Add asset and value blinding functionality

kallewoof · Mar 20, 2019 · f81122f · f81122f
1 parent 8b8256d
commit f81122f
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Showing 6 changed files with 432 additions and 8 deletions.
diff --git a/src/Makefile.am b/src/Makefile.am
@@ -97,6 +97,7 @@ BITCOIN_CORE_H = \
   base58.h \
   bech32.h \
   blech32.h \
+  blind.h \
   bloom.h \
   blockencodings.h \
   blockfilter.h \
@@ -114,6 +115,7 @@ BITCOIN_CORE_H = \
   compat/endian.h \
   compat/sanity.h \
   compressor.h \
+  confidential_validation.h \
   consensus/consensus.h \
   consensus/tx_verify.h \
   core_io.h \
@@ -233,6 +235,7 @@ libbitcoin_server_a_SOURCES = \
   block_proof.cpp \
   chain.cpp \
   checkpoints.cpp \
+  confidential_validation.cpp \
   consensus/tx_verify.cpp \
   httprpc.cpp \
   httpserver.cpp \
@@ -412,6 +415,7 @@ libbitcoin_common_a_SOURCES = \
   base58.cpp \
   bech32.cpp \
   blech32.cpp \
+  blind.cpp \
   chainparams.cpp \
   coins.cpp \
   compressor.cpp \

diff --git a/src/blind.cpp b/src/blind.cpp
@@ -0,0 +1,190 @@
+// Copyright (c) 2017-2019 The Elements Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <blind.h>
+
+#include <hash.h>
+#include <primitives/transaction.h>
+#include <primitives/confidential.h>
+#include <issuance.h>
+#include <random.h>
+#include <util.h>
+
+static secp256k1_context* secp256k1_blind_context = NULL;
+
+class Blind_ECC_Init {
+public:
+    Blind_ECC_Init() {
+        assert(secp256k1_blind_context == NULL);
+
+        secp256k1_context *ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
+        assert(ctx != NULL);
+
+        secp256k1_blind_context = ctx;
+    }
+
+    ~Blind_ECC_Init() {
+        secp256k1_context *ctx = secp256k1_blind_context;
+        secp256k1_blind_context = NULL;
+
+        if (ctx) {
+            secp256k1_context_destroy(ctx);
+        }
+    }
+};
+
+static Blind_ECC_Init ecc_init_on_load;
+
+bool UnblindConfidentialPair(const CKey& blinding_key, const CConfidentialValue& conf_value, const CConfidentialAsset& conf_asset, const CConfidentialNonce& nonce_commitment, const CScript& committedScript, const std::vector<unsigned char>& vchRangeproof, CAmount& amount_out, uint256& blinding_factor_out, CAsset& asset_out, uint256& asset_blinding_factor_out)
+{
+    if (!blinding_key.IsValid() || vchRangeproof.size() == 0) {
+        return false;
+    }
+    CPubKey ephemeral_key(nonce_commitment.vchCommitment);
+    if (nonce_commitment.vchCommitment.size() > 0 && !ephemeral_key.IsFullyValid()) {
+        return false;
+    }
+
+    // ECDH or not depending on if nonce commitment is non-empty
+    uint256 nonce;
+    bool blank_nonce = false;
+    if (nonce_commitment.vchCommitment.size() > 0) {
+        nonce = blinding_key.ECDH(ephemeral_key);
+        CSHA256().Write(nonce.begin(), 32).Finalize(nonce.begin());
+    } else {
+        // Use blinding key directly, and don't commit to a scriptpubkey
+        // This is used for issuance inputs.
+        blank_nonce = true;
+        nonce = uint256(std::vector<unsigned char>(blinding_key.begin(), blinding_key.end()));
+    }
+
+    // API-prescribed sidechannel maximum size, though we only use 64 bytes
+    unsigned char msg[4096] = {0};
+    // 32 bytes of asset type, 32 bytes of asset blinding factor in sidechannel
+    size_t msg_size = 64;
+
+    // If value is unblinded, we don't support unblinding just the asset
+    if (!conf_value.IsCommitment()) {
+        return false;
+    }
+
+    // Valid asset commitment?
+    secp256k1_generator observed_gen;
+    if (conf_asset.IsCommitment()) {
+        if (secp256k1_generator_parse(secp256k1_blind_context, &observed_gen, &conf_asset.vchCommitment[0]) != 1)
+            return false;
+    } else if (conf_asset.IsExplicit()) {
+        if (secp256k1_generator_generate(secp256k1_blind_context, &observed_gen, conf_asset.GetAsset().begin()) != 1)
+            return false;
+    }
+
+    // Valid value commitment?
