diff --git a/src/gf256.py b/src/gf256.py
--- a/src/gf256.py
+++ b/src/gf256.py
@@ -35,3 +35,16 @@ def evaluate(coefs,x):
 		res^=ffmul(a,xK)
 		xK=ffmul(xK,x)
 	return res
+
+
+def getConstantCoef(k,*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=ffmul(prod, (ffmul(xj,inv[xj^x])))
+		res^=ffmul(y,prod)
+	return res