bulletproofs: reject points not in the main subgroup

3 files changed, 33 insertions, 0 deletions

diff --git a/src/ringct/bulletproofs.cc b/src/ringct/bulletproofs.cc
index bc7e15f35..7ec87378b 100644
--- a/src/ringct/bulletproofs.cc
+++ b/src/ringct/bulletproofs.cc
@@ -922,6 +922,19 @@ bool bulletproof_VERIFY(const std::vector<const Bulletproof*> &proofs)
   for (const Bulletproof *p: proofs)
   {
     const Bulletproof &proof = *p;
+
+    // check subgroup
+    for (const rct::key &k: proof.V)
+      CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(k), false, "Input point not in subgroup");
+    for (const rct::key &k: proof.L)
+      CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(k), false, "Input point not in subgroup");
+    for (const rct::key &k: proof.R)
+      CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(k), false, "Input point not in subgroup");
+    CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(proof.A), false, "Input point not in subgroup");
+    CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(proof.S), false, "Input point not in subgroup");
+    CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(proof.T1), false, "Input point not in subgroup");
+    CHECK_AND_ASSERT_MES(rct::isInMainSubgroup(proof.T2), false, "Input point not in subgroup");
+
     CHECK_AND_ASSERT_MES(proof.V.size() >= 1, false, "V does not have at least one element");
     CHECK_AND_ASSERT_MES(proof.L.size() == proof.R.size(), false, "Mismatched L and R sizes");
     CHECK_AND_ASSERT_MES(proof.L.size() > 0, false, "Empty proof");
diff --git a/src/ringct/rctOps.cpp b/src/ringct/rctOps.cpp
index fe0ad8747..df027f4b6 100644
--- a/src/ringct/rctOps.cpp
+++ b/src/ringct/rctOps.cpp
@@ -60,6 +60,17 @@ namespace rct {
 
     //Various key generation functions
 
+    bool toPointCheckOrder(ge_p3 *P, const unsigned char *data)
+    {
+        if (ge_frombytes_vartime(P, data))
+            return false;
+        ge_p2 R;
+        ge_scalarmult(&R, curveOrder().bytes, P);
+        key tmp;
+        ge_tobytes(tmp.bytes, &R);
+        return tmp == identity();
+    }
+
     //generates a random scalar which can be used as a secret key or mask
     void skGen(key &sk) {
         random32_unbiased(sk.bytes);
@@ -200,6 +211,12 @@ namespace rct {
         return aP;
     }
 
+    //Computes aL where L is the curve order
+    bool isInMainSubgroup(const key & a) {
+        ge_p3 p3;
+        return toPointCheckOrder(&p3, a.bytes);
+    }
+
     //Curve addition / subtractions
 
     //for curve points: AB = A + B
diff --git a/src/ringct/rctOps.h b/src/ringct/rctOps.h
index f8889af5c..f0320f333 100644
--- a/src/ringct/rctOps.h
+++ b/src/ringct/rctOps.h
@@ -83,6 +83,7 @@ namespace rct {
     keyM keyMInit(size_t rows, size_t cols);
 
     //Various key generation functions        
+    bool toPointCheckOrder(ge_p3 *P, const unsigned char *data);
 
     //generates a random scalar which can be used as a secret key or mask
     key skGen();
@@ -119,6 +120,8 @@ namespace rct {
     key scalarmultKey(const key &P, const key &a);
     //Computes aH where H= toPoint(cn_fast_hash(G)), G the basepoint
     key scalarmultH(const key & a);
+    // checks a is in the main subgroup (ie, not a small one)
+    bool isInMainSubgroup(const key & a);
 
     //Curve addition / subtractions