Fix link to Maurer's multiplication proof for pedersen commitments

Author: jonasnick

md/pedersen-swap.md

@@ -28,8 +28,8 @@ This is a non-interactive zero knowledge proof that for given Pedersen
 commitments `Q = r*G + x*H`, `T1 = t1*G` and `T2 = t2*G` it holds that `r =
 t1*t2`. The given construction is a special case (commitment to 0) of the proof
 from the paper [Zero-knowledge proofs of knowledge for group
-homomorphisms](https://sci-hub.la/10.1007/s10623-015-0103-5) by Ueli Maurer
-section 6.7 with the addition of the Fiat-Shamir heuristic.
+homomorphisms](https://www.crypto.ethz.ch/teaching/lectures/KP19/papers/Maurer15.pdf)
+by Ueli Maurer section 6.7 with the addition of the Fiat-Shamir heuristic.
 
 Informally, the scheme consists of the two algorithms "generate" and "check":