# I want forgery ECDSA when prime p is form of 2*n+1, n is order of Elliptic curve

I have parameters of an elliptic curve s.t $p=2n+1$ , when $n$ = order of elliptic curve over finite field of order prime p.

If I want to forge any message for such ECDSA, What can I do? Maybe the condition $p=2n+1$ will be helpful, but I have no idea for this.

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 $n>8$.
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.