Why does Blockcypher signer tool return some extra characters than bip32 dart package?

Question

I'm trying to sign a transaction skeleton Blockcypher returns, in order to send it along, following https://www.blockcypher.com/dev/bitcoin/#creating-transactions.

For this example, I'll use the completely-unsafe 'raw raw raw raw raw raw raw raw raw raw raw raw' mnemonic, which using dart bip32 package creates a BIP32 with private key 0x05a2716a8eb37eb2aaa72594573165349498aa6ca20c71346fb15d82c0cbbf7c and address mpQfiFFq7SHvzS9ebxMRGVohwHTRJJf9ra for BTC testnet.

Blockcypher Tx Skeleton tosign is 1cbbb4d229dcafe6dc3363daab8de99d6d38b043ce62b7129a8236e40053383e.

Using Blockcypher signer tool:

$ ./signer 1cbbb4d229dcafe6dc3363daab8de99d6d38b043ce62b7129a8236e40053383e 05a2716a8eb37eb2aaa72594573165349498aa6ca20c71346fb15d82c0cbbf7c

304402202711792b72547d2a1730a319bd219854f0892451b8bc2ab8c17ec0c6cba4ecc4022058f675ca0af3db455913e59dadc7c5e0bd0bf1b8ef8c13e830a627a18ac375ab

On the other hand, using bip32 I get:

String toSign = txSkel['tosign'][0];
var uToSign = crypto.hexToBytes(toSign);
var signed = fromNode.sign(uToSign);
var signedHex = bufferToHex(signed);
var signedHexNo0x = signedHex.substring(2);

where fromNode is the bip32.BIP32 node. Output is signedHexNo0x = 2711792b72547d2a1730a319bd219854f0892451b8bc2ab8c17ec0c6cba4ecc458f675ca0af3db455913e59dadc7c5e0bd0bf1b8ef8c13e830a627a18ac375ab.

At first sight, they seem completely different buffers, but after a detailed look, Blockcypher signer output only has some extra characters than that of bip32:

Blockcypher signer output (I split it into several lines for you to see it clearly):
30440220
2711792b72547d2a1730a319bd219854f0892451b8bc2ab8c17ec0c6cba4ecc4
0220
58f675ca0af3db455913e59dadc7c5e0bd0bf1b8ef8c13e830a627a18ac375ab

bip32 output (also intentionally split):
2711792b72547d2a1730a319bd219854f0892451b8bc2ab8c17ec0c6cba4ecc4
58f675ca0af3db455913e59dadc7c5e0bd0bf1b8ef8c13e830a627a18ac375ab

I'd expect two 64-character numbers to give a 128-characters signature, which bip32 output accomplishes. Hence, Blockcypher signer output has 140 characters, i.e. 12 more than the former, which is clear when seen as split into lines as above.

I'd be really thankful to anyone throwing some light on this issue, which I need to understand and correct. I need to implement the solution in dart, I cannot use the signer script other than for testing.

Answer 1

The dart bip32 package doesn't seem to encode the signature in DER format, but rather in a simple (r, s) encoding. However DER is required for Bitcoin. For more information see:

https://bitcoin.stackexchange.com/questions/92680/what-are-the-der-signature-and-sec-format

You can either add the DER extra bytes yourself according to your r and s or check if there's a DER encoding in the dart bip32 library.