add special case

modules/math.js

@@ -163,8 +163,6 @@ function modsqrt(n, modulus) {
 		throw new Error("modulus must be prime to compute square root");
 	}
 
-	// TODO: add special case for modulus = 3 (mod 4)
-	
 	n %= modulus;
 	if (n < 0n) {
 		n += modulus;
@@ -176,6 +174,8 @@ function modsqrt(n, modulus) {
 		throw new Error("radicand is not a quadratic residue of the modulus");
 	} else if (modulus === 2n) {
 		return n % 2n;
+	} else if (modulus % 4n === 3n) {
+		return modpow(n, (modulus+1n)/4n, modulus);
 	}
 
 	return tonelliShanks(n, modulus);