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

@@ -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) {
+int ge_p3_is_point_at_infinity_vartime(const ge_p3 *p) {
-  // X = 0 and Y == Z
+  // https://eprint.iacr.org/2008/522
-  int n;
+  // X == T == 0 and Y/Z == 1
-  for (n = 0; n < 10; ++n)
+  // 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])
       return 0;
   }
-  return 1;
+
+  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;
+  }
+
+  // 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;
 }

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);

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
 

tests/crypto/crypto-tests.h

@@ -46,4 +46,6 @@ void random_scalar(crypto::ec_scalar &res);
 void hash_to_scalar(const void *data, std::size_t length, crypto::ec_scalar &res);
 void hash_to_point(const crypto::hash &h, crypto::ec_point &res);
 void hash_to_ec(const crypto::public_key &key, crypto::ec_point &res);
+bool check_ge_p3_identity_failure(const crypto::public_key &point);
+bool check_ge_p3_identity_success(const crypto::public_key &point);
 #endif

tests/crypto/crypto.cpp

@@ -32,6 +32,36 @@
 
 #include "crypto-tests.h"
 
+static void get_ge_p3_for_identity_test(const crypto::public_key &point, crypto::ge_p3 &result_out_p3)
+{
+  // compute (K + K) - K - K to get a specific ge_p3 point representation of identity
+  crypto::ge_cached temp_cache;
+  crypto::ge_p1p1 temp_p1p1;
+
+  crypto::ge_frombytes_vartime(&result_out_p3, &point);  // K
+  crypto::ge_p3_to_cached(&temp_cache, &result_out_p3);
+  crypto::ge_add(&temp_p1p1, &result_out_p3, &temp_cache);  // K + K
+  crypto::ge_p1p1_to_p3(&result_out_p3, &temp_p1p1);
+  crypto::ge_sub(&temp_p1p1, &result_out_p3, &temp_cache);  // (K + K) - K
+  crypto::ge_p1p1_to_p3(&result_out_p3, &temp_p1p1);
+  crypto::ge_sub(&temp_p1p1, &result_out_p3, &temp_cache);  // ((K + K) - K) - K
+  crypto::ge_p1p1_to_p3(&result_out_p3, &temp_p1p1);
+}
+
+static int ge_p3_is_point_at_infinity_vartime_bad(const crypto::ge_p3 *p) {
+  // X = 0 and Y == Z
+  // bad: components of 'p' are not reduced mod q
+  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;
+}
+
 bool check_scalar(const crypto::ec_scalar &scalar) {
   return crypto::sc_check(crypto::operator &(scalar)) == 0;
 }
@@ -55,3 +85,19 @@ void hash_to_ec(const crypto::public_key &key, crypto::ec_point &res) {
   crypto::hash_to_ec(key, tmp);
   crypto::ge_p3_tobytes(crypto::operator &(res), &tmp);
 }
+
+bool check_ge_p3_identity_failure(const crypto::public_key &point)
+{
+  crypto::ge_p3 ident_p3;
+  get_ge_p3_for_identity_test(point, ident_p3);
+
+  return ge_p3_is_point_at_infinity_vartime_bad(&ident_p3) == 1;
+}
+
+bool check_ge_p3_identity_success(const crypto::public_key &point)
+{
+  crypto::ge_p3 ident_p3;
+  get_ge_p3_for_identity_test(point, ident_p3);
+
+  return crypto::ge_p3_is_point_at_infinity_vartime(&ident_p3) == 1;
+}

tests/crypto/main.cpp

@@ -259,6 +259,16 @@ int main(int argc, char *argv[]) {
       if (expected != actual) {
         goto error;
       }
+    } else if (cmd == "check_ge_p3_identity") {
+      cerr << "Testing: " << cmd << endl;
+      public_key point;
+      bool expected_bad, expected_good, result_badfunc, result_goodfunc;
+      get(input, point, expected_bad, expected_good);
+      result_badfunc = check_ge_p3_identity_failure(point);
+      result_goodfunc = check_ge_p3_identity_success(point);
+      if (expected_bad != result_badfunc || expected_good != result_goodfunc) {
+        goto error;
+      }
     } else {
       throw ios_base::failure("Unknown function: " + cmd);
     }