diff --git a/src/shamira/gf256.py b/src/shamira/gf256.py
--- a/src/shamira/gf256.py
+++ b/src/shamira/gf256.py
@@ -2,6 +2,9 @@
 
 """Arithmetic operations on Galois Field 2**8. See https://en.wikipedia.org/wiki/Finite_field_arithmetic"""
 
+from functools import reduce
+import operator
+
 
 def _gfmul(a, b):
 	"""Basic multiplication. Russian peasant algorithm."""
@@ -47,18 +50,25 @@ def evaluate(coefs, x):
 	return res
 
 
-def get_constant_coef(*points):
+def get_constant_coef(weights, y_coords):
 	"""Compute constant polynomial coefficient given the points.
 
 	See https://en.wikipedia.org/wiki/Shamir's_Secret_Sharing#Computationally_Efficient_Approach"""
-	k = len(points)
-	res = 0
-	for i in range(k):
-		(x, y) = points[i]
-		prod = 1
-		for j in range(k):
-			if i==j: continue
-			(xj, yj) = points[j]
-			prod = gfmul(prod, (gfmul(xj, INV[xj^x])))
-		res ^= gfmul(y, prod)
+	return reduce(
+		operator.xor,
+		map(lambda ab: gfmul(*ab), zip(weights, y_coords))
+	)
+
+
+def compute_weights(x_coords):
+	assert x_coords
+
+	res = [
+			reduce(
+				gfmul,
+				(gfmul(xj, INV[xj^xi]) for xj in x_coords if xi!=xj),
+				1
+			) for xi in x_coords
+	]
+
 	return res