7 ms·

Can someone explain, in clear layman terms, what the difference is between a password hash and a KDF? I have went through this whole thread and tried to look ar

Can someone explain, in clear layman terms, what the difference is between a password hash and a KDF? I have went through this whole thread and tried to look around online but I still don't understand.

pjc50 2y ago
Password hash is designed for matching: take the salt, add it to the password, run it through the hash, compare it to the stored hash. The important properties are: - MUST be non-reversible, including against tricks like "rainbow tables" - should be somewhat expensive to discourage just trying all possible passwords against a (leaked) hash KDF is a key derivation function. The value will be used as a key in, say, AES. The important properties are: - should distribute entropy as well as possible, across the required width of output bits - reversibility less important as the derived key shouldn't be stored anywhere - may or may not want artificially inflated cost to discourage cracking
- Lvl999Noob 2y ago
  I still have no idea now haha. Your answer and Fabbari's are total opposites. If I am understanding right, you are saying that Password Hash is how a password should be stored while a KDF is not meant for storing passwords. Fabbari is saying the opposite of this, that KDF should be used for storing passwords while password hashes should not.
  - pjc50 2y ago
    Further discussion upthread under https://news.ycombinator.com/item?id=42957300 https://news.ycombinator.com/item?id=42957300
fabbari 2y ago
A password hash is a simple hash of a password. Hash algorithms are made to be fast. KDF - key deriving functions - are slow by design and are made to derive a key from a given string. They are designed to be slow to make password searching slower. This is a 2c tour of the topic.
- johnisgood 2y ago
  There are 2 comments to OP, and now the person can wonder which one is supposed to be slow or not.

pjc50 2y ago

Password hash is designed for matching: take the salt, add it to the password, run it through the hash, compare it to the stored hash. The important properties are: - MUST be non-reversible, including against tricks like "rainbow tables" - should be somewhat expensive to discourage just trying all possible passwords against a (leaked) hash KDF is a key derivation function. The value will be used as a key in, say, AES. The important properties are: - should distribute entropy as well as possible, across the required width of output bits - reversibility less important as the derived key shouldn't be stored anywhere - may or may not want artificially inflated cost to discourage cracking

Lvl999Noob 2y ago
I still have no idea now haha. Your answer and Fabbari's are total opposites. If I am understanding right, you are saying that Password Hash is how a password should be stored while a KDF is not meant for storing passwords. Fabbari is saying the opposite of this, that KDF should be used for storing passwords while password hashes should not.
- pjc50 2y ago
  Further discussion upthread under https://news.ycombinator.com/item?id=42957300 https://news.ycombinator.com/item?id=42957300

fabbari 2y ago

A password hash is a simple hash of a password. Hash algorithms are made to be fast. KDF - key deriving functions - are slow by design and are made to derive a key from a given string. They are designed to be slow to make password searching slower. This is a 2c tour of the topic.

johnisgood 2y ago
There are 2 comments to OP, and now the person can wonder which one is supposed to be slow or not.