Merge #13756: wallet: "avoid_reuse" wallet flag for improved privacy

5ebc6b0eb267e0552c66fffc5e5afe7df8becf80 bitcoind: update -avoidpartialspends description to account for auto-enable for avoid_reuse wallets (Karl-Johan Alm)
ada258f8c8f92d44d893cf9f22d15acdeca40b1a doc: release notes for avoid_reuse (Karl-Johan Alm)
27669551da52099e4a6a401acd7aa32b32832423 wallet: enable avoid_partial_spends by default if avoid_reuse is set (Karl-Johan Alm)
8f2e208f7c0468f9ba92bc789a698281b1c81284 test: add test for avoidreuse feature (Karl-Johan Alm)
0bdfbd34cf4015de87741ff549db35e5064f4e16 wallet/rpc: add 'avoid_reuse' option to RPC commands (Karl-Johan Alm)
f904723e0d5883309cb0dd14b826bc45c5e776fb wallet/rpc: add setwalletflag RPC and MUTABLE_WALLET_FLAGS (Karl-Johan Alm)
8247a0da3a46d7c38943ee0304343ab7465305bd wallet: enable avoid_reuse feature (Karl-Johan Alm)
eec15662fad917b169f5e3b8baaf4301dcf00a7b wallet: avoid reuse flags (Karl-Johan Alm)
58928098c299efdc7c5ddf2dc20716ca5272f21b wallet: make IsWalletFlagSet() const (Karl-Johan Alm)
129a5bafd9a3efa2fa16d780885048a06566d262 wallet: rename g_known_wallet_flags constant to KNOWN_WALLET_FLAGS (Karl-Johan Alm)

Pull request description:

  Add a new wallet flag called `avoid_reuse` which, when enabled, will keep track of when a specific destination has been spent from, and will actively "blacklist" any new UTXOs which send to an already-spent-from destination.

  This improves privacy, as a payer could otherwise begin tracking a payee's wallet by regularly peppering a known UTXO with dust outputs, which would then be scooped up and used in payments by the payee, allowing the payer to map out (1) the inputs owned by the payee and (2) the destinations to which the payee is making payments.

  This replaces #10386 and together with the (now merged) #12257 it addresses #10065 in full. The concerns raised in https://github.com/bitcoin/bitcoin/pull/10386#issuecomment-302361381 are also addressed due to #12257.

  ~~Note: this builds on top of #15780.~~ (merged)

ACKs for commit 5ebc6b:
  jnewbery:
    ACK 5ebc6b0eb
  laanwj:
    Concept and code-review ACK 5ebc6b0eb267e0552c66fffc5e5afe7df8becf80
  meshcollider:
    Code review ACK https://github.com/bitcoin/bitcoin/pull/13756/commits/5ebc6b0eb267e0552c66fffc5e5afe7df8becf80
  achow101:
    ACK 5ebc6b0eb267e0552c66fffc5e5afe7df8becf80 modulo above nits

Tree-SHA512: fdef45826af544cbbb45634ac367852cc467ec87081d86d08b53ca849e588617e9a0a255b7e7bb28692d15332de58d6c3d274ac003355220e4213d7d9070742e

1 files changed, 114 insertions, 8 deletions

diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index f9baabeda4..cfa36ad40c 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -41,6 +41,17 @@
 
 static const std::string WALLET_ENDPOINT_BASE = "/wallet/";
 
