wallet: Add addhdkey RPC

diff --git a/src/wallet/rpc/wallet.cpp b/src/wallet/rpc/wallet.cpp
index b6c9179d..8aa15c7e 100644
--- a/src/wallet/rpc/wallet.cpp
+++ b/src/wallet/rpc/wallet.cpp
@@ -840,6 +840,82 @@ static RPCMethod createwalletdescriptor()
     };
 }
 
+RPCMethod addhdkey()
+{
+    return RPCMethod{
+        "addhdkey",
+        "Add a BIP 32 HD key to the wallet that can be used with 'createwalletdescriptor'\n",
+        {
+            {"hdkey", RPCArg::Type::STR, RPCArg::DefaultHint{"Automatically generated new key"}, "The BIP 32 extended private key to add. If none is provided, a randomly generated one will be added."},
+        },
+        RPCResult{
+            RPCResult::Type::OBJ, "", "",
+            {
+                {RPCResult::Type::STR, "xpub", "The xpub of the HD key that was added to the wallet"}
+            },
+        },
+        RPCExamples{
+            HelpExampleCli("addhdkey", "xprv") + HelpExampleRpc("addhdkey", "xprv")
+        },
+        [&](const RPCMethod& self, const JSONRPCRequest& request) -> UniValue
+        {
+            std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
+            if (!wallet) return UniValue::VNULL;
+
+            if (wallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
+                throw JSONRPCError(RPC_WALLET_ERROR, "addhdkey is not available for wallets without private keys");
+            }
+
+            EnsureWalletIsUnlocked(*wallet);
+
+            CExtKey hdkey;
+            if (request.params[0].isNull()) {
+                CKey seed_key = GenerateRandomKey();
+                hdkey.SetSeed(seed_key);
+            } else {
+                hdkey = DecodeExtKey(request.params[0].get_str());
+                if (!hdkey.key.IsValid()) {
+                    // Check if the user gave us an xpub and give a more descriptive error if so
+                    CExtPubKey xpub = DecodeExtPubKey(request.params[0].get_str());
+                    if (xpub.pubkey.IsValid()) {
+                        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Extended public key (xpub) provided, but extended private key (xprv) is required");
+                    } else {
+                        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Could not parse HD key");
+                    }
+                }
+            }
+
+            LOCK(wallet->cs_wallet);
+            std::string desc_str = "unused(" + EncodeExtKey(hdkey) + ")";
+            FlatSigningProvider keys;
+            std::string error;
+            std::vector<std::unique_ptr<Descriptor>> descs = Parse(desc_str, keys, error, false);
+            CHECK_NONFATAL(!descs.empty());
+            WalletDescriptor w_desc(std::move(descs.at(0)), GetTime(), 0, 0, 0);
+            if (wallet->GetDescriptorScriptPubKeyMan(w_desc) != nullptr) {
+                throw JSONRPCError(RPC_WALLET_ERROR, "HD key already exists");
+            }
+
+            auto spkm = wallet->AddWalletDescriptor(w_desc, keys, /*label=*/"", /*internal=*/false);
+            if (!spkm) {
+                throw JSONRPCError(RPC_WALLET_ERROR, util::ErrorString(spkm).original);
+            }
+
+            UniValue response(UniValue::VOBJ);
+            const DescriptorScriptPubKeyMan& desc_spkm = spkm->get();
+            LOCK(desc_spkm.cs_desc_man);
+            std::set<CPubKey> pubkeys;
+            std::set<CExtPubKey> extpubs;
+            desc_spkm.GetWalletDescriptor().descriptor->GetPubKeys(pubkeys, extpubs);
+            CHECK_NONFATAL(pubkeys.size() == 0);
+            CHECK_NONFATAL(extpubs.size() == 1);
+            response.pushKV("xpub", EncodeExtPubKey(*extpubs.begin()));
+
+            return response;
+        },
+    };
+}
+
 // addresses
 RPCMethod getaddressinfo();
 RPCMethod getnewaddress();
@@ -907,6 +983,7 @@ std::span<const CRPCCommand> GetWalletRPCCommands()
         {"rawtransactions", &fundrawtransaction},
         {"wallet", &abandontransaction},
         {"wallet", &abortrescan},
+        {"wallet", &addhdkey},
         {"wallet", &backupwallet},
         {"wallet", &bumpfee},
         {"wallet", &psbtbumpfee},
diff --git a/test/functional/wallet_hd.py b/test/functional/wallet_hd.py
index c130c731..e5227e6f 100755
--- a/test/functional/wallet_hd.py
+++ b/test/functional/wallet_hd.py
@@ -7,10 +7,12 @@
 import shutil
 
