add ct_lsb_prop_u16

Author: tomato42

tlslite/utils/constanttime.py

@@ -23,6 +23,7 @@ def ct_lt_u32(val_a, val_b):
 
     return (val_a^((val_a^val_b)|(((val_a-val_b)&0xffffffff)^val_b)))>>31
 
+
 def ct_gt_u32(val_a, val_b):
     """
     Return 1 if val_a > val_b, 0 otherwise. Constant time.
@@ -35,6 +36,7 @@ def ct_gt_u32(val_a, val_b):
     """
     return ct_lt_u32(val_b, val_a)
 
+
 def ct_le_u32(val_a, val_b):
     """
     Return 1 if val_a <= val_b, 0 otherwise. Constant time.
@@ -47,14 +49,26 @@ def ct_le_u32(val_a, val_b):
     """
     return 1 ^ ct_gt_u32(val_a, val_b)
 
+
 def ct_lsb_prop_u8(val):
-    """Propagate LSB to all 8 bits of the returned byte. Constant time."""
+    """Propagate LSB to all 8 bits of the returned int. Constant time."""
+    val &= 0x01
+    val |= val << 1
+    val |= val << 2
+    val |= val << 4
+    return val
+
+
+def ct_lsb_prop_u16(val):
+    """Propagate LSB to all 16 bits of the returned int. Constant time."""
     val &= 0x01
     val |= val << 1
     val |= val << 2
     val |= val << 4
+    val |= val << 8
     return val
 
+
 def ct_isnonzero_u32(val):
     """
     Returns 1 if val is != 0, 0 otherwise. Constant time.
@@ -66,6 +80,7 @@ def ct_isnonzero_u32(val):
     val &= 0xffffffff
     return (val|(-val&0xffffffff)) >> 31
 
+
 def ct_neq_u32(val_a, val_b):
     """
     Return 1 if val_a != val_b, 0 otherwise. Constant time.

unit_tests/test_tlslite_utils_constanttime.py

@@ -11,7 +11,7 @@
 
 from tlslite.utils.constanttime import ct_lt_u32, ct_gt_u32, ct_le_u32, \
         ct_lsb_prop_u8, ct_isnonzero_u32, ct_neq_u32, ct_eq_u32, \
-        ct_check_cbc_mac_and_pad, ct_compare_digest
+        ct_check_cbc_mac_and_pad, ct_compare_digest, ct_lsb_prop_u16
 
 from hypothesis import given, example
 import hypothesis.strategies as st
@@ -80,6 +80,14 @@ def test_ct_lsb_prop_u8(self, i):
         self.assertEqual(((i & 0x1) == 1), (ct_lsb_prop_u8(i) == 0xff))
         self.assertEqual(((i & 0x1) == 0), (ct_lsb_prop_u8(i) == 0x00))
 
+    @given(i=st.integers(0, 2**16-1))
+    @example(i=0)
+    @example(i=255)
+    @example(i=2**16-1)
+    def test_ct_lsb_prop_u16(self, i):
+        self.assertEqual(((i & 0x1) == 1), (ct_lsb_prop_u16(i) == 0xffff))
+        self.assertEqual(((i & 0x1) == 0), (ct_lsb_prop_u16(i) == 0x0000))
+
     @given(i=st.integers(0,2**32 - 1))
     @example(i=0)
     def test_ct_isnonzero_u32(self, i):