I want forgery ECDSA when prime p is form of 2*n+1, n is order of Elliptic curve
Hasse's theorem on elliptic curves tells us that, using your notation:
$|n-p+1|\le2\sqrt{p} \Rightarrow |n-(2n+1)+1)|\le2\sqrt{2n+1}\Rightarrow n\le2\sqrt{2n+1}$ which doesn't hold for $n&gt;8$.

So either your curve is so small you can solve the ECDLP by hand or you are not defining an elliptic curve.

Could it be that the order of the curve is $2n$ but you are just working in the large prime order group (of order $n$)?
Because in that case $n$ divides $p-1=2n$ and you can mount a MOV attack.