+static inline bool GetAvoidReuseFlag(CWallet * const pwallet, const UniValue& param) {
+    bool can_avoid_reuse = pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE);
+    bool avoid_reuse = param.isNull() ? can_avoid_reuse : param.get_bool();
+
+    if (avoid_reuse && !can_avoid_reuse) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "wallet does not have the \"avoid reuse\" feature enabled");
+    }
+
+    return avoid_reuse;
+}
+
 bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest& request, std::string& wallet_name)
 {
     if (request.URI.substr(0, WALLET_ENDPOINT_BASE.size()) == WALLET_ENDPOINT_BASE) {
@@ -304,7 +315,7 @@ static UniValue setlabel(const JSONRPCRequest& request)
 
 static CTransactionRef SendMoney(interfaces::Chain::Lock& locked_chain, CWallet * const pwallet, const CTxDestination &address, CAmount nValue, bool fSubtractFeeFromAmount, const CCoinControl& coin_control, mapValue_t mapValue)
 {
-    CAmount curBalance = pwallet->GetBalance().m_mine_trusted;
+    CAmount curBalance = pwallet->GetBalance(0, coin_control.m_avoid_address_reuse).m_mine_trusted;
 
     // Check amount
     if (nValue <= 0)
@@ -351,7 +362,7 @@ static UniValue sendtoaddress(const JSONRPCRequest& request)
         return NullUniValue;
     }
 
-    if (request.fHelp || request.params.size() < 2 || request.params.size() > 8)
+    if (request.fHelp || request.params.size() < 2 || request.params.size() > 9)
         throw std::runtime_error(
             RPCHelpMan{"sendtoaddress",
                 "\nSend an amount to a given address." +
@@ -372,6 +383,8 @@ static UniValue sendtoaddress(const JSONRPCRequest& request)
             "       \"UNSET\"\n"
             "       \"ECONOMICAL\"\n"
             "       \"CONSERVATIVE\""},
+                    {"avoid_reuse", RPCArg::Type::BOOL, /* default */ pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE) ? "true" : "unavailable", "Avoid spending from dirty addresses; addresses are considered\n"
+            "                             dirty if they have previously been used in a transaction."},
                 },
                 RPCResult{
             "\"txid\"                  (string) The transaction id.\n"
@@ -428,6 +441,9 @@ static UniValue sendtoaddress(const JSONRPCRequest& request)
         }
     }
 
+    coin_control.m_avoid_address_reuse = GetAvoidReuseFlag(pwallet, request.params[8]);
+    // We also enable partial spend avoidance if reuse avoidance is set.
+    coin_control.m_avoid_partial_spends |= coin_control.m_avoid_address_reuse;
 
     EnsureWalletIsUnlocked(pwallet);
 
@@ -717,7 +733,7 @@ static UniValue getbalance(const JSONRPCRequest& request)
         return NullUniValue;
     }
 
-    if (request.fHelp || (request.params.size() > 3 ))
+    if (request.fHelp || request.params.size() > 4)
         throw std::runtime_error(
             RPCHelpMan{"getbalance",
                 "\nReturns the total available balance.\n"
@@ -727,6 +743,7 @@ static UniValue getbalance(const JSONRPCRequest& request)
                     {"dummy", RPCArg::Type::STR, RPCArg::Optional::OMITTED_NAMED_ARG, "Remains for backward compatibility. Must be excluded or set to \"*\"."},
                     {"minconf", RPCArg::Type::NUM, /* default */ "0", "Only include transactions confirmed at least this many times."},
                     {"include_watchonly", RPCArg::Type::BOOL, /* default */ "false", "Also include balance in watch-only addresses (see 'importaddress')"},
+                    {"avoid_reuse", RPCArg::Type::BOOL, /* default */ pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE) ? "true" : "unavailable", "Do not include balance in dirty outputs; addresses are considered dirty if they have previously been used in a transaction."},
                 },
                 RPCResult{
             "amount              (numeric) The total amount in " + CURRENCY_UNIT + " received for this wallet.\n"
@@ -763,7 +780,9 @@ static UniValue getbalance(const JSONRPCRequest& request)
         include_watchonly = true;
     }
 
