From 27e2d2d2bc6e4a3ca1d61ed9117421e2bbcefcf8 Mon Sep 17 00:00:00 2001
From: mratsim <mamy_github@numforge.co>
Date: Sun, 2 Dec 2018 19:28:42 +0100
Subject: [PATCH] Implement to Montgomery domain conversion

---
 README.md                  | 5 +++--
 constantine/field_fp.nim   | 4 ++--
 constantine/montgomery.nim | 9 ++++++++-
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index dc854b0..0460f82 100644
--- a/README.md
+++ b/README.md
@@ -6,8 +6,9 @@
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 ![Stability: experimental](https://img.shields.io/badge/stability-experimental-orange.svg)
 
-This library provides constant time finite field primitives.
-The main use will be for implementation of elliptic curve cryptography
+This library provides constant time finite field primitives for implementation of elliptic curve cryptography.
+
+Warning ⚠️: The library is in development state.
 
 ## Installation
 
diff --git a/constantine/field_fp.nim b/constantine/field_fp.nim
index a442654..a9a79ff 100644
--- a/constantine/field_fp.nim
+++ b/constantine/field_fp.nim
@@ -160,14 +160,14 @@ template scaleadd_impl(a: var Fp, c: Limb) =
     sub(a, Fp.P, tooBig)
 
 func scaleadd*(a: var Fp, c: Limb) =
-  ## Scale-accumulate
+  ## Scale-accumulate modulo P
   ##
   ## With a word W = 2^LimbBitSize and a field Fp
   ## Does a <- a * W + c (mod p)
   scaleadd_impl(a, c)
 
 func scaleadd*(a: var Fp, c: static Limb) =
-  ## Scale-accumulate
+  ## Scale-accumulate modulo P
   ##
   ## With a word W = 2^LimbBitSize and a field Fp
   ## Does a <- a * W + c (mod p)
diff --git a/constantine/montgomery.nim b/constantine/montgomery.nim
index 64a009e..934bc21 100644
--- a/constantine/montgomery.nim
+++ b/constantine/montgomery.nim
@@ -69,5 +69,12 @@ func montyMagic*(M: static BigInt): static Limb =
   for _ in static(0 ..< k):
     result *= 2 + M * result # x' = x(2 + ax) (`+` to avoid negating at the end)
 
-# func toMonty*[P: static BigInt](a: Fp[P], montyMagic: Limb): Montgomery[P] =
+func toMonty*[P: static BigInt](a: Fp[P]): Montgomery[P] =
+  ## Convert a big integer over Fp to it's montgomery representation
+  ## over Fp.
+  ## i.e. Does "a * (2^LimbSize)^W (mod p), where W is the number
+  ## of words needed to represent p in base 2^LimbSize
 
+  result = a
+  for i in static(countdown(P.limbs.high, 0)):
+    scaleadd(result, 0)