Support Range Proof for ConfidentialMPTSend (#6404)

- proving send amount m is in the range [0, 2^64)
- proving remaining balance b-m is in the range [0, 2^64)

cspell.config.yaml

@@ -54,6 +54,7 @@ words:
   - autobridging
   - bimap
   - bindir
+  - blindings
   - bookdir
   - Bougalis
   - Britto
@@ -249,6 +250,7 @@ words:
   - stvar
   - stvector
   - stxchainattestations
+  - summands
   - superpeer
   - superpeers
   - takergets

include/xrpl/protocol/ConfidentialTransfer.h

@@ -469,6 +469,24 @@ verifyAggregatedBulletproof(
     std::vector<Slice> const& compressedCommitments,
     uint256 const& contextHash);
 
+/**
+ * @brief Computes the remainder commitment for ConfidentialMPTSend.
+ *
+ * Given a balance commitment PC_bal = m_bal*G + rho_bal*H and an amount
+ * commitment PC_amt = m_amt*G + rho_amt*H, this function computes:
+ * PC_rem = PC_bal - PC_amt = (m_bal - m_amt)*G + (rho_bal - rho_amt)*H
+ *
+ * This derived commitment is used in an aggregated range proof to ensure
+ * the sender maintains a non-negative balance (m_bal - m_amt >= 0).
+ *
+ * @param balanceCommitment The compressed Pedersen commitment to the balance (33 bytes).
+ * @param amountCommitment  The compressed Pedersen commitment to the amount (33 bytes).
+ * @param out               Output buffer for the resulting remainder commitment (33 bytes).
+ * @return tesSUCCESS on success, tecINTERNAL on failure.
+ */
+TER
+computeSendRemainder(Slice const& balanceCommitment, Slice const& amountCommitment, Buffer& out);
+
 /**
  * @brief Computes the remainder commitment for ConvertBack.
  *

src/libxrpl/protocol/ConfidentialTransfer.cpp

@@ -579,6 +579,41 @@ verifyAggregatedBulletproof(
     return tesSUCCESS;
 }
 
+TER
+computeSendRemainder(Slice const& balanceCommitment, Slice const& amountCommitment, Buffer& out)
+{
+    if (balanceCommitment.size() != ecPedersenCommitmentLength || amountCommitment.size() != ecPedersenCommitmentLength)
+        return tecINTERNAL;
+
+    auto const ctx = secp256k1Context();
+
+    secp256k1_pubkey pcBalance;
+    if (secp256k1_ec_pubkey_parse(ctx, &pcBalance, balanceCommitment.data(), ecPedersenCommitmentLength) != 1)
+        return tecINTERNAL;
+
+    secp256k1_pubkey pcAmount;
+    if (secp256k1_ec_pubkey_parse(ctx, &pcAmount, amountCommitment.data(), ecPedersenCommitmentLength) != 1)
+        return tecINTERNAL;
+
+    // Negate PC_amount point to get -PC_amount
+    if (!secp256k1_ec_pubkey_negate(ctx, &pcAmount))
+        return tecINTERNAL;
+
+    // Compute pcRem = pcBalance + (-pcAmount)
+    secp256k1_pubkey const* summands[2] = {&pcBalance, &pcAmount};
+    secp256k1_pubkey pcRem;
+    if (!secp256k1_ec_pubkey_combine(ctx, &pcRem, summands, 2))
+        return tecINTERNAL;
+
+    // Serialize result to compressed format
+    out.alloc(ecPedersenCommitmentLength);
+    size_t outLen = ecPedersenCommitmentLength;
+    if (secp256k1_ec_pubkey_serialize(ctx, out.data(), &outLen, &pcRem, SECP256K1_EC_COMPRESSED) != 1)
+        return tecINTERNAL;
+
+    return tesSUCCESS;
+}
+
 TER
 computeConvertBackRemainder(Slice const& commitment, std::uint64_t amount, Buffer& out)
 {

src/test/app/ConfidentialTransfer_test.cpp

@@ -72,7 +72,7 @@ class ConfidentialTransfer_test : public beast::unit_test::suite
     getTrivialSendProofHex(size_t nRecipients)
     {
         size_t const sizeEquality = getMultiCiphertextEqualityProofSize(nRecipients);
-        size_t const totalSize = sizeEquality + (2 * ecPedersenProofLength);
+        size_t const totalSize = sizeEquality + (2 * ecPedersenProofLength) + ecDoubleBulletproofLength;
 
