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the answer to this question suggests that the x-coordinate of an ECDH shared secret is itself usable as an encryption key without apply a KDF.... if so, then what additional security is provided by the KDF???

i'm operating in a very resource-constrained embedded environment, where eliminating code normally used in a KDF (eg., SHA256) is critical... i should also add that ECDH private key is generated from a truly random 256-bit seed....

i'm already using 128-bit AES-CCM for encryption/authentication.... perhaps i can use the AES engine in some manner to "hash" the shared secret if that is advantageous....

that gives me three choices to produce a 128-bit AES key from my ECDH shared secret:

  1. use the secret's x-coordinate (trimming its size)
  2. applying a SHA-256 hash
  3. using AES-CCM for hashing (with some known key/IV)

what are the tradeoffs here and are there any other options worth considering???

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  • $\begingroup$ In this case, using KDF can only slow down the key-search. One can use both. $\endgroup$
    – kelalaka
    May 18, 2020 at 21:35

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