How to find Z value of multisig transaction?

Question

for example this transaction: http://2coin.org/txinfo.aspx?txid=6c3dc603da32bba3f56f2b33053aaf0f0f17322386c0c6846786bffe49f6ef22&cur=BTC

Script not of right size to be a scriptSig, expecting 2 but got 4

Answer 1

What you're asking for is how to figure out what to sign for a multisig transaction.

Multisig is actually very close to this answer. There are a few subtle differences, however, so let me try to summarize step-by-step using the transaction as seen on the blockchain

Note that for each input, the actual hash to be signed is going to be different. Namely, you will need to fill in the redeem script for that input only and nulls for every other input. To illustrate, let's figure out what you need to sign for the very first input.

1. Transaction version: 01000000
2. Number of inputs: 03
3. Tx input #1 hash: fdb1fe0b4506f8d412f8498a0d747701bc5ed8c009e779ee670c82361c1d1dd5
4. Tx input #1 index: 01000000
5. Tx input #1 redeem script length: 47 (71 bytes)
6. Tx input #1 redeem script: 522102cebf6ab580948d146b7cc771d8e646974349d3d7b11f3e03287d0997a477d3b921037ba651485b7a2cb222191eb64a55926e62bbabfe9b5ed2a9488aad547b20428252ae
7. Tx input #1 sequence: ffffffff
8. Tx input #2 hash: a614d26f1878078a00a3c296085576cd7e6361234ea82c865681041fcfdacea8
9. Tx input #2 index: 01000000
10. Tx input #2 redeem script length: 00 (nothing)
11. Tx input #2 sequence: ffffffff
12. Tx input #3 hash: d064d2f9cf9e5196a9d81dd87718c9cfbec97f3ccac7164946d956421597c7f1
13. Tx input #3 index: 01000000
14. Tx input #3 redeem script length: 00 (nothing)
15. Tx input #3 sequence: ffffffff
16. Number of outputs: 01000000
17. Amount being sent to the first (and only) output: e068704600000000
18. Output script length: 19
19. Output script: 76a9142c76e6fdd1a81c902afa62e78ec71435708d9d9d88ac
20. Lock time field: 00000000
21. SIGHASH_ALL: 01000000

Now, if you double-sha256 these bytes you get:

9c4b551f37f4b383af9216045d80b2fcd4ed57bddca8df388ec29601cbd2a4f1

And indeed when you check against the embedded signature of that transaction, you can see that that is indeed the hash that was signed. Here's a code sample to verify using the excellent btcd library written in go:

package main

import (
    "encoding/hex"
    "fmt"
    "hash"

    "github.com/btcsuite/btcec"
    "github.com/btcsuite/fastsha256"
)

// Calculate the hash of hasher over buf.
func calcHash(buf []byte, hasher hash.Hash) []byte {
    hasher.Write(buf)
    return hasher.Sum(nil)
}

// Hash160 calculates the hash ripemd160(sha256(b)).
func Hash256(buf []byte) []byte {
    return calcHash(calcHash(buf, fastsha256.New()), fastsha256.New())
}

func main() {
    x := "0100000003fdb1fe0b4506f8d412f8498a0d747701bc5ed8c009e779ee670c82361c1d1dd50100000047522102cebf6ab580948d146b7cc771d8e646974349d3d7b11f3e03287d0997a477d3b921037ba651485b7a2cb222191eb64a55926e62bbabfe9b5ed2a9488aad547b20428252aeffffffffa614d26f1878078a00a3c296085576cd7e6361234ea82c865681041fcfdacea80100000000ffffffffd064d2f9cf9e5196a9d81dd87718c9cfbec97f3ccac7164946d956421597c7f10100000000ffffffff01e0687046000000001976a9142c76e6fdd1a81c902afa62e78ec71435708d9d9d88ac0000000001000000"

    b, _ := hex.DecodeString(x)

    hash := Hash256(b)
    fmt.Printf("hash of thing to sign: %x\n", hash)

    pubkeyStr := "02cebf6ab580948d146b7cc771d8e646974349d3d7b11f3e03287d0997a477d3b9"
    pubkeyStr2 := "037ba651485b7a2cb222191eb64a55926e62bbabfe9b5ed2a9488aad547b204282"
    pubkeyBytes, _ := hex.DecodeString(pubkeyStr)
    pubkeyBytes2, _ := hex.DecodeString(pubkeyStr2)

    p, _ := btcec.ParsePubKey(pubkeyBytes, btcec.S256())
    p2, _ := btcec.ParsePubKey(pubkeyBytes2, btcec.S256())

    fmt.Printf("pubkeys: %s, %s\n", p, p2)

    sigStr := "3044022025332b6dabf11e493fbc62c93e7302c48666512e1cf88157c26176f4af6d064702201ee7ec25d0917244e514c402e8751f112dfd1bef2b22ec5e496fbafabb52bf010148"

    sigStr2 := "3045022100fa1f17bf59bee0ac33ae5f682711c5471c73a4aeb898aee218478289a4c7aa6e02207b40dfeae3fa4a50dc147bd42be40370d76a35d72c0b27b27c4ba2439a565fb901"

    sigBytes, _ := hex.DecodeString(sigStr)
    sigBytes2, _ := hex.DecodeString(sigStr2)

    s, _ := btcec.ParseDERSignature(sigBytes, btcec.S256())
    s2, _ := btcec.ParseDERSignature(sigBytes2, btcec.S256())

    fmt.Printf("sig: %s\n", s)

    fmt.Printf("signature valid: %v\n", s.Verify(hash, p))
    fmt.Printf("signature valid: %v\n", s2.Verify(hash, p2))

}

You can do the same thing with input #2 and input #3 to get the hash that gets signed by changing #5 and #6 (combined) to be 00 and #10 or #14 in the steps above to be 47522102cebf6ab580948d146b7cc771d8e646974349d3d7b11f3e03287d0997a477d3b921037ba651485b7a2cb222191eb64a55926e62bbabfe9b5ed2a9488aad547b20428252ae