-    const auto bal = pwallet->GetBalance(min_depth);
+    bool avoid_reuse = GetAvoidReuseFlag(pwallet, request.params[3]);
+
+    const auto bal = pwallet->GetBalance(min_depth, avoid_reuse);
 
     return ValueFromAmount(bal.m_mine_trusted + (include_watchonly ? bal.m_watchonly_trusted : 0));
 }
@@ -2461,6 +2480,7 @@ static UniValue getwalletinfo(const JSONRPCRequest& request)
             "  \"paytxfee\": x.xxxx,                (numeric) the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB\n"
             "  \"hdseedid\": \"<hash160>\"            (string, optional) the Hash160 of the HD seed (only present when HD is enabled)\n"
             "  \"private_keys_enabled\": true|false (boolean) false if privatekeys are disabled for this wallet (enforced watch-only wallet)\n"
+            "  \"avoid_reuse\": true|false          (boolean) whether this wallet tracks clean/dirty coins in terms of reuse\n"
             "  \"scanning\":                        (json object) current scanning details, or false if no scan is in progress\n"
             "    {\n"
             "      \"duration\" : xxxx              (numeric) elapsed seconds since scan start\n"
@@ -2509,6 +2529,7 @@ static UniValue getwalletinfo(const JSONRPCRequest& request)
         obj.pushKV("hdseedid", seed_id.GetHex());
     }
     obj.pushKV("private_keys_enabled", !pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS));
+    obj.pushKV("avoid_reuse", pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE));
     if (pwallet->IsScanning()) {
         UniValue scanning(UniValue::VOBJ);
         scanning.pushKV("duration", pwallet->ScanningDuration() / 1000);
@@ -2637,6 +2658,76 @@ static UniValue loadwallet(const JSONRPCRequest& request)
     return obj;
 }
 
+static UniValue setwalletflag(const JSONRPCRequest& request)
+{
+    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
+    CWallet* const pwallet = wallet.get();
+
+    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
+        return NullUniValue;
+    }
+
+    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2) {
+        std::string flags = "";
+        for (auto& it : WALLET_FLAG_MAP)
+            if (it.second & MUTABLE_WALLET_FLAGS)
+                flags += (flags == "" ? "" : ", ") + it.first;
+        throw std::runtime_error(
+            RPCHelpMan{"setwalletflag",
+                "\nChange the state of the given wallet flag for a wallet.\n",
+                {
+                    {"flag", RPCArg::Type::STR, RPCArg::Optional::NO, "The name of the flag to change. Current available flags: " + flags},
+                    {"value", RPCArg::Type::BOOL, /* default */ "true", "The new state."},
+                },
+                RPCResult{
+            "{\n"
+            "    \"flag_name\": string   (string) The name of the flag that was modified\n"
+            "    \"flag_state\": bool    (bool) The new state of the flag\n"
+            "    \"warnings\": string    (string) Any warnings associated with the change\n"
+            "}\n"
+                },
+                RPCExamples{
+                    HelpExampleCli("setwalletflag", "avoid_reuse")
+                  + HelpExampleRpc("setwalletflag", "\"avoid_reuse\"")
+                },
+            }.ToString());
+    }
+
+    std::string flag_str = request.params[0].get_str();
+    bool value = request.params[1].isNull() || request.params[1].get_bool();
+
+    if (!WALLET_FLAG_MAP.count(flag_str)) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Unknown wallet flag: %s", flag_str));
+    }
+
+    auto flag = WALLET_FLAG_MAP.at(flag_str);
+
+    if (!(flag & MUTABLE_WALLET_FLAGS)) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Wallet flag is immutable: %s", flag_str));
+    }
+
+    UniValue res(UniValue::VOBJ);
+
+    if (pwallet->IsWalletFlagSet(flag) == value) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Wallet flag is already set to %s: %s", value ? "true" : "false", flag_str));
+    }
+
+    res.pushKV("flag_name", flag_str);
+    res.pushKV("flag_state", value);
+
+    if (value) {
+        pwallet->SetWalletFlag(flag);
+    } else {
+        pwallet->UnsetWalletFlag(flag);
+    }
+
+    if (flag && value && WALLET_FLAG_CAVEATS.count(flag)) {
+        res.pushKV("warnings", WALLET_FLAG_CAVEATS.at(flag));
+    }
+
+    return res;
+}
+
 static UniValue createwallet(const JSONRPCRequest& request)
 {
     const RPCHelpMan help{
@@ -2647,6 +2738,7 @@ static UniValue createwallet(const JSONRPCRequest& request)
             {"disable_private_keys", RPCArg::Type::BOOL, /* default */ "false", "Disable the possibility of private keys (only watchonlys are possible in this mode)."},
             {"blank", RPCArg::Type::BOOL, /* default */ "false", "Create a blank wallet. A blank wallet has no keys or HD seed. One can be set using sethdseed."},
             {"passphrase", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "Encrypt the wallet with this passphrase."},
+            {"avoid_reuse", RPCArg::Type::BOOL, /* default */ "false", "Keep track of coin reuse, and treat dirty and clean coins differently with privacy considerations in mind."},
         },
         RPCResult{
             "{\n"
@@ -2688,6 +2780,10 @@ static UniValue createwallet(const JSONRPCRequest& request)
         flags |= WALLET_FLAG_BLANK_WALLET;
     }
 
