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I am trying to understand why is bcrypt called a Key Derivation Function?

I looked up the details of Ekfblowfish on Usenix article here:

http://static.usenix.org/event/usenix99/provos/provos_html/node4.html#SECTION00040000000000000000

After reading it, I understood that:

  1. the password is the encryption key. The length of the password can vary and so the length of the encryption key can vary (max is 56 bytes or 448 bits).
  2. 128 bit salt, it does not provide the details about how this salt is generated? It only says, that the 128 bit salt along with the variable length encryption key is used to modify the S Boxes and the P Array.
  3. When it says, Key Schedule, it refers to both, S Boxes and P Arrays. The Key Schedule keeps changing using the bits of 128 bit salt and the encryption key.

Now, what key are we deriving here? It is not clear, why is bcrypt called a Key Derivation Function?

So, does it mean that at the end of Expensive Key Schedule, we have a unique value for S boxes and P array contents which are further used to encrypt "OrpheanBeholderScryDoubt" using ECB mode.

So, aren't we deriving the Key Schedule (S boxes + P Arrays) instead of the Encryption Key (which is the same as the password that was supplied as an argument in the starting)?

Which key has been derived using Eksblowfish?

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1 Answer 1

It's called key derivation because it is used to obtain a "strong" key based on a key you own and is not so strong. Suppose a user has a password 12345 and an online service needs authentication. In order to verify the correctness the server doesn't store 12345 but it stores bcrypt(12345+salt) which further makes is more difficult for an attacker to break its one-wayness.

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