[bug]: Questions about LND's derivation path of watch only wallet

[hanxianlong]

Background

I'd like to import several watch only account to LND, and these accounts are generated from different mnemonics. However, the [account]' of LND's p2tr address derivation path seems not seperated these watch only accounts from each other. But the account of them are 0、1、2...etc.
LND wallet: m/86'/0'/0'/0/0
watch only wallet A: m/86'/0'/1'/0/0
watch only wallet B: m/86'/0'/2'/0/0

`"account_with_addresses": [
       {
            "name": "default",
            "address_type": "TAPROOT_PUBKEY",
            "derivation_path": "m/86'/0'/0'",
            "addresses": [
{
                    "address": "bcrt1p4r5unsafwxl5xx88v6556vnyclzsr9zaxqs9tq8ln0ed2hynxq4snglrzc",
                    "is_internal": false,
                    "balance": "0",
                    "derivation_path": "m/86'/0'/**0**'/0/0",
                    "public_key": "035f2cc3ff54cb55c71e5f86229ed9170231185c22686b03fb43fc5b9ac285ebb8"
                }
]},
 {
            "name": "A",
            "address_type": "TAPROOT_PUBKEY",
            "derivation_path": "m/86'/0'/0'",
            "addresses": [
                {
                    "address": "bcrt1p08xkyq5uwfgcelxp6nngdlcph6qugsqq7g8z99djn6n98plz8vsqgqfn5g",
                    "is_internal": false,
                    "balance": "0",
                    "derivation_path": "m/86'/0'/**1**'/0/0",
                    "public_key": "027a5f85279682a055145c0b0bf86b30c37e67d46e88f0129fa55c978a87975f6a"
                }]
},
 {
            "name": "B",
            "address_type": "TAPROOT_PUBKEY",
            "derivation_path": "m/86'/0'/0'",
            "addresses": [
                {
                    "address": "bcrt1p4crj4ccaa884pcgqztg3uxkxkxauewq4mklvn3hmy4pqsntruekqnzr7qm",
                    "is_internal": false,
                    "balance": "0",
                    "derivation_path": "m/86'/0'/**2**'/0/0",
                    "public_key": "0398c2060721128c534ba4c3c8425ea509dbcab8a4c7be691b30f2fa109ff81783"
                }]
}
]

Your environment

• lnd 0.18.5-beta
• MacOS
• bitcoind

Steps to reproduce

Step1. Create a LND wallet (let's call it wallet A), and then create a new P2TR address with the command: lncli newaddress p2tr , and it derives a new p2tr address with the drivation path:m/86'/0'/0'/0/0

Step2. Import a watchonly wallet(let's call it wallet B, which is generated by another mnemonic) into LND with the following command: lncli wallet accounts import EXTENDED_PUBLIC_KEY B
, then create a new P2TR address with the following command:lncli newaddress --account B p2tr and then it derives a new p2tr address with the derivation path m/86'/0'/1'/0/0

Expected behaviour

Since wallet B is a completed new watchonly wallet, it has nothing to do with the wallet A, so I think wallet B's p2tr address derivation path's account index should not be related with wallet A.
That means, the watch only wallet's address derivation path's [account index]' should be 0, which is m/86'/0'/0'/0/0

Actual behaviour

account B p2tr and then it derives a new p2tr address with the derivation path m/86'/0'/1'/0/0
account B p2tr and then it derives a new p2tr address with the derivation path m/86'/0'/2'/0/0

Could anybody help me whether this is an issue/bug, or is it by design?

[guggero]

Unfortunately this is a consequence of the very much dated architecture of the btcwallet library lnd uses. There only is one scoped manager for all imported accounts. And for each new one that is added, the internal account number is incremented and that number is used in the derivation path. This can't be changed easily because that account number is also the unique identifier.

[hanxianlong]

Thanks @guggero Is there any other way to get the correct account of a completed new wallet(I mean it starts from 0)? There is a method named DeriveNextKey in the WalletKitService of LND, https://lightning.engineering/api-docs/api/lnd/wallet-kit/derive-next-key/
and the method requires two parameters, the first one is fingerprint, and the second one is the keyFamily. It seems we can specifiy the keyfamily(account) via this method. However, what confuses me is that the type of fingerPrint parameter is int32, and when I converted the wallet hex fingerprint to int, and call this method, it says proto: (line 1:21): invalid value for int32 type.
for example, my wallet's fingerprint is d7de188f, and I converted it to a number 3621656719, and then call the method DeriveNextKey with the following code:

  let requestBody = {
        'key_finger_print': 3621656719,
        'key_family': 0,
    };

 let options = {
        url: 'http://localhost:8080/v2/wallet/address/next',
        // Work-around for self-signed certificates.
        rejectUnauthorized: false,
        json: true,
        headers: {
            'Grpc-Metadata-macaroon': fs.readFileSync(LND_MACAROON_PATH).toString('hex'),
        },
        form: JSON.stringify(requestBody),
    }

    request.post(options, function(error: any, response:any, body:any) {
        console.log(JSON.stringify(body))
    });

and I got the following reponse:
{"code":3,"message":"proto: (line 1:21): invalid value for int32 type: 3621656719","details":[]}

Could you please kindly tell me why the fingerprint type is int32? While the parameter master_key_fingerprint of ImportAccount method is string.

[guggero]

I don't think the DeriveNextKey is what you want, since that will derive from the lnd internal key path of m/1017'/coin_type'/family'/0/index path.

fingerprint is the big-endian encoding of the hex fingerprint. You were very close, but needed to convert it to int32 (signed), so -673310577 should work.

[hanxianlong]

Thank you soooooo much! @guggero
BTW, I investigated what you said in your last reply "This can't be changed easily because that account number is also the unique identifier." and got a solution for my requirement by modifying the low-level library that LND referenced . I'll investigate it further and if it works well I'll come back to you for another discussion. Thanks again! @guggero