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I'm studying segwit and I'm doing some experiment in regtest. I create a raw transaction with P2SH address.

My Transaction data without signature is:

0200000001faf19017fe7e20747535ac41009922e4beb76e17a7fd5423bc60e75fbc8381c40000000000ffffffff01606b042a0100000017a914b63007b51b936175644dcf654f77c528e35f01178700000000

that's my message to sign, instead to use signrawtransactionwithkey I try to use signmessagewithprivkey and I expect the signature that I can find in txwitness field (first row).

signmessagewithprivkey return base64 signature then I convert it in hex. result is:

20db4febb769c43c9072daf30934a345b2327b6107e701f21c7fe052a040180d4129dc3079dc7dad29db37c31fa6401013819e386beb9396e330b3b6e9f7fbcf5b

signrawtransactionwithkey return:

"txinwitness": [
      "3044022077f49ad0371ea5baf0ce60d3d15175c9bb5e37ff7f3bc023b76863b2a91d379e022073010deba5eef8619745cd79d94a73c4a4527391253731104fc734449322d85b01",            "02985586bf43d26229a97092f76086cfe6f9bf09d93bef16a92f65185c8f7f0b17"
      ],

I wonder, why the result of signmessagewithprivkey isn't equal at first raw of txwitness?

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From Elliptic Curve Cryptography perspective everything you sign is a "message" but the term "message" in bitcoin context means a random string that user wants to sign with his key (usually used for proof of ownership or something similar).
This means when you use sign*message*withprivkey the code interprets the input as a string and signs it with a different method that generates and entirely different digest ("some_fixed_string" + UTF8.Decode("0200000001f...")). But when you use sign*rawtransaction*withkey the code interprets the given string as a transaction and signs it based on its input scripts with different methods ("Transaction.Decode("0200000001f...") then Serialize() based on BIP143 for SegWit).

  • Thanks for reply. If I understand correctly, before sign with sign*rawtransaction*withkey the string (transaction data) is altered by some serialization right? – monkeyUser Jan 21 at 14:23
  • That is correct. You can see the details of the process in BIP143 for SegWit and here for legacy transactions OP_CheckSig. – Coding Enthusiast Jan 21 at 14:26
  • In legacy transaction if we use SIGHASH_NONE we will sign the whole transaction data (already serialized). If I Sign this transaction with method signmessagewithprivkey I didn't give the same digest. Where can I find in Github the code apply on transaction data before sign? I didn't find. – monkeyUser Jan 21 at 15:23
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    You will never be able to get the same signature from signmessagewithprivkey because a fixed string is being added to the "message" before hashing intentionally to make the result different. (not sure where the code is in bitcoin core project though). – Coding Enthusiast Jan 21 at 19:21
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    In addition to the difference in the input data, txn sigs are in ASN.1 DER (variable) format plus a sighash byte at the end while msg sigs are in P1363/PKCS11 (fixed) format plus a recovery byte at the beginning; see bitcoin.stackexchange.com/questions/90799/… – dave_thompson_085 Jan 22 at 10:43

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