Add FixedWindowLTRMultiplier.

1 files changed, 63 insertions, 0 deletions

diff --git a/pyecsca/ec/scalar.py b/pyecsca/ec/scalar.py
new file mode 100644
index 0000000..06058a0
--- /dev/null
+++ b/pyecsca/ec/scalar.py
@@ -0,0 +1,63 @@
+"""Provides functions for computing various scalar representations (like NAF, or different bases)."""
+from typing import List
+
+from public import public
+
+
+@public
+def convert(i: int, base: int) -> List[int]:
+    """
+    Convert an integer to base.
+
+    :param i:
+    :param base:
+    :return:
+    """
+    if i == 0:
+        return [0]
+    res = []
+    while i:
+        i, r = divmod(i, base)
+        res.append(r)
+    return res
+
+
+@public
+def wnaf(k: int, w: int) -> List[int]:
+    """
+    Compute width `w` NAF (Non-Adjacent Form) of the scalar `k`.
+
+    :param k: The scalar.
+    :param w: The width.
+    :return: The NAF.
+    """
+    half_width = 2 ** (w - 1)
+    full_width = half_width * 2
+
+    def mods(val: int) -> int:
+        val_mod = val % full_width
+        if val_mod > half_width:
+            val_mod -= full_width
+        return val_mod
+
+    result: List[int] = []
+    while k >= 1:
+        if k & 1:
+            k_val = mods(k)
+            result.insert(0, k_val)
+            k -= k_val
+        else:
+            result.insert(0, 0)
+        k >>= 1
+    return result
+
+
+@public
+def naf(k: int) -> List[int]:
+    """
+    Compute the NAF (Non-Adjacent Form) of the scalar `k`.
+
+    :param k: The scalar.
+    :return: The NAF.
+    """
+    return wnaf(k, 2)