blockchain – How to build a Block Header from pool data?

and thanks in advance for your help!

I’m new to the forum, but I’ve read quite a bit of documentation, and I still can’t figure out how to properly construct the block header from the data provided by a mining pool. I’m really interested in mining lately and would like to understand the process in detail.

Let’s go through a practical example:

1. Subscribing to the Pool

When I send mining.subscribe, I receive:

[[["mining.set_difficulty","0HN9B1NBLO9DQ"],["mining.notify","0HN9B1NBLO9DQ"]],"2006be21",4]

From this, I get:

Extranonce1 = "2006be21"
Extranonce2 must be 4 bytes (e.g., "c95fc1a9").

2. Receiving Work from the Pool

After mining.authorize, I receive a mining.notify message:

["00059c50","cd6f21657c8eb8301949442a3fff2e78a9cb857e01b2a0120000000000000000",
"01000000010000000000000000000000000000000000000000000000000000000000000000ffffffff1503e4810d0474d9ad6700",
"027a690000000001bae6a212000000001976a9143e5509507b08938f58bdb9d9d8e860e0d0d084dc88ac00000000",
["6cffdce483ddf6d154b90eaa262e63e0f4fddd17bea7730af0cb070a2482e800",
"11f5139edbd86f6497e8341b195d435694c0b482babf9e6ff9c68f3d2cfd9cc8",
"879bf96a7f01f34dd6f9e14e91cd4153da922e136ded76c4d76376dc4aa2bffb",
"579a25a2a4e4844aca8d357d0ef3ab4269cb06aeba874e3a7945ec04b6a207eb",
"4f4cd551e8ab18716ac53a1460c77f795c1999dd983a98f030500ca646790f86",
"2806126621f7fe529e08b0cdf7f502ccae77a27d2f9c9f1c3f39d5849f885c70",
"d1a498f3adc3db8f7b3bca34427d7d5e22caee7ef8ef63e681f92e64bd9bc0c4"],
"20000000","1802b50e","67add974",false]

From this, I extract the following:

jobID: "00059c50"
previous hash: "cd6f21657c8eb8301949442a3fff2e78a9cb857e01b2a0120000000000000000"
Coinbase1: "01000000010000000000000000000000000000000000000000000000000000000000000000ffffffff1503e4810d0474d9ad6700"
Coinbase2: "027a690000000001bae6a212000000001976a9143e5509507b08938f58bdb9d9d8e860e0d0d084dc88ac00000000"
Merkle branch: [...]
Version: "20000000"
nBits: "1802b50e"
nTime: "67add974"

3. Constructing the Coinbase Transaction

I concatenate:

Coinbase1 + Extranonce1 + Extranonce2 + Coinbase2

Resulting in:

"01000000010000000000000000000000000000000000000000000000000000000000000000ffffffff1503e4810d0474d9ad67002006be21c95fc1a9027a690000000001bae6a212000000001976a9143e5509507b08938f58bdb9d9d8e860e0d0d084dc88ac00000000"

4. Computing the Merkle Root

I compute sha256(sha256(CoinbaseTransaction)), then hash it with the Merkle branch using:

DoubleSha256(merkleBranch[i] + merkleBranch[i + 1])

This results in the Merkle root:

"6e7770d16e88b3effd401a58e47a22ac1eaba562543620bf673ff8d40548ae56"

5. Constructing the Block Header

I format the block header as:

"00000020" + previousHash + merkleRoot + nTime + nBits + nonce

Example with nonce d1002500 (- are added to make it more readable):

"00000020-cd6f21657c8eb8301949442a3fff2e78a9cb857e01b2a0120000000000000000-56ae4805d4f83f67bf20365462a5ab1eac227ae4581a40fdefb3886ed170776e-74d9ad67-0eb50218-d1002500"

Computing the double SHA-256:

cc8c028e85d85cdfd07541cc50d7e5c7e10fa708c39a3c942d0083238e020000

6. Submitting the Work

I send a mining.submit with:

jobID: "00059c50"
ExtraNonce2: "c95fc1a9"
nTime: "67add974"
nOnce: "d1002500"

Issue

Even when I submit a hash with a higher difficulty, the pool eventually returns a “low difficulty” error (e.g., E-10, E-9). This suggests I’m incorrectly constructing the block header or calculating the Merkle root incorrectly.

Can anyone help me figure out what I’m doing wrong?

Thanks a lot! 😊