         Buffer buf(totalSize);
         std::memset(buf.data(), 0, totalSize);
@@ -1721,6 +1721,71 @@ class ConfidentialTransfer_test : public beast::unit_test::suite
         }
     }
 
+    void
+    testSendRangeProof(FeatureBitset features)
+    {
+        testcase("test ConfidentialMPTSend Range Proof");
+
+        using namespace test::jtx;
+        Env env{*this, features};
+        Account const alice("alice");
+        Account const bob("bob");
+        Account const carol("carol");
+        MPTTester mptAlice(env, alice, {.holders = {bob, carol}});
+
+        mptAlice.create({.ownerCount = 1, .flags = tfMPTCanLock | tfMPTCanPrivacy | tfMPTCanTransfer});
+        mptAlice.authorize({.account = bob});
+        mptAlice.authorize({.account = carol});
+
+        mptAlice.pay(alice, bob, 1000);
+        mptAlice.pay(alice, carol, 1000);
+
+        mptAlice.generateKeyPair(alice);
+        mptAlice.generateKeyPair(bob);
+        mptAlice.generateKeyPair(carol);
+
+        mptAlice.set({.account = alice, .issuerPubKey = mptAlice.getPubKey(alice)});
+
+        {
+            // Bob converts 60
+            mptAlice.convert({.account = bob, .amt = 60, .holderPubKey = mptAlice.getPubKey(bob)});
+            mptAlice.mergeInbox({.account = bob});
+
+            mptAlice.convert({.account = carol, .amt = 50, .holderPubKey = mptAlice.getPubKey(carol)});
+            mptAlice.mergeInbox({.account = carol});
+
+            // Bob has 60, tries to send 70. Invalid remaining balance.
+            mptAlice.send({.account = bob, .dest = carol, .amt = 70, .err = tecBAD_PROOF});
+
+            // Bob has 60, tries to send 61. Invalid remaining balance.
+            mptAlice.send({.account = bob, .dest = carol, .amt = 61, .err = tecBAD_PROOF});
+
+            // Bob has 60, sends 60. Remainder is exactly 0. Valid remaining balance.
+            mptAlice.send({.account = bob, .dest = carol, .amt = 60, .err = tesSUCCESS});
+        }
+
+        {
+            // Bob converts 100.
+            mptAlice.convert({.account = bob, .amt = 100});
+            mptAlice.mergeInbox({.account = bob});
+
+            // Bob has 100, tries to send 2^64-1. Invalid remaining balance.
+            mptAlice.send(
+                {.account = bob,
+                 .dest = carol,
+                 .amt = 0xFFFFFFFFFFFFFFFF,  // Max uint64
+                 .err = tecBAD_PROOF});
+
+            // Bob sends 1, remaining 99.
+            mptAlice.send({.account = bob, .dest = carol, .amt = 1, .err = tesSUCCESS});
+
+            // Bob sends 100, but only has 99. Invalid remaining balance.
+            mptAlice.send({.account = bob, .dest = carol, .amt = 100, .err = tecBAD_PROOF});
+        }
+
+        // todo: test m exceeding range, require using scala and refactor
+    }
+
     void
     testDelete(FeatureBitset features)
     {
@@ -3668,11 +3733,13 @@ class ConfidentialTransfer_test : public beast::unit_test::suite
     void
     testWithFeats(FeatureBitset features)
     {
+        // ConfidentialMPTConvert
         testConvert(features);
         testConvertPreflight(features);
         testConvertPreclaim(features);
         testConvertWithAuditor(features);
 
