bip32 hd wallets – I’m not having the ability to signal PSBT transactions with Zprv in uBitcoin

I’m simply toying round with this in my Arduino ESP32 Nano, and for some motive I’m not having the ability to export a sound PSBT when utilizing a non-public key within the Zprv format, the next method:

#embody "Bitcoin.h"
#embody "PSBT.h"

#embody "Secrets and techniques.h"

#outline MAX_PSBT_SIZE 3000

HDPrivateKey hd(XPRV); // Only a string like "ZprvAhrnh(...)"
PSBT psbt;

void setup() {
  Serial.start(115200);
  whereas (!Serial) ;
  Serial.println(LABEL);
  Serial.println("Please enter the bottom 64 PSBT:");
}

void loop() {
  if (Serial.accessible() > 0) {

    String enter = "";
    char c;
    int charCount = 0;

    whereas (true) {
      c = Serial.learn();
      if(c == 'n' || charCount >= MAX_PSBT_SIZE) {
        break;
      } else if ((int)c < (int)'!' || (int)c > (int)'z') {
        proceed;
      }

      enter += c;
      charCount++;
    }

    Serial.println("Acquired PSBT:");
    Serial.println(enter);
    // Parse the enter as base64 PSBT
    psbt.parseBase64(enter);

    // Verify parsing is OK
    if (!psbt) {
      Serial.println("Failed parsing transaction");
      Serial.println("Please enter the bottom 64 PSBT:");
      return;
    }

    Serial.println("Transactions particulars:");

    // going via all outputs to print data
    Serial.println("Outputs:");
    for(int i=0; i<psbt.tx.outputsNumber; i++){
      // print addresses
      Serial.print(psbt.tx.txOuts[i].tackle(&Regtest));
      Serial.print(" -> ");
      Serial.print(psbt.tx.txOuts[i].btcAmount()*1e3);
      Serial.println(" mBTC");
    }
    Serial.print("Price: ");
    Serial.print(float(psbt.price()));
    Serial.println(" sats");
    
    psbt.signal(hd);
    Serial.println("============================================");
    Serial.println(psbt.toBase64()); // now you possibly can mix and finalize PSBTs in Bitcoin Core
    Serial.println("============================================");

    Serial.println(LABEL);
    Serial.println("Please enter the bottom 64 PSBT:");
    return;
  }
}

This mainly receives a PSBT from the serial comm, and outputs the signed PSBT.

Nonetheless, I imagine the parsing of Zprv is buggy, I attempted to debug the fingerprint half and the parsed fingerprint appears completely completely different, then from that time on every thing fails, the PSBT.signal perform can not discover a match public key matching my non-public key and proceeds with out signing.

PS. Neglect “Secrets and techniques.h”, it simply defines the XPRV fixed (I ought to really name it ZPRV).

This is what I used to be capable of debug:

For instance, for this key:

(Discover the Grasp fingerprint of 5A:21:88:34).

Then I export this with Sparrow to the Electrum json format to acquire the xprv:

{
  "keystore": {
    "xpub": "zpub6rgGh4Q21dLzrac8fqhtCubP5JpfDSf6BJLyJKjfn3D2ojp56PFn7nD7pwVRMtJk9EkkFDW4UrGeDWyFnBfptgvC8uvjWY6PoutNMLwzTyu",
    "xprv": "zprvAY3WiCK4YLmi4neGTqnauNMd8cFLhobtcP3ANMi4CtbLeCeVRzRWCw9qivm3stUnXQ3hwDfCSBzY5qUWcLeatCjadMLmaebwR2fS2suNaHL",
    "kind": "bip32",
    "pw_hash_version": 1
  },
  "wallet_type": "normal",
  "use_encryption": false,
  "seed_type": "bip39"
}

Now, if I present

HDPrivateKey hd("zprvAY3WiCK4YLmi4neGTqnauNMd8cFLhobtcP3ANMi4CtbLeCeVRzRWCw9qivm3stUnXQ3hwDfCSBzY5qUWcLeatCjadML");

The fingerprint I’ll discover at this a part of the code shall be: B9:40:6D:D9

And on this a part of the code shall be: FF:73:D5:35

And as you possibly can see from the photographs above, they don’t have anything to do with the true grasp fingerprint of 5A:21:88:34.

I don’t know how the initally parsed B9:40:6D:D9 turns into FF:73:D5:35 when it reaches the PSBT signing perform, however each values don’t match the anticipated 5A:21:88:34.

Thus, I imagine there is a bug parsing these Zprv keys. Perhaps even parsing different elements as effectively, I did not validate additional as a result of the fingerprint alone is motive for it to fail.

It makes the PSBT.signal() perform fail at this situation, thus ignore signing any PSBT, as a result of it’ll by no means match any fingerprint within the PSBT metadata.