+    secp256k1_pedersen_commitment value_commit;
+    if (secp256k1_pedersen_commitment_parse(secp256k1_blind_context, &value_commit, conf_value.vchCommitment.data()) != 1) {
+        return false;
+    }
+
+    // Rewind rangeproof
+    uint64_t min_value, max_value, amount;
+    if (!secp256k1_rangeproof_rewind(secp256k1_blind_context, blinding_factor_out.begin(), &amount, msg, &msg_size, nonce.begin(), &min_value, &max_value, &value_commit, &vchRangeproof[0], vchRangeproof.size(), (committedScript.size() && !blank_nonce)? &committedScript.front(): NULL, blank_nonce ? 0 : committedScript.size(), &observed_gen)) {
+        return false;
+    }
+
+    // Value sidechannel must be a transaction-valid amount (should be belt-and-suspenders check)
+    if (amount > (uint64_t)MAX_MONEY || !MoneyRange((CAmount)amount)) {
+        return false;
+    }
+
+    // Convienience pointers to starting point of each recovered 32 byte message
+    unsigned char *asset_type = msg;
+    unsigned char *asset_blinder = msg+32;
+
+    // Asset sidechannel of asset type + asset blinder
+    secp256k1_generator recalculated_gen;
+    if (msg_size != 64 || secp256k1_generator_generate_blinded(secp256k1_blind_context, &recalculated_gen, asset_type, asset_blinder) != 1) {
+        return false;
+    }
+
+    // Serialize both generators then compare
+    unsigned char observed_generator[33];
+    unsigned char derived_generator[33];
+    secp256k1_generator_serialize(secp256k1_blind_context, observed_generator, &observed_gen);
+    secp256k1_generator_serialize(secp256k1_blind_context, derived_generator, &recalculated_gen);
+    if (memcmp(observed_generator, derived_generator, sizeof(observed_generator))) {
+        return false;
+    }
+
+    amount_out = (CAmount)amount;
+    asset_out = CAsset(std::vector<unsigned char>(asset_type, asset_type+32));
+    asset_blinding_factor_out = uint256(std::vector<unsigned char>(asset_blinder, asset_blinder+32));
+    return true;
+}
+
+// Create surjection proof
+bool SurjectOutput(CTxOutWitness& txoutwit, const std::vector<secp256k1_fixed_asset_tag>& surjection_targets, const std::vector<secp256k1_generator>& target_asset_generators, const std::vector<uint256 >& target_asset_blinders, const std::vector<const unsigned char*> asset_blindptrs, const secp256k1_generator& output_asset_gen, const CAsset& asset)
+{
+    int ret;
+    // 1 to 3 targets
+    size_t nInputsToSelect = std::min((size_t)3, surjection_targets.size());
+    unsigned char randseed[32];
+    GetStrongRandBytes(randseed, 32);
+    size_t input_index;
+    secp256k1_surjectionproof proof;
+    secp256k1_fixed_asset_tag tag;
+    memcpy(&tag, asset.begin(), 32);
+    // Find correlation between asset tag and listed input tags
+    if (secp256k1_surjectionproof_initialize(secp256k1_blind_context, &proof, &input_index, &surjection_targets[0], surjection_targets.size(), nInputsToSelect, &tag, 100, randseed) == 0) {
+        return false;
+    }
+    // Using the input chosen, build proof
+    ret = secp256k1_surjectionproof_generate(secp256k1_blind_context, &proof, target_asset_generators.data(), target_asset_generators.size(), &output_asset_gen, input_index, target_asset_blinders[input_index].begin(), asset_blindptrs[asset_blindptrs.size()-1]);
+    assert(ret == 1);
+    // Double-check answer
+    ret = secp256k1_surjectionproof_verify(secp256k1_blind_context, &proof, target_asset_generators.data(), target_asset_generators.size(), &output_asset_gen);
+    assert(ret != 0);
+
+    // Serialize into output witness structure
+    size_t output_len = secp256k1_surjectionproof_serialized_size(secp256k1_blind_context, &proof);
+    txoutwit.vchSurjectionproof.resize(output_len);
