minting cNFTs with Solana Pay QR code

Question

I'm trying to create a qr code so people can scan it and mint a cNFT. I'm using a transaction request as shown in the Solana Pay docs and using Metaplex Bubblegum for state compression. This is on Devnet for now.

My question here is that I'm signing a transaction with the authority Keypair, but the API logs return 2 signatures, and I don’t understand why...nor why a null one (the null one corresponds to the pubkey of the wallet requesting the transaction) ?!?!?!

Can someone explain to me what the problem is?

Thanks a lot :)

Here’s the relevant part of my code:

let accountField = req.body?.account;
if (!accountField) throw new Error('missing account');
const user = new PublicKey(accountField)

const authoritySecret = JSON.parse(process.env.AUTHORITY_KEY ?? "") as number[]
const authoritySecretKey = Uint8Array.from(authoritySecret)
const authority = Keypair.fromSecretKey(authoritySecretKey)

const tree = new PublicKey("ERkz...X97T");
const collectionMint = new PublicKey("3Xfk...Fvfz");
const ix = await createMintCNFTInstruction(tree, user, authority.publicKey, collectionMint);

let transaction = new Transaction();
transaction.add(ix);

const connection = new Connection(devnet url here)
const bh = await connection.getLatestBlockhash();
transaction.recentBlockhash = bh.blockhash;
transaction.feePayer = user

transaction = Transaction.from(transaction.serialize({
    verifySignatures: false,
    requireAllSignatures: false,
}));

transaction.sign(authority)

Here's the createMintCNFTInstruction function:

async function createMintCNFTInstruction(merkleTree: PublicKey, user: PublicKey, authority: PublicKey, collectionMint: PublicKey) {

    const [treeAuthority, _bump] = PublicKey.findProgramAddressSync(
        [merkleTree.toBuffer()],
        BUBBLEGUM_PROGRAM_ID,
    );

    const [collectionMetadataAccount, _b1] = PublicKey.findProgramAddressSync(
        [
            Buffer.from("metadata", "utf8"),
            TOKEN_METADATA_PROGRAM_ID.toBuffer(),
            collectionMint.toBuffer(),
        ],
        TOKEN_METADATA_PROGRAM_ID
    );

    const [collectionEditionAccount, _b2] = PublicKey.findProgramAddressSync(
        [
            Buffer.from("metadata", "utf8"),
            TOKEN_METADATA_PROGRAM_ID.toBuffer(),
            collectionMint.toBuffer(),
            Buffer.from("edition", "utf8"),
        ],
        TOKEN_METADATA_PROGRAM_ID
    );

    const [bgumSigner, __] = PublicKey.findProgramAddressSync(
        [Buffer.from("collection_cpi", "utf8")],
        BUBBLEGUM_PROGRAM_ID
    );

    const tix = await createMintToCollectionV1Instruction({
        treeAuthority: treeAuthority,
        leafOwner: user,
        leafDelegate: user,
        merkleTree: merkleTree,
        payer: user,
        treeDelegate: authority,
        logWrapper: SPL_NOOP_PROGRAM_ID,
        compressionProgram: SPL_ACCOUNT_COMPRESSION_PROGRAM_ID,
        collectionAuthority: authority,
        collectionAuthorityRecordPda: BUBBLEGUM_PROGRAM_ID,
        collectionMint: collectionMint,
        collectionMetadata: collectionMetadataAccount,
        editionAccount: collectionEditionAccount,
        bubblegumSigner: bgumSigner,
        tokenMetadataProgram: TOKEN_METADATA_PROGRAM_ID,
    }, {
        metadataArgs: {
            collection: { key: collectionMint, verified: false },
            creators: [],
            isMutable: true,
            name: "NameHere",
            primarySaleHappened: true,
            sellerFeeBasisPoints: 0,
            symbol: "CAS",
            uri: "https://shdw-drive.genesysgo.net/....json",
            uses: null,
            tokenStandard: TokenStandard.NonFungible,
            editionNonce: null,
            tokenProgramVersion: TokenProgramVersion.Original
        }
    });
    return tix;


}

Here’s the API logs:

the transaction.signature:  [
  {
    signature: null,
    publicKey: PublicKey [PublicKey(c9M…GHK)] {
      _bn: <BN: 900c8…ecce>
    }
  },
  {
    signature: <Buffer 2a b1 ... 14 more bytes>,
    publicKey: PublicKey [PublicKey(E8a…pD)] {
      _bn: <BN: c3189…196>
    }
  }
]

Answer 1

You are adding the user as feepayer, which means it needs to sign, but looks like you are not signing with it. So the signature would be empty.

So you could probably use the authority key as feepayer which should solve it. Or partially sign with the user keypair as well.

For the second signature: Can you add the code from the createMintCNFTInstruction its probably adding another signer there or its the signature of the authority.

Answer 2

If you're encountering an error when scanning on mobile (Solflare) that says, Invalid transaction: transaction failed to sanitize accounts offsets correctly, the issue may be due to trying to add a signer after serializing the transaction. Try adding the additional signer before serializing the tx like this:

transaction.feePayer = user
transaction.sign(authority)

transaction = Transaction.from(transaction.serialize({
    verifySignatures: false,
    requireAllSignatures: false,
}));

Since you're setting the feepayer as the user scanning the qrcode, the first signature should be null as you're seeing. This is because the user hasn't signed yet when the transaction is built.

For your transaction, two signatures are expected since the payer is the user and the treeDelegate and collectionAuthority both the authority. createMintToCollectionV1Instruction requires the payer, treeDelegate, and collectionAuthority to be signers.

Also, if you're testing Solana Pay QrCodes locally, you'll need something like https://ngrok.com/ for an https URL. (If you just use http://localhost:3000/ scanning won't work)

Lastly, it seems that compressed nfts don't currently show up on mobile wallets (solflare/phantom) when connected to devnet (they work fine for mainnet though). So if you don't see them show up in your mobile wallet after scanning but the transaction goes through, try adding the wallet to a browser wallet instead to check.