+    if (!request.params[4].isNull() && request.params[4].get_bool()) {
+        flags |= WALLET_FLAG_AVOID_REUSE;
+    }
+
     WalletLocation location(request.params[0].get_str());
     if (location.Exists()) {
         throw JSONRPCError(RPC_WALLET_ERROR, "Wallet " + location.GetName() + " already exists.");
@@ -2789,6 +2885,8 @@ static UniValue listunspent(const JSONRPCRequest& request)
         return NullUniValue;
     }
 
+    bool avoid_reuse = pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE);
+
     if (request.fHelp || request.params.size() > 5)
         throw std::runtime_error(
             RPCHelpMan{"listunspent",
@@ -2828,6 +2926,9 @@ static UniValue listunspent(const JSONRPCRequest& request)
             "    \"witnessScript\" : \"script\" (string) witnessScript if the scriptPubKey is P2WSH or P2SH-P2WSH\n"
             "    \"spendable\" : xxx,        (bool) Whether we have the private keys to spend this output\n"
             "    \"solvable\" : xxx,         (bool) Whether we know how to spend this output, ignoring the lack of keys\n"
+            + (avoid_reuse ?
+            "    \"reused\" : xxx,           (bool) Whether this output is reused/dirty (sent to an address that was previously spent from)\n" :
+            "") +
             "    \"desc\" : xxx,             (string, only when solvable) A descriptor for spending this output\n"
             "    \"safe\" : xxx              (bool) Whether this output is considered safe to spend. Unconfirmed transactions\n"
             "                              from outside keys and unconfirmed replacement transactions are considered unsafe\n"
@@ -2907,9 +3008,11 @@ static UniValue listunspent(const JSONRPCRequest& request)
     UniValue results(UniValue::VARR);
     std::vector<COutput> vecOutputs;
     {
+        CCoinControl cctl;
+        cctl.m_avoid_address_reuse = false;
         auto locked_chain = pwallet->chain().lock();
         LOCK(pwallet->cs_wallet);
-        pwallet->AvailableCoins(*locked_chain, vecOutputs, !include_unsafe, nullptr, nMinimumAmount, nMaximumAmount, nMinimumSumAmount, nMaximumCount, nMinDepth, nMaxDepth);
+        pwallet->AvailableCoins(*locked_chain, vecOutputs, !include_unsafe, &cctl, nMinimumAmount, nMaximumAmount, nMinimumSumAmount, nMaximumCount, nMinDepth, nMaxDepth);
     }
 