+    secp256k1_surjectionproof_serialize(secp256k1_blind_context, &txoutwit.vchSurjectionproof[0], &output_len, &proof);
+    assert(output_len == txoutwit.vchSurjectionproof.size());
+    return true;
+}
+
+bool GenerateRangeproof(std::vector<unsigned char>& rangeproof, const std::vector<unsigned char*>& value_blindptrs, const uint256& nonce, const CAmount amount, const CScript& scriptPubKey, const secp256k1_pedersen_commitment& value_commit, const secp256k1_generator& gen, const CAsset& asset, std::vector<const unsigned char*>& asset_blindptrs)
+{
+    // Prep range proof
+    size_t nRangeProofLen = 5134;
+    rangeproof.resize(nRangeProofLen);
+
+    // Compose sidechannel message to convey asset info (ID and asset blinds)
+    unsigned char asset_message[64];
+    memcpy(asset_message, asset.begin(), 32);
+    memcpy(asset_message+32, asset_blindptrs[asset_blindptrs.size()-1], 32);
+
+    // Sign rangeproof
+    // If min_value is 0, scriptPubKey must be unspendable
+    int res = secp256k1_rangeproof_sign(secp256k1_blind_context, rangeproof.data(), &nRangeProofLen, scriptPubKey.IsUnspendable() ? 0 : 1, &value_commit, value_blindptrs.back(), nonce.begin(), std::min(std::max((int)gArgs.GetArg("-ct_exponent", 0), -1),18), std::min(std::max((int)gArgs.GetArg("-ct_bits", 36), 1), 51), amount, asset_message, sizeof(asset_message), scriptPubKey.size() ? &scriptPubKey.front() : NULL, scriptPubKey.size(), &gen);
+    rangeproof.resize(nRangeProofLen);
+    return (res == 1);
+}
+
+void BlindAsset(CConfidentialAsset& conf_asset, secp256k1_generator& asset_gen, const CAsset& asset, const unsigned char* asset_blindptr)
+{
+    conf_asset.vchCommitment.resize(CConfidentialAsset::nCommittedSize);
+    int ret = secp256k1_generator_generate_blinded(secp256k1_blind_context, &asset_gen, asset.begin(), asset_blindptr);
+    assert(ret == 1);
+    ret = secp256k1_generator_serialize(secp256k1_blind_context, conf_asset.vchCommitment.data(), &asset_gen);
+    assert(ret != 0);
+}
+
+void CreateValueCommitment(CConfidentialValue& conf_value, secp256k1_pedersen_commitment& value_commit, const unsigned char* value_blindptr, const secp256k1_generator& asset_gen, const CAmount amount)
+{
+    int ret;
+    conf_value.vchCommitment.resize(CConfidentialValue::nCommittedSize);
+    ret = secp256k1_pedersen_commit(secp256k1_blind_context, &value_commit, value_blindptr, amount, &asset_gen);
+    assert(ret != 0);
+    secp256k1_pedersen_commitment_serialize(secp256k1_blind_context, conf_value.vchCommitment.data(), &value_commit);
+    assert(conf_value.IsValid());
+}
diff --git a/src/blind.h b/src/blind.h
@@ -0,0 +1,38 @@
+// Copyright (c) 2017-2019 The Elements Core developers
+// // Distributed under the MIT software license, see the accompanying
+// // file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_WALLET_BLIND_H
+#define BITCOIN_WALLET_BLIND_H
+
+#include <key.h>
+#include <pubkey.h>
+#include <primitives/transaction.h>
+#include <primitives/confidential.h>
+
+#include <secp256k1.h>
+#include <secp256k1_rangeproof.h>
+#include <secp256k1_surjectionproof.h>
+
+//! ELEMENTS: 36-bit rangeproof size
+static const size_t DEFAULT_RANGEPROOF_SIZE = 2893;
+
+/*
+ * Unblind a pair of confidental asset and value.
+ * Note that unblinded data will only be outputted if *BOTH* asset and value could be unblinded.
+ *
+ * blinding_key is used to create the nonce to rewind the rangeproof in conjunction with the nNonce commitment. In the case of a 0-length nNonce, the blinding key is directly used as the nonce.
+ * Currently there is only a sidechannel message in the rangeproof so a valid rangeproof must
+ * be included in the pair to recover value and asset data.