+        // ConfidentialMPTMergeInbox
         testMergeInbox(features);
         testMergeInboxPreflight(features);
         testMergeInboxPreclaim(features);
@@ -3683,6 +3750,7 @@ class ConfidentialTransfer_test : public beast::unit_test::suite
         testSend(features);
         testSendPreflight(features);
         testSendPreclaim(features);
+        testSendRangeProof(features);
         testSendDepositPreauth(features);
         testSendWithAuditor(features);
 
@@ -3695,6 +3763,7 @@ class ConfidentialTransfer_test : public beast::unit_test::suite
 
         testDelete(features);
 
+        // ConfidentialMPTConvertBack
         testConvertBack(features);
         testConvertBackPreflight(features);
         testConvertBackPreclaim(features);

src/test/jtx/impl/mpt.cpp

@@ -820,10 +820,27 @@ MPTTester::getConfidentialSendProof(
 
     auto const balanceLinkageProof = getBalanceLinkageProof(sender, contextHash, *senderPubKey, balanceParams);
 
+    std::uint64_t const remainingBalance = balanceParams.amt - amount;
+
+    // Compute the blinding factor for the remaining balance: rho_rem = rho_balance - rho_amount
+    unsigned char rho_rem[32];
+    unsigned char neg_rho_m[32];
+
+    secp256k1_mpt_scalar_negate(neg_rho_m, amountParams.blindingFactor.data());
+    secp256k1_mpt_scalar_add(rho_rem, balanceParams.blindingFactor.data(), neg_rho_m);
+
+    // Generate bulletproof for the amount and remaining balance
+    Buffer const bulletproof =
+        getBulletproof({amount, remainingBalance}, {amountParams.blindingFactor, Buffer(rho_rem, 32)}, contextHash);
+
+    OPENSSL_cleanse(neg_rho_m, 32);
+    OPENSSL_cleanse(rho_rem, 32);
+
     auto const sizeAmountLinkage = amountLinkageProof.size();
     auto const sizeBalanceLinkage = balanceLinkageProof.size();
+    auto const sizeBulletproof = bulletproof.size();
 
-    size_t const proofSize = sizeEquality + sizeAmountLinkage + sizeBalanceLinkage;
+    size_t const proofSize = sizeEquality + sizeAmountLinkage + sizeBalanceLinkage + sizeBulletproof;
     Buffer proof(proofSize);
 
     auto ptr = proof.data();
@@ -834,6 +851,9 @@ MPTTester::getConfidentialSendProof(
     ptr += sizeAmountLinkage;
 
     std::memcpy(ptr, balanceLinkageProof.data(), sizeBalanceLinkage);
+    ptr += sizeBalanceLinkage;
+
+    std::memcpy(ptr, bulletproof.data(), sizeBulletproof);
 
     return proof;
 }
@@ -1298,7 +1318,8 @@ MPTTester::send(MPTConfidentialSend const& arg)
             jv[sfZKProof.jsonName] = strHex(*proof);
         else
         {
-            size_t const dummySize = secp256k1_mpt_prove_same_plaintext_multi_size(nRecipients);
+            size_t const sizeEquality = secp256k1_mpt_prove_same_plaintext_multi_size(nRecipients);
+            size_t const dummySize = sizeEquality + 2 * ecPedersenProofLength + ecDoubleBulletproofLength;
 
             jv[sfZKProof.jsonName] = strHex(Buffer(dummySize));
         }

src/xrpld/app/tx/detail/ConfidentialMPTSend.cpp

@@ -44,7 +44,7 @@ ConfidentialMPTSend::preflight(PreflightContext const& ctx)
     auto const sizeEquality = getMultiCiphertextEqualityProofSize(recipientCount);
     auto const sizePedersenLinkage = 2 * ecPedersenProofLength;
 
