As I know from the Elliptic Curve Diffie Hellman (ECDH), you can create a "Shared Key": https://asecuritysite.com/encryption/ecdh2

This will look like this:

dA × dB × G = Shared Key

QB = dB × G

QA = dA × G

But is it possible to get the sum of dA and dB? (Plusing the value of each private key?)

dA + dB × G = ???

How to get a "Shared Key" when adding each private key (plus)?

1 Answer 1


Yes, QA + QB = (dA + dB) × G, but this isn't used in ECDH as QA + QB can be calculated by third parties.

You can read about this property here.

  • QA + QB can not be calculated by third parties? That is, the point G does not allow this to be done. Or how can QA + QB be calculated? Maybe in a different way?
    – Izi Tors
    Oct 25, 2019 at 10:24
  • If you know the points QA and QB, what you should do is draw the curve, mark the points QA and QB, connect them with a line, find where the line intersects the curve (apart from QA and QB). The third intersection will be -(QA + QB). You can then reflect the point over the X axis to find the point QA + QB. You can read more about addition here. (note that there are also some modulo operations) @IziTors
    – MCCCS
    Oct 25, 2019 at 10:45
  • How on the graph of the curve can they get the sum of the total private keys (dA + dB) by calculating QA + QB?
    – Izi Tors
    Oct 25, 2019 at 11:19
  • It’s assumed to be impossible to find dA + dB from QA + QB, but you can find (dA + dB) x G if you know QA and QB by adding them. If you know QA and QB or QA + QB you can’t find dA or dB or their sum.
    – MCCCS
    Oct 25, 2019 at 12:03
  • And there are minus points QA - QB, and accordingly minus (dA - dB) x G ?
    – Izi Tors
    Nov 1, 2019 at 12:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.