rpc: add descriptorprocesspsbt rpc

This RPC can be the Updater, Signer, and optionally the Input
Finalizer for a psbt, and has no interaction with the Bitcoin
Core wallet.

Author: ishaanam

src/rpc/client.cpp

@@ -133,6 +133,9 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "walletprocesspsbt", 1, "sign" },
     { "walletprocesspsbt", 3, "bip32derivs" },
     { "walletprocesspsbt", 4, "finalize" },
+    { "descriptorprocesspsbt", 1, "descriptors"},
+    { "descriptorprocesspsbt", 3, "bip32derivs" },
+    { "descriptorprocesspsbt", 4, "finalize" },
     { "createpsbt", 0, "inputs" },
     { "createpsbt", 1, "outputs" },
     { "createpsbt", 2, "locktime" },

src/rpc/rawtransaction.cpp

@@ -172,8 +172,9 @@ static std::vector<RPCArg> CreateTxDoc()
     };
 }
 
-// Update PSBT with information from the mempool, the UTXO set, the txindex, and the provided descriptors
-PartiallySignedTransaction ProcessPSBT(const std::string& psbt_string, const std::any& context, const HidingSigningProvider& provider)
+// Update PSBT with information from the mempool, the UTXO set, the txindex, and the provided descriptors.
+// Optionally, sign the inputs that we can using information from the descriptors.
+PartiallySignedTransaction ProcessPSBT(const std::string& psbt_string, const std::any& context, const HidingSigningProvider& provider, int sighash_type, bool finalize)
 {
     // Unserialize the transactions
     PartiallySignedTransaction psbtx;
@@ -242,9 +243,10 @@ PartiallySignedTransaction ProcessPSBT(const std::string& psbt_string, const std
         }
 
         // Update script/keypath information using descriptor data.
-        // Note that SignPSBTInput does a lot more than just constructing ECDSA signatures
-        // we don't actually care about those here, in fact.
-        SignPSBTInput(provider, psbtx, /*index=*/i, &txdata, /*sighash=*/1);
+        // Note that SignPSBTInput does a lot more than just constructing ECDSA signatures.
+        // We only actually care about those if our signing provider doesn't hide private
+        // information, as is the case with `descriptorprocesspsbt`
+        SignPSBTInput(provider, psbtx, /*index=*/i, &txdata, sighash_type, /*out_sigdata=*/nullptr, finalize);
     }
 
     // Update script/keypath information using descriptor data.
@@ -1697,7 +1699,9 @@ static RPCHelpMan utxoupdatepsbt()
     const PartiallySignedTransaction& psbtx = ProcessPSBT(
         request.params[0].get_str(),
         request.context,
-        HidingSigningProvider(&provider, /*hide_secret=*/true, /*hide_origin=*/false));
+        HidingSigningProvider(&provider, /*hide_secret=*/true, /*hide_origin=*/false),
+        /*sighash_type=*/SIGHASH_ALL,
+        /*finalize=*/false);
 
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
     ssTx << psbtx;
@@ -1916,6 +1920,82 @@ static RPCHelpMan analyzepsbt()
     };
 }
 
