--- a/viff/bedoza.py	Tue Jul 06 15:00:30 2010 +0200
+++ b/viff/bedoza.py	Tue Jul 06 16:17:16 2010 +0200
@@ -317,3 +317,37 @@
         zks = xks - yks
         zms = xms - yms
         return (zi, zks, zms)
+
+
+    def _cmul(self, share_x, share_y, field):
+        """Multiplication of a share with a constant.
+
+        Either share_x or share_y must be a BeDOZaShare but not
+        both. Returns None if both share_x and share_y are
+        BeDOZaShares.
+        """
+        if not isinstance(share_x, Share):
+            # Then share_y must be a Share => local multiplication. We
+            # clone first to avoid changing share_y.
+            assert isinstance(share_y, Share), \
+                "At least one of the arguments must be a share."
+            result = share_y.clone()
+            result.addCallback(self._constant_multiply, share_x)
+            return result
+        if not isinstance(share_y, Share):
+            # Likewise when share_y is a constant.
+            assert isinstance(share_x, Share), \
+                "At least one of the arguments must be a share."
+            result = share_x.clone()
+            result.addCallback(self._constant_multiply, share_y)
+            return result
+        return None
+
+    def _constant_multiply(self, x, c):
+        """Multiplication of a share-tuple with a constant c."""
+        assert(isinstance(c, FieldElement))
+        xi, xks, xms = x
+        zi = c * xi
+        zks = BeDOZaKeyList(xks.alpha, map(lambda k: c * k, xks.keys))
+        zms = BeDOZaMessageList(map(lambda m: c * m, xms.auth_codes))
+        return (zi, zks, zms)

--- a/viff/test/test_bedoza_runtime.py	Tue Jul 06 15:00:30 2010 +0200
+++ b/viff/test/test_bedoza_runtime.py	Tue Jul 06 16:17:16 2010 +0200
@@ -276,19 +276,39 @@
         return d
 
     @protocol
-    def test_sub_constant_left(self, runtime):
-        """Test subtraction of a public number and secret shared number."""
+    def test_constant_multiplication_constant_left(self, runtime):
+        """Test multiplication of two numbers."""
 
         self.Zp = GF(6277101735386680763835789423176059013767194773182842284081)
 
-        x1 = 42
+        x1 = 6
         y1 = 7
 
         def check(v):
-            self.assertEquals(v, 1)
+            self.assertEquals(v, x1 * y1)
 
         x2 = runtime.random_share(self.Zp)
-        z2 = y1 - x2
+
+        z2 = runtime._cmul(self.Zp(y1), x2, self.Zp)
         d = runtime.open(z2)
         d.addCallback(check)
         return d
+
+    @protocol
+    def test_constant_multiplication_constant_right(self, runtime):
+        """Test multiplication of two numbers."""
+
+        self.Zp = GF(6277101735386680763835789423176059013767194773182842284081)
+
+        x1 = 6
+        y1 = 7
+
+        def check(v):
+            self.assertEquals(v, x1 * y1)
+
+        x2 = runtime.random_share(self.Zp)
+
+        z2 = runtime._cmul(x2, self.Zp(y1), self.Zp)
+        d = runtime.open(z2)
+        d.addCallback(check)
+        return d