     LOCK(pwallet->cs_wallet);
@@ -2918,6 +3021,7 @@ static UniValue listunspent(const JSONRPCRequest& request)
         CTxDestination address;
         const CScript& scriptPubKey = out.tx->tx->vout[out.i].scriptPubKey;
         bool fValidAddress = ExtractDestination(scriptPubKey, address);
+        bool reused = avoid_reuse && pwallet->IsUsedDestination(address);
 
         if (destinations.size() && (!fValidAddress || !destinations.count(address)))
             continue;
@@ -2974,6 +3078,7 @@ static UniValue listunspent(const JSONRPCRequest& request)
             auto descriptor = InferDescriptor(scriptPubKey, *pwallet);
             entry.pushKV("desc", descriptor->ToString());
         }
+        if (avoid_reuse) entry.pushKV("reused", reused);
         entry.pushKV("safe", out.fSafe);
         results.push_back(entry);
     }
@@ -4185,13 +4290,13 @@ static const CRPCCommand commands[] =
     { "wallet",             "addmultisigaddress",               &addmultisigaddress,            {"nrequired","keys","label","address_type"} },
     { "wallet",             "backupwallet",                     &backupwallet,                  {"destination"} },
     { "wallet",             "bumpfee",                          &bumpfee,                       {"txid", "options"} },
-    { "wallet",             "createwallet",                     &createwallet,                  {"wallet_name", "disable_private_keys", "blank", "passphrase"} },
+    { "wallet",             "createwallet",                     &createwallet,                  {"wallet_name", "disable_private_keys", "blank", "passphrase", "avoid_reuse"} },
     { "wallet",             "dumpprivkey",                      &dumpprivkey,                   {"address"}  },
     { "wallet",             "dumpwallet",                       &dumpwallet,                    {"filename"} },
     { "wallet",             "encryptwallet",                    &encryptwallet,                 {"passphrase"} },
     { "wallet",             "getaddressesbylabel",              &getaddressesbylabel,           {"label"} },
     { "wallet",             "getaddressinfo",                   &getaddressinfo,                {"address"} },
-    { "wallet",             "getbalance",                       &getbalance,                    {"dummy","minconf","include_watchonly"} },
+    { "wallet",             "getbalance",                       &getbalance,                    {"dummy","minconf","include_watchonly","avoid_reuse"} },
     { "wallet",             "getnewaddress",                    &getnewaddress,                 {"label","address_type"} },
     { "wallet",             "getrawchangeaddress",              &getrawchangeaddress,           {"address_type"} },
     { "wallet",             "getreceivedbyaddress",             &getreceivedbyaddress,          {"address","minconf"} },
@@ -4222,10 +4327,11 @@ static const CRPCCommand commands[] =
     { "wallet",             "removeprunedfunds",                &removeprunedfunds,             {"txid"} },
     { "wallet",             "rescanblockchain",                 &rescanblockchain,              {"start_height", "stop_height"} },
     { "wallet",             "sendmany",                         &sendmany,                      {"dummy","amounts","minconf","comment","subtractfeefrom","replaceable","conf_target","estimate_mode"} },
-    { "wallet",             "sendtoaddress",                    &sendtoaddress,                 {"address","amount","comment","comment_to","subtractfeefromamount","replaceable","conf_target","estimate_mode"} },
+    { "wallet",             "sendtoaddress",                    &sendtoaddress,                 {"address","amount","comment","comment_to","subtractfeefromamount","replaceable","conf_target","estimate_mode","avoid_reuse"} },
     { "wallet",             "sethdseed",                        &sethdseed,                     {"newkeypool","seed"} },
     { "wallet",             "setlabel",                         &setlabel,                      {"address","label"} },
     { "wallet",             "settxfee",                         &settxfee,                      {"amount"} },
+    { "wallet",             "setwalletflag",                    &setwalletflag,                 {"flag","value"} },
     { "wallet",             "signmessage",                      &signmessage,                   {"address","message"} },
     { "wallet",             "signrawtransactionwithwallet",     &signrawtransactionwithwallet,  {"hexstring","prevtxs","sighashtype"} },
     { "wallet",             "unloadwallet",                     &unloadwallet,                  {"wallet_name"} },