-    if (ctx.tx[sfZKProof].length() != sizeEquality + sizePedersenLinkage)
+    if (ctx.tx[sfZKProof].length() != sizeEquality + sizePedersenLinkage + ecDoubleBulletproofLength)
         return temMALFORMED;
 
     // Check the Pedersen commitments are valid
@@ -105,7 +105,14 @@ verifySendProofs(
     currentOffset += ecPedersenProofLength;
     remainingLength -= ecPedersenProofLength;
 
-    // todo: Extract range proof once the lib is ready
+    // Extract range proof
+    if (remainingLength < ecDoubleBulletproofLength)
+        return tecINTERNAL;
+
+    auto const rangeProof = proof.substr(currentOffset, ecDoubleBulletproofLength);
+    currentOffset += ecDoubleBulletproofLength;
+    remainingLength -= ecDoubleBulletproofLength;
+
     if (remainingLength != 0)
         return tecINTERNAL;  // LCOV_EXCL_LINE
 
@@ -114,9 +121,7 @@ verifySendProofs(
     recipients.reserve(recipientCount);
 
     recipients.push_back({(*sleSenderMPToken)[sfHolderElGamalPublicKey], ctx.tx[sfSenderEncryptedAmount]});
-
     recipients.push_back({(*sleDestinationMPToken)[sfHolderElGamalPublicKey], ctx.tx[sfDestinationEncryptedAmount]});
-
     recipients.push_back({(*sleIssuance)[sfIssuerElGamalPublicKey], ctx.tx[sfIssuerEncryptedAmount]});
 
     if (hasAuditor)
@@ -132,12 +137,15 @@ verifySendProofs(
         ctx.tx[sfDestination],
         (*sleSenderMPToken)[~sfConfidentialBalanceVersion].value_or(0));
 
+    // Use a boolean flag to track validity instead of returning early on failure to prevent leaking information about
+    // which proof failed through timing differences
+    bool valid = true;
+
     // Verify the multi-ciphertext equality proof
     if (auto const ter = verifyMultiCiphertextEqualityProof(equalityProof, recipients, recipientCount, contextHash);
         !isTesSuccess(ter))
     {
-        JLOG(ctx.j.trace()) << "ConfidentialMPTSend: Equality proof failed.";
-        return ter;
+        valid = false;
     }
 
     // Verify amount linkage
@@ -149,8 +157,7 @@ verifySendProofs(
             contextHash);
         !isTesSuccess(ter))
     {
-        JLOG(ctx.j.trace()) << "ConfidentialMPTSend: Amount linkage proof failed.";
-        return ter;
+        valid = false;
     }
 
     // Verify balance linkage
@@ -162,8 +169,38 @@ verifySendProofs(
             contextHash);
         !isTesSuccess(ter))
     {
-        JLOG(ctx.j.trace()) << "ConfidentialMPTSend: Balance linkage proof failed.";
-        return ter;
+        valid = false;
+    }
+
+    // Verify Range Proof
+    {
+        Buffer pcRem;
+
+        // Derive PC_rem = PC_balance - PC_amount
+        if (auto const ter = computeSendRemainder(ctx.tx[sfBalanceCommitment], ctx.tx[sfAmountCommitment], pcRem);
+            !isTesSuccess(ter))
+        {
+            valid = false;
+        }
+        else
+        {
+            // Aggregated commitments: [PC_amount, PC_rem]
+            // Prove that both the transfer amount and the remaining balance are in range
+            std::vector<Slice> commitments;
+            commitments.push_back(ctx.tx[sfAmountCommitment]);
+            commitments.push_back(Slice{pcRem.data(), pcRem.size()});
+
+            if (auto const ter = verifyAggregatedBulletproof(rangeProof, commitments, contextHash); !isTesSuccess(ter))
+            {
+                valid = false;
+            }
+        }
+    }
+
+    if (!valid)
+    {
+        JLOG(ctx.j.trace()) << "ConfidentialMPTSend: One or more cryptographic proofs failed.";
+        return tecBAD_PROOF;
     }
 
     return tesSUCCESS;