File hash doesn't match (#ND2)

[joelburger]

I’m stuck on Stage #ND2 =(

I’ve tried downloading the piece, but the file hash doesn’t match the expected hash from the torrent info.

Here are my logs:

[your_program] Response received: 16397 bytes
[your_program] Socket 0, Piece 0, Offset 229376 successfully downloaded in 92ms
[your_program] Sending message. Piece index 0, Block offset: 245760, Block size: 16384
[your_program] Response received: 16397 bytes
[your_program] Socket 0, Piece 0, Offset 245760 successfully downloaded in 91ms
[your_program] Download finished. Saving to /tmp/torrents2671258469/piece-0
[your_program] Socket end
[your_program] Connection closed
[tester::#ND2] File SHA-1 does not match expected SHA-1. Expected: 3c34309faebf01e49c0f63c90b7edcc2259b6ad0 Actual: 2354d053bb7ce7f85337594b7306df07ae436ae7
[tester::#ND2] Test failed

I’ve pushed all my changes to the Git repo. Any help would be much appreciated.

[andy1li]

I’ll take a look shortly.

[andy1li]

Hi @joelburger, I added a few logs like the following:

async function downloadBlock(socket, torrent, pieceIndex, blockOffset) {
  const { messageId, payload: blockPayload } = parsePeerMessageResponse(response);
  console.log('Block payload length:', blockPayload.length);
  console.log('Block payload [0:32]:', blockPayload.subarray(0, 32));

Looks like all the data are being parsed correctly:

The bug occurs when the whole blockPayload (length: 16392 = 16K + 8) is copied directly into pieceBuffer.

You will need to remove (and ideally verify) index and begin (combined length of 8 bytes), and copy only block (length: 16k):

[joelburger]

Thanks Andy.

I’ve managed to match the expected hashes for pieces before the last one.

The last piece from the sample.torrent file doesn’t seem to match though. The total size I have for this last piece is 22625.

Sending message. Piece index 2, Block offset: 0, Block size: 16384
Response received: 14480 bytes
Response received: 1917 bytes
Piece 2, Offset 0, Block size: 16384 successfully fetched in 609ms
Sending message. Piece index 2, Block offset: 16384, Block size: 6241
Response received: 6254 bytes
Piece 2, Offset 16384, Block size: 6241 successfully fetched in 406ms
Failed to download piece: Error: Invalid piece hash. Size: 22625, Expected: f00d937a0213df1982bc8d097227ad9e909acc17, Actual: abab7072869e6861fb035dffb90206666c015fe1

[joelburger]

Thanks again for getting back so quickly Andy