more comprehensive test for ge_p3 comparison to identity/point at infinity

Reported by QuarksLab.

src/crypto/crypto-ops.c

@@ -3729,3 +3729,16 @@ int sc_isnonzero(const unsigned char *s) {
     s[18] | s[19] | s[20] | s[21] | s[22] | s[23] | s[24] | s[25] | s[26] |
     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)
+  {
+    if (p->X[n] | p->T[n])
+      return 0;
+    if (p->Y[n] != p->Z[n])
+      return 0;
+  }
+  return 1;
+}

src/crypto/crypto-ops.h

@@ -159,3 +159,5 @@ void ge_sub(ge_p1p1 *r, const ge_p3 *p, const ge_cached *q);
 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);

src/ringct/multiexp.cc

@@ -216,7 +216,7 @@ 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()) && memcmp(&data[n].point, &ge_p3_identity, sizeof(ge_p3)))
+    if (!(data[n].scalar == rct::zero()) && !ge_p3_is_point_at_infinity(&data[n].point))
       heap.push_back(n);
   }
   points = heap.size();
@@ -442,7 +442,7 @@ 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() || !memcmp(&data[i].point, &ge_p3_identity, sizeof(ge_p3));
+    skip[i] = data[i].scalar == rct::zero() || ge_p3_is_point_at_infinity(&data[i].point);
   MULTIEXP_PERF(PERF_TIMER_STOP(skip));
 #endif
 
@@ -611,7 +611,7 @@ rct::key pippenger(const std::vector<MultiexpData> &data, const std::shared_ptr<
 
   for (size_t k = groups; k-- > 0; )
   {
-    if (memcmp(&result, &ge_p3_identity, sizeof(ge_p3)))
+    if (!ge_p3_is_point_at_infinity(&result))
     {
       ge_p2 p2;
       ge_p3_to_p2(&p2, &result);
@@ -638,7 +638,7 @@ rct::key pippenger(const std::vector<MultiexpData> &data, const std::shared_ptr<
       if (bucket == 0)
         continue;
       CHECK_AND_ASSERT_THROW_MES(bucket < (1u<<c), "bucket overflow");
-      if (memcmp(&buckets[bucket], &ge_p3_identity, sizeof(ge_p3)))
+      if (!ge_p3_is_point_at_infinity(&buckets[bucket]))
       {
         add(buckets[bucket], local_cache->cached[i]);
       }
@@ -650,9 +650,9 @@ rct::key pippenger(const std::vector<MultiexpData> &data, const std::shared_ptr<
     ge_p3 pail = ge_p3_identity;
     for (size_t i = (1<<c)-1; i > 0; --i)
     {
-      if (memcmp(&buckets[i], &ge_p3_identity, sizeof(ge_p3)))
+      if (!ge_p3_is_point_at_infinity(&buckets[i]))
         add(pail, buckets[i]);
-      if (memcmp(&pail, &ge_p3_identity, sizeof(ge_p3)))
+      if (!ge_p3_is_point_at_infinity(&pail))
         add(result, pail);
     }
   }