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I want to make DH-like key exchange calculating something like this:

b = H(b0B1B2)

But the problem is multiplication between two ec-points is not defined on ec-curve.

I know monero makes some transformations like public key -> secret key here https://github.com/monero-project/monero/blob/4b728d7dd48584987f53995a141baac4f886f017/src/multisig/multisig.cpp#L77:

  rct::key sk = rct::scalarmultKey(rct::pk2rct(k), rct::sk2rct(blinded_skey));
  crypto::secret_key msk = get_multisig_blinded_secret_key(rct::rct2sk(sk));

Can I use the same trick? Is it secure?

  • 2
    There is no such thing as multiplying two EC points together. You can only add EC points, or multiply an EC point with a scalar. You can hash an EC point to produce a scalar. Can you elaborate on what you're trying to achieve? A 3-way DH exchange? If so, see crypto.stackexchange.com/questions/1025/… – knaccc May 17 '18 at 15:35
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knacc is right - there's no way no just multiply to ec-points. I solved my problem by making key exchange:

b = b0b1B2

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