 from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.descriptors import descsum_create
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
     wallet_importprivkey,
+    assert_raises_rpc_error,
 )
 
 
@@ -25,6 +27,56 @@ class WalletHDTest(BitcoinTestFramework):
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()
 
+    def test_addhdkey(self):
+        self.log.info("Test addhdkey")
+        def_wallet = self.nodes[0].get_wallet_rpc(self.default_wallet_name)
+        self.nodes[0].createwallet("hdkey")
+        wallet = self.nodes[0].get_wallet_rpc("hdkey")
+
+        assert_equal(len(wallet.gethdkeys()), 1)
+
+        wallet.addhdkey()
+        xpub_info = wallet.gethdkeys()
+        assert_equal(len(xpub_info), 2)
+        for x in xpub_info:
+            if len(x["descriptors"]) == 1 and x["descriptors"][0]["desc"].startswith("unused("):
+                break
+        else:
+            assert False, "Did not find HD key with no descriptors"
+
+        imp_xpub_info = def_wallet.gethdkeys(private=True)[0]
+        imp_xpub = imp_xpub_info["xpub"]
+        imp_xprv = imp_xpub_info["xprv"]
+
+        assert_raises_rpc_error(-5, "Extended public key (xpub) provided, but extended private key (xprv) is required", wallet.addhdkey, imp_xpub)
+        add_res = wallet.addhdkey(imp_xprv)
+        expected_unused_desc = descsum_create(f"unused({imp_xpub})")
+        assert_equal(add_res["xpub"], imp_xpub)
+        xpub_info = wallet.gethdkeys()
+        assert_equal(len(xpub_info), 3)
+        for x in xpub_info:
+            if x["xpub"] == imp_xpub:
+                assert_equal(len(x["descriptors"]), 1)
+                assert_equal(x["descriptors"][0]["desc"], expected_unused_desc)
+                break
+        else:
+            assert False, "Added HD key was not found in wallet"
+
+        for d in wallet.listdescriptors()["descriptors"]:
+            if d["desc"] == expected_unused_desc:
+                assert_equal(d["active"], False)
+                break
+        else:
+            assert False, "Added HD key's descriptor was not found in wallet"
+
+        assert_raises_rpc_error(-4, "HD key already exists", wallet.addhdkey, imp_xprv)
+
+    def test_addhdkey_noprivs(self):
+        self.log.info("Test addhdkey is not available for wallets without privkeys")
+        self.nodes[0].createwallet("hdkey_noprivs", disable_private_keys=True)
+        wallet = self.nodes[0].get_wallet_rpc("hdkey_noprivs")
+        assert_raises_rpc_error(-4, "addhdkey is not available for wallets without private keys", wallet.addhdkey)
+
     def run_test(self):
         # Make sure we use hd, keep masterkeyid
         hd_fingerprint = self.nodes[1].getaddressinfo(self.nodes[1].getnewaddress())['hdmasterfingerprint']
@@ -124,6 +176,8 @@ class WalletHDTest(BitcoinTestFramework):
 
         assert_equal(keypath[0:14], "m/84h/1h/0h/1/")
 
+        self.test_addhdkey()
+        self.test_addhdkey_noprivs()
 
 if __name__ == '__main__':
     WalletHDTest(__file__).main()