+ */
+bool UnblindConfidentialPair(const CKey& blinding_key, const CConfidentialValue& conf_value, const CConfidentialAsset& conf_asset, const CConfidentialNonce& nonce_commitment, const CScript& committedScript, const std::vector<unsigned char>& vchRangeproof, CAmount& amount_out, uint256& blinding_factor_out, CAsset& asset_out, uint256& asset_blinding_factor_out);
+
+bool GenerateRangeproof(std::vector<unsigned char>& rangeproof, const std::vector<unsigned char*>& value_blindptrs, const uint256& nonce, const CAmount amount, const CScript& scriptPubKey, const secp256k1_pedersen_commitment& value_commit, const secp256k1_generator& gen, const CAsset& asset, std::vector<const unsigned char*>& asset_blindptrs);
+
+bool SurjectOutput(CTxOutWitness& txoutwit, const std::vector<secp256k1_fixed_asset_tag>& surjection_targets, const std::vector<secp256k1_generator>& target_asset_generators, const std::vector<uint256 >& target_asset_blinders, const std::vector<const unsigned char*> asset_blindptrs, const secp256k1_generator& output_asset_gen, const CAsset& asset);
+
+void BlindAsset(CConfidentialAsset& conf_asset, secp256k1_generator& asset_gen, const CAsset& asset, const unsigned char* asset_blindptr);
+
+void CreateValueCommitment(CConfidentialValue& conf_value, secp256k1_pedersen_commitment& value_commit, const unsigned char* value_blindptr, const secp256k1_generator& asset_gen, const CAmount amount);
+
+#endif //BITCOIN_WALLET_BLIND_H
diff --git a/src/confidential_validation.cpp b/src/confidential_validation.cpp
@@ -0,0 +1,107 @@
+
+#include <confidential_validation.h>
+#include <issuance.h>
+#include <pegins.h>
+#include <script/sigcache.h>
+#include <blind.h>
+
+namespace {
+static secp256k1_context *secp256k1_ctx_verify_amounts;
+
+class CSecp256k1Init {
+public:
+    CSecp256k1Init() {
+        assert(secp256k1_ctx_verify_amounts == NULL);
+        secp256k1_ctx_verify_amounts = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY | SECP256K1_CONTEXT_SIGN);
+        assert(secp256k1_ctx_verify_amounts != NULL);
+    }
+    ~CSecp256k1Init() {
+        assert(secp256k1_ctx_verify_amounts != NULL);
+        secp256k1_context_destroy(secp256k1_ctx_verify_amounts);
+        secp256k1_ctx_verify_amounts = NULL;
+    }
+};
+static CSecp256k1Init instance_of_csecp256k1;
+}
+
+bool CRangeCheck::operator()() {
+    if (val->IsExplicit()) {
+        return true;
+    }
+
+    if (!CachingRangeProofChecker(store).VerifyRangeProof(rangeproof, val->vchCommitment, assetCommitment, scriptPubKey, secp256k1_ctx_verify_amounts)) {
+        error = SCRIPT_ERR_RANGEPROOF;
+        return false;
+    }
+
+    return true;
+};
+
+bool CBalanceCheck::operator()() {
+    if (!secp256k1_pedersen_verify_tally(secp256k1_ctx_verify_amounts, vpCommitsIn.data(), vpCommitsIn.size(), vpCommitsOut.data(), vpCommitsOut.size())) {
+        error = SCRIPT_ERR_PEDERSEN_TALLY;
+        return false;
+    }
+
+    return true;
+}
+
+bool CSurjectionCheck::operator()() {
+    return CachingSurjectionProofChecker(store).VerifySurjectionProof(proof, vTags, gen, secp256k1_ctx_verify_amounts, wtxid);
+}
+
+// Destroys the check in the case of no queue, or passes its ownership to the queue.
+ScriptError QueueCheck(std::vector<CCheck*>* queue, CCheck* check) {
+    if (queue != NULL) {
+        queue->push_back(check);
+        return SCRIPT_ERR_OK;
+    }
+    bool success = (*check)();
+    ScriptError err = check->GetScriptError();
+    delete check;
+    return success ? SCRIPT_ERR_OK : err;
+}
+
+// Helper function for VerifyAmount(), not exported
+static bool VerifyIssuanceAmount(secp256k1_pedersen_commitment& value_commit, secp256k1_generator& asset_gen,
+                    const CAsset& asset, const CConfidentialValue& value, const std::vector<unsigned char>& rangeproof,
+                    std::vector<CCheck*>* checks, const bool store_result)
+{
+    // This is used to add in the explicit values
+    unsigned char explicit_blinds[32];
+    memset(explicit_blinds, 0, sizeof(explicit_blinds));
+    int ret;
+
+    ret = secp256k1_generator_generate(secp256k1_ctx_verify_amounts, &asset_gen, asset.begin());
+    assert(ret == 1);
+
+    // Build value commitment
+    if (value.IsExplicit()) {
+        if (!MoneyRange(value.GetAmount()) || value.GetAmount() == 0) {
+            return false;
+        }
+        if (!rangeproof.empty()) {
+            return false;
+        }
+
+
+        ret = secp256k1_pedersen_commit(secp256k1_ctx_verify_amounts, &value_commit, explicit_blinds, value.GetAmount(), &asset_gen);
+        // The explicit_blinds are all 0, and the amount is not 0. So secp256k1_pedersen_commit does not fail.
+        assert(ret == 1);
+    } else if (value.IsCommitment()) {
+        // Verify range proof
+        std::vector<unsigned char> vchAssetCommitment(CConfidentialAsset::nExplicitSize);
+        secp256k1_generator_serialize(secp256k1_ctx_verify_amounts, vchAssetCommitment.data(), &asset_gen);
+        if (QueueCheck(checks, new CRangeCheck(&value, rangeproof, vchAssetCommitment, CScript(), store_result)) != SCRIPT_ERR_OK) {
+            return false;
+        }
+
+        if (secp256k1_pedersen_commitment_parse(secp256k1_ctx_verify_amounts, &value_commit, value.vchCommitment.data()) != 1) {
+            return false;
+        }
+    } else {
+        return false;
+    }
+
+    return true;
+}