From 6d12cb9a74a3f03521063e648304ee68fd8a15dc Mon Sep 17 00:00:00 2001
From: Jack Grigg <jack@z.cash>
Date: Fri, 18 May 2018 14:38:32 +1200
Subject: [PATCH] Extract LEOS2IP and I2LEOSP functions

---
 sapling_jubjub.py   | 13 +++++--------
 sapling_pedersen.py |  8 +++-----
 sapling_utils.py    | 10 ++++++++++
 3 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/sapling_jubjub.py b/sapling_jubjub.py
index 1e70df2..bc4529c 100644
--- a/sapling_jubjub.py
+++ b/sapling_jubjub.py
@@ -1,7 +1,5 @@
 #!/usr/bin/env python3
-from sapling_utils import i2lebsp
-
-ENDIANNESS = 'little'
+from sapling_utils import i2lebsp, leos2ip, i2leosp
 
 q_j = 52435875175126190479447740508185965837690552500527637822603658699938581184513
 r_j = 6554484396890773809930967563523245729705921265872317281365359162392183254199
@@ -49,7 +47,8 @@ class FieldElement(object):
         return i2lebsp(l, self.s)
 
     def __bytes__(self):
-        return self.s.to_bytes(32, byteorder=ENDIANNESS)
+        # TODO: Check length
+        return i2leosp(256, self.s)
 
     def __eq__(self, a):
         return self.s == a.s
@@ -59,8 +58,7 @@ class FieldElement(object):
 class Fq(FieldElement):
     @staticmethod
     def from_bytes(buf):
-        s = int.from_bytes(buf, byteorder=ENDIANNESS)
-        return Fq(s)
+        return Fq(leos2ip(buf))
 
     def __init__(self, s):
         FieldElement.__init__(self, Fq, s, q_j)
@@ -111,8 +109,7 @@ class Fq(FieldElement):
 class Fr(FieldElement):
     @staticmethod
     def from_bytes(buf):
-        s = int.from_bytes(buf, byteorder=ENDIANNESS)
-        return Fr(s)
+        return Fr(leos2ip(buf))
 
     def __init__(self, s):
         FieldElement.__init__(self, Fr, s, r_j)
diff --git a/sapling_pedersen.py b/sapling_pedersen.py
index e11ea5c..6ca1721 100644
--- a/sapling_pedersen.py
+++ b/sapling_pedersen.py
@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 from sapling_generators import find_group_hash, NOTE_POSITION_BASE, WINDOWED_PEDERSEN_RANDOMNESS_BASE
-from sapling_jubjub import ENDIANNESS, Fr
+from sapling_jubjub import Fr
+from sapling_utils import cldiv, i2leosp
 
 
 #
@@ -8,7 +9,7 @@ from sapling_jubjub import ENDIANNESS, Fr
 #
 
 def I_D_i(D, i):
-    return find_group_hash(D, (i - 1).to_bytes(4, byteorder=ENDIANNESS))
+    return find_group_hash(D, i2leosp(32, i - 1))
 
 def encode_chunk(mj):
     (s0, s1, s2) = mj
@@ -20,9 +21,6 @@ def encode_segment(Mi):
     assert(len(Michunks) == ki)
     return Fr(sum([encode_chunk(Michunks[j-1]) * 2**(4*(j-1)) for j in range(1, ki + 1)]))
 
-def cldiv(n, divisor):
-    return (n + (divisor - 1)) // divisor
-
 c = 63
 
 def pedersen_hash_to_point(D, M):
diff --git a/sapling_utils.py b/sapling_utils.py
index 0a32dff..dea7215 100644
--- a/sapling_utils.py
+++ b/sapling_utils.py
@@ -1,4 +1,14 @@
 #!/usr/bin/env python3
 
+def cldiv(n, divisor):
+    return (n + (divisor - 1)) // divisor
+
 def i2lebsp(l, x):
     return [int(c) for c in format(x, '0%sb' % l)[::-1]]
+
+def leos2ip(S):
+    return int.from_bytes(S, byteorder='little')
+
+# This should be equivalent to LEBS2OSP(I2LEBSP(l, x))
+def i2leosp(l, x):
+    return x.to_bytes(cldiv(l, 8), byteorder='little')