update key_exchange.md

remove the link to a paper

docs/mpc/key_exchange.md

@@ -30,7 +30,7 @@ in such a way that
 1. Neither party learns the other party's $x$ value
 2. Neither party learns $x_r$, only their respective shares of $x_r$.
 
-We will use two maliciously secure protocols described on p.25 in the paper [Efficient Secure Two-Party Exponentiation](https://www.cs.umd.edu/~fenghao/paper/modexp.pdf):
+We will use two share conversion protocols:
 
 - `A2M` protocol, which converts additive shares into multiplicative shares, i.e. given shares `a` and `b` such that `a + b = c`, it converts them into shares `d` and `e` such that `d * e = c`    
 - `M2A` protocol, which converts multiplicative shares into additive shares
@@ -51,4 +51,4 @@ Now we apply `M2A` to $C_q * C_p$ to get $D_q + D_p$, which leads us to two fina
 
 $$
 x_r = (D_q - x_q) + (D_p - x_p)
-$$
+$$