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What does 'salt' refer to in string-to-key (s2k) specifier?

Answer 1

+2 A:

The salt can be any consistent value.

Either a constant, or the user ID. Better if it includes both.

This is used to prevent pre generated rainbow tables from working.

Lo'oris 2010-05-16 21:29:04

What is a 'pre generated rainbow table'?

WilliamKF 2010-05-16 23:24:27

A rainbow table for hashes without any salt, i.e. created hashing just the plain password. Yes, people still do that.

Lo'oris 2010-05-17 08:59:08

Answer 2

+3 A:

From Wikipedia:

In cryptography, a salt comprises of random bits that are used as one of the inputs to a key derivation function. The other input is usually a password or passphrase. The output of the key derivation function is stored as the encrypted version of the password.

A salt is just some bits that are used to increase the security of the system. They help prevent pre-computed dictionary attacks.

Mark Byers 2010-05-16 21:31:44

Calling them "random" is not really fair, and that wikipedia article it questionable to say the least. I think they might be called "salt" even if they are a const, and I'm pretty sure they can be called "salt" even if they are not random but taken from a constant value such as the user ID.

Lo'oris 2010-05-16 21:41:08

To be more precise: if the salt is constant for every entry, it still prevents pregenerated rainbow tables from working; if the salt also changes for each user, it also prevents the attacker from generating a rainbow table and using it on every entry.

Lo'oris 2010-05-16 21:42:53

@Lo'oris: I think using user_id as a salt would be a bad idea in many cases. Root for example has user_id 0 on some systems and you could pre-calculate a dictionary for that salt. The salt shouldn't be predictable in advance. That is why a random number is good, although I admit that there are other ways of generating unpredictable numbers, such as using a hash.

Mark Byers 2010-05-16 22:33:18

@Mark: clever! this would not work if you add a constant value *and* the ID, though (and now I understand why they said to use both, somewhere)

Lo'oris 2010-05-16 23:05:42

@Mark - unless you derive the salt from the userid somehow. eg. `SHA256(userid + 9978)`

kibibu 2010-05-16 23:13:43

@kibibu: yes, but its much simpler and as effective to just add a constant eg. `hash(CONST+ID+PASSWD)`

Lo'oris 2010-05-16 23:21:26

Answer 3

+1 A:

I think you're asking the origin of the term, not the definition.

Time for a round of folk etymology! (Until someone gives the real answer.)

My guess is that it is an analogy from cooking: the salt is an improving additive. And a little bit goes a long way.

harpo 2010-05-16 23:14:49

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What does 'salt' refer to in string-to-key (s2k) specifier?

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