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Alice and Bob need to share public keys to sign/verify ephemeral keys. They have a secret key $K$.

  1. Alice generates a 32 bytes random number $A$, computes $\operatorname{HMAC-SHA256}(K, M||A)$ and sends Bob the hash and plain $M$, $A$
  2. Bob computes $\operatorname{HMAC-SHA256}(K, M||A)$ to check the hashes
  3. If hashes match, Bob computes $N=\operatorname{HKDF}(A||K)$
  4. Bob encrypts his public key $E_N(\mathit{PubKeyBob})$ using AES 256 GCM, and sends the cipher to Alice
  5. Alice computes $N$ and decrypts the cipher, encrypts her public key $E_N(\mathit{PubKeyAlice})$, and sends Bob the cipher
  6. Bob decrypts the cipher and they both have each other's public key

Is this a safe way to share public keys?

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    $\begingroup$ Welcome to crypto.stackexchange - May I ask why the public keys need to be encrypted at all? Why can you assume the existence of a pre-shared secret, but not the existence of a pre-shared public key? $\endgroup$ – Ella Rose Sep 29 '19 at 22:26
  • $\begingroup$ To prevent man in the middle attack. They have a pre-shared secret because they know each other. $\endgroup$ – civ15 Sep 29 '19 at 22:30
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    $\begingroup$ If you have pre-shared $K$ why don't you just send the public keys encrypted with AES? $\endgroup$ – kelalaka Sep 29 '19 at 23:02
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If they have a pre-shared key $K$ and want to authenticate public keys, the simplest way would be for Alice to compute $T = HMAC(K, PubKeyAlice)$ and send $(PubKeyAlice,T)$ to Bob. This authenticates the public key of Alice. There is no need to encrypt it, since it is a public value anyway.

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  • $\begingroup$ AES-GCM can be faster. $\endgroup$ – kelalaka Sep 30 '19 at 9:08
  • $\begingroup$ @kelalaka: Yes, depending on the platform GHASH could be faster. I could have written it in more general form T = MAC(K; PubKeyAlice) but since HMAC-SHA256 was explicitly mentioned in the question, I just used that instantiation. $\endgroup$ – Krystian Sep 30 '19 at 12:37

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