+RPCHelpMan descriptorprocesspsbt()
+{
+    return RPCHelpMan{"descriptorprocesspsbt",
+                "\nUpdate all segwit inputs in a PSBT with information from output descriptors, the UTXO set or the mempool. \n"
+                "Then, sign the inputs we are able to with information from the output descriptors. ",
+                {
+                    {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "The transaction base64 string"},
+                    {"descriptors", RPCArg::Type::ARR, RPCArg::Optional::NO, "An array of either strings or objects", {
+                        {"", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "An output descriptor"},
+                        {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "An object with an output descriptor and extra information", {
+                             {"desc", RPCArg::Type::STR, RPCArg::Optional::NO, "An output descriptor"},
+                             {"range", RPCArg::Type::RANGE, RPCArg::Default{1000}, "Up to what index HD chains should be explored (either end or [begin,end])"},
+                        }},
+                    }},
+                    {"sighashtype", RPCArg::Type::STR, RPCArg::Default{"DEFAULT for Taproot, ALL otherwise"}, "The signature hash type to sign with if not specified by the PSBT. Must be one of\n"
+            "       \"DEFAULT\"\n"
+            "       \"ALL\"\n"
+            "       \"NONE\"\n"
+            "       \"SINGLE\"\n"
+            "       \"ALL|ANYONECANPAY\"\n"
+            "       \"NONE|ANYONECANPAY\"\n"
+            "       \"SINGLE|ANYONECANPAY\""},
+                    {"bip32derivs", RPCArg::Type::BOOL, RPCArg::Default{true}, "Include BIP 32 derivation paths for public keys if we know them"},
+                    {"finalize", RPCArg::Type::BOOL, RPCArg::Default{true}, "Also finalize inputs if possible"},
+                },
+                RPCResult{
+                    RPCResult::Type::OBJ, "", "",
+                    {
+                        {RPCResult::Type::STR, "psbt", "The base64-encoded partially signed transaction"},
+                        {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
+                    }
+                },
+                RPCExamples{
+                    HelpExampleCli("descriptorprocesspsbt", "\"psbt\" \"[\\\"descriptor1\\\", \\\"descriptor2\\\"]\"") +
+                    HelpExampleCli("descriptorprocesspsbt", "\"psbt\" \"[{\\\"desc\\\":\\\"mydescriptor\\\", \\\"range\\\":21}]\"")
+                },
+        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
+{
+    // Add descriptor information to a signing provider
+    FlatSigningProvider provider;
+
+    auto descs = request.params[1].get_array();
+    for (size_t i = 0; i < descs.size(); ++i) {
+        EvalDescriptorStringOrObject(descs[i], provider, /*expand_priv=*/true);
+    }
+
+    int sighash_type = ParseSighashString(request.params[2]);
+    bool bip32derivs = request.params[3].isNull() ? true : request.params[3].get_bool();
+    bool finalize = request.params[4].isNull() ? true : request.params[4].get_bool();
+
+    const PartiallySignedTransaction& psbtx = ProcessPSBT(
+        request.params[0].get_str(),
+        request.context,
+        HidingSigningProvider(&provider, /*hide_secret=*/false, !bip32derivs),
+        sighash_type,
+        finalize);
+
+    // Check whether or not all of the inputs are now signed
+    bool complete = true;
+    for (const auto& input : psbtx.inputs) {
+        complete &= PSBTInputSigned(input);
+    }
+
+    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
+    ssTx << psbtx;
+
+    UniValue result(UniValue::VOBJ);
+
+    result.pushKV("psbt", EncodeBase64(ssTx));
+    result.pushKV("complete", complete);
+
+    return result;
+},
+    };
+}
+
 void RegisterRawTransactionRPCCommands(CRPCTable& t)
 {
     static const CRPCCommand commands[]{
@@ -1931,6 +2011,7 @@ void RegisterRawTransactionRPCCommands(CRPCTable& t)
         {"rawtransactions", &createpsbt},
         {"rawtransactions", &converttopsbt},
         {"rawtransactions", &utxoupdatepsbt},
+        {"rawtransactions", &descriptorprocesspsbt},
         {"rawtransactions", &joinpsbts},
         {"rawtransactions", &analyzepsbt},
     };

src/rpc/util.cpp

@@ -1126,7 +1126,7 @@ std::pair<int64_t, int64_t> ParseDescriptorRange(const UniValue& value)
     return {low, high};
 }
 
-std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, FlatSigningProvider& provider)
+std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, FlatSigningProvider& provider, const bool expand_priv)
 {
     std::string desc_str;
     std::pair<int64_t, int64_t> range = {0, 1000};
@@ -1159,6 +1159,9 @@ std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, Fl
         if (!desc->Expand(i, provider, scripts, provider)) {
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Cannot derive script without private keys: '%s'", desc_str));
         }
+        if (expand_priv) {
+            desc->ExpandPrivate(/*pos=*/i, provider, /*out=*/provider);
+        }
         std::move(scripts.begin(), scripts.end(), std::back_inserter(ret));
     }
     return ret;

src/rpc/util.h

@@ -110,7 +110,7 @@ UniValue JSONRPCTransactionError(TransactionError terr, const std::string& err_s
 std::pair<int64_t, int64_t> ParseDescriptorRange(const UniValue& value);
 
 /** Evaluate a descriptor given as a string, or as a {"desc":...,"range":...} object, with default range of 1000. */
-std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, FlatSigningProvider& provider);
+std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, FlatSigningProvider& provider, const bool expand_priv = false);
 
 /** Returns, given services flags, a list of humanly readable (known) network services */
 UniValue GetServicesNames(ServiceFlags services);

src/test/fuzz/rpc.cpp

@@ -97,6 +97,7 @@ const std::vector<std::string> RPC_COMMANDS_SAFE_FOR_FUZZING{
     "decoderawtransaction",
     "decodescript",
     "deriveaddresses",
+    "descriptorprocesspsbt",
     "disconnectnode",
     "echo",
     "echojson",