Can somebody help me to understand the LOGIC behind Key-Derivation Methods (SP800 56A CH5.8)? This is my common understanding and please correct me if any wrong..
- Alice and Bob can general individual key-pair, by any public cryptography method (RSA, ECC, DL, etc..);
- Based on the Key Exchange protocol (for exp ECDH), Alice and Bob can obtain the shared secret Z between them, and using Z as AES key for the afterwards communication.
- If they like they can generate the new shared secret Z as they want--that is named as Ephemeral ECDH.
OK my question would be: here I don't really understand the Key-Derivation Methods (SP800 56A CH5.8) application scenarios--it looks like based on the common key agreement scheme (like ECDH), this KDM introduce a "FixedInfo" (which can be a readable text or just some hex string) concept.
I didn't find detail infor about how this "FixedInfo" works:
Does it use some Hash to process the "FixedInfo" and then derive new Secrete Key?
And does that mean each time a new FixedInfo shall be applied for both Alice and Bob?
Anyway, adding this "FixedInfo" looks NOT enhance security if compare to an Ephemeral ECDH--from my point-of-view..
---self update: OK after some study I have one explain but am not sure if that is correct--add such "FixedInfo" is actually a kinds of Ephemeral ECDH--it does not require a new ECC key-pair to be generated, so would reduce the key leakage risk? or from computing point-of-view this would save computing power?