Elliptic Curve-based Diffie Hellman: How is the shared key (mnp) calculated?
So my question is, how do you calculate $mnP$?


For Michael, he takes the value $nP$ he received from Nikita, and adds it to itself $m$ times; that is, it's $\underbrace{nP + nP + \dots + nP}_\text{$m$ times}$, that is the value $m(nP)$.

And, of course Nikita takes the value $mP$ he received from Michael, and adds it to itself $n$ times; that is, it's $\underbrace{mP + mP + \dots + mP}_\text{$n$ times}$, that is the value $n(mP)$.

And, because we perform these operations in a mathematical group (that is, associativity holds), we have $m(nP) = n(mP) = mnP$, so they come up with the same value.