From c36ea26e5c6b94f107cc1aeb02a45cfc5a7c38ef Mon Sep 17 00:00:00 2001
From: koe <ukoe@protonmail.com>
Date: Mon, 8 Nov 2021 18:14:17 -0600
Subject: fix ge_p3_is_point_at_infinity(), which is evaluating field elements
 that haven't been reduced by the field order

---
 src/crypto/crypto-ops.c | 50 ++++++++++++++++++++++++++++++++++++++++++-------
 src/crypto/crypto-ops.h |  2 +-
 src/ringct/multiexp.cc  |  4 ++--
 3 files changed, 46 insertions(+), 10 deletions(-)

(limited to 'src')

diff --git a/src/crypto/crypto-ops.c b/src/crypto/crypto-ops.c
index 508709280..54ca39775 100644
--- a/src/crypto/crypto-ops.c
+++ b/src/crypto/crypto-ops.c
@@ -3830,15 +3830,51 @@ int sc_isnonzero(const unsigned char *s) {
     s[27] | s[28] | s[29] | s[30] | s[31]) - 1) >> 8) + 1;
 }
 
-int ge_p3_is_point_at_infinity(const ge_p3 *p) {
-  // X = 0 and Y == Z
-  int n;
-  for (n = 0; n < 10; ++n)
+int ge_p3_is_point_at_infinity_vartime(const ge_p3 *p) {
+  // https://eprint.iacr.org/2008/522
+  // X == T == 0 and Y/Z == 1
+  // note: convert all pieces to canonical bytes in case rounding is required (i.e. an element is > q)
+  // note2: even though T = XY/Z is true for valid point representations (implying it isn't necessary to
+  //        test T == 0), the input to this function might NOT be valid, so we must test T == 0
+  char result_X_bytes[32];
+  fe_tobytes((unsigned char*)&result_X_bytes, p->X);
+
+  // X != 0
+  for (int i = 0; i < 32; ++i)
   {
-    if (p->X[n] | p->T[n])
+    if (result_X_bytes[i])
       return 0;
-    if (p->Y[n] != p->Z[n])
+  }
+
+  char result_T_bytes[32];
+  fe_tobytes((unsigned char*)&result_T_bytes, p->T);
+
+  // T != 0
+  for (int i = 0; i < 32; ++i)
+  {
+    if (result_T_bytes[i])
+      return 0;
+  }
+
+  char result_Y_bytes[32];
+  char result_Z_bytes[32];
+  fe_tobytes((unsigned char*)&result_Y_bytes, p->Y);
+  fe_tobytes((unsigned char*)&result_Z_bytes, p->Z);
+
+  // Y != Z
+  for (int i = 0; i < 32; ++i)
+  {
+    if (result_Y_bytes[i] != result_Z_bytes[i])
       return 0;
   }
-  return 1;
+
+  // is Y nonzero? then Y/Z == 1
+  for (int i = 0; i < 32; ++i)
+  {
+    if (result_Y_bytes[i] != 0)
+      return 1;
+  }
+
+  // Y/Z = 0/0
+  return 0;
 }
diff --git a/src/crypto/crypto-ops.h b/src/crypto/crypto-ops.h
index 22f76974b..96da16cbd 100644
--- a/src/crypto/crypto-ops.h
+++ b/src/crypto/crypto-ops.h
@@ -162,4 +162,4 @@ void fe_add(fe h, const fe f, const fe g);
 void fe_tobytes(unsigned char *, const fe);
 void fe_invert(fe out, const fe z);
 
-int ge_p3_is_point_at_infinity(const ge_p3 *p);
+int ge_p3_is_point_at_infinity_vartime(const ge_p3 *p);
diff --git a/src/ringct/multiexp.cc b/src/ringct/multiexp.cc
index 784c90a4e..f256325a1 100644
--- a/src/ringct/multiexp.cc
+++ b/src/ringct/multiexp.cc
@@ -235,7 +235,7 @@ rct::key bos_coster_heap_conv_robust(std::vector<MultiexpData> data)
   heap.reserve(points);
   for (size_t n = 0; n < points; ++n)
   {
-    if (!(data[n].scalar == rct::zero()) && !ge_p3_is_point_at_infinity(&data[n].point))
+    if (!(data[n].scalar == rct::zero()) && !ge_p3_is_point_at_infinity_vartime(&data[n].point))
       heap.push_back(n);
   }
   points = heap.size();
@@ -457,7 +457,7 @@ rct::key straus(const std::vector<MultiexpData> &data, const std::shared_ptr<str
   MULTIEXP_PERF(PERF_TIMER_START_UNIT(skip, 1000000));
   std::vector<uint8_t> skip(data.size());
   for (size_t i = 0; i < data.size(); ++i)
-    skip[i] = data[i].scalar == rct::zero() || ge_p3_is_point_at_infinity(&data[i].point);
+    skip[i] = data[i].scalar == rct::zero() || ge_p3_is_point_at_infinity_vartime(&data[i].point);
   MULTIEXP_PERF(PERF_TIMER_STOP(skip));
 #endif
 
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