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I created a multi signature wallet 2 of 3. I want to get the xpub key used to generate the addresses. As I understand all 3 public keys are used to generate the address. Is it possible to use those 3 keys to generate a single xpub key for further addresses?
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No, it's not possible to get a single public seed for all future addresses.
Let's say you have three private seeds, Sa, Sb and Sc with public seeds (xpubs) Pa, Pb and Pc that generate the addresses Px1, Px2, ...
Then, the way to calculate the multi-sig addresses for a 2-of-3 multisig for these three seeds using P2SH is the following:
You create a P2SH redeem script that looks as follows:
OP_2 [Pa1] [Pb1] [Pc1] OP_3 OP_CHECKMULTISIG
Then, the "multisig address" is the RIPEMD-160 hash of that script.
Thus, to know the next address, you will need to calculate a new RIPEMD-160 hash with the script now containing Pa2, Pb2 and Pc2. There is not a multi-seed of any kind.
