P2SH address as a product of public keys

Question

Assuming the multisig 2 of 3, if I do have extended public keys (xpub) for all 3 cosigners (with proper derivations), can I somewhat get the result address (P2SH)? How to calculate the "script" out of these public keys?

Would be cool to have scheme for doing so or a code/pseudocode in python if possible.

Answer 1

Let's start from multisig 2-of-3 redeem_script template:

OP_2 OP_DATA_33 key1.public_key OP_DATA_33 key2.public_key OP_DATA_33 key3.public_key OP_3 OP_CHECKMULTISIG

which ties the keys together. All you need to do here is generate some 3 keys (33 bytes each) from those xpubs (BIP-0032) and concatenate the keys with opcode bytes.

Raw bytes of this script would be (note: the + operator below indicates byte concatenation):

0x52 + 0x21 + key1.public_key + 0x21 + key2.public_key + 0x21 + key3.public_key + 0x53 + 0xae

From that, you compute:

redeem_script_hash = ripemd160(sha256(redeem_script))

Then, you can construct the raw output locking_script:

OP_HASH160 OP_DATA_20 redeem_script_hash OP_EQUAL

by again concatenating the raw bytes:

0xa9 + 0x14 + redeem_script_hash + 0x87

That's it! You can either use the raw bytes directly if you're constructing a raw transaction yourself, or encode it into a Bitcoin address so other wallets can pay into it.

When time comes to spend, the input script will be of the form:

OP_0 OP_DATA_71 key1.signature OP_DATA_71 key2.signature OP_PUSHDATA1 105 redeem_script

and its raw bytes will be:

0x00 + 0x47 + key1.signature + 0x47 + key2.signature + 0x4c + 0x69 + redeem_script

Here's is a great reference for P2SH & multisig: https://btcinformation.org/en/developer-guide#multisig