking of is a secret-splitting scheme; where you split a secret among multiple parties, where you need the information from both parties to reconstruct the secret. That's a valid cryptographic technique, but that's not what's meant when you say "salted password". The problem is, for passwords, that approach isn't really workable. Whatever machine is verifying the passwords will need access to the full hash to do so, so if it's compromised, you could get access to the full hash. I suppose you could split the verification into two parts, on two separate machines. Each one would have it's own password database, with independent salts, and the machine trying to authenticate the user would request verification from each of the other machines before allowing you in. However, that would make your password system slower and more fragile; now if either of those machines goes offline, your login functionality breaks (and yes, you could do a "two out of three" system to avoid that, but now you're talking about tripling your hardware requirements just for your password verification system). Really the best bang for your buck will come from (a) securing your system so that it doesn't reveal your password database in the first place and (b) using bcrypt, scrypt, or PBKDF2 to make brute forcing a database that is disclosed much more difficult. It's far more likely that you will build a secure system by following basic security practices like isolating services and giving each the minimum privilege that it needs, doing code review, keeping inessential services off of important machines, using prepared statements rather than building SQL queries from user-supplied strings, and so on, as well as using off-the-shelf security components that have been well tested like the above hash algorithms, than you will by designing your own password system using some novel technique to spread the secret out among multiple machines.
- PwdRsch 13y ago
  There actually has been research on how to split passwords across multiple servers (one example http://www.passwordresearch.com/papers/paper270.html http://www.passwordresearch.com/papers/paper270.html), and RSA is currently marketing a commercial product that does this. While I agree that most people probably aren't concerned enough about password exposure to do this, there are effective solutions to accommodate those that are.