How to resolve error when trying to sign and broadcast PSBT transaction via electrum - (Witness program hash mismatch code -26)?

Question

I'm trying to generate a 2-of-2 multisig transaction as a PSBT so that both parties can easily sign their transaction. I'm doing this all on btc testnet as a p2sh p2wsh transaction

When initially generating my PSBT, I get the following

70736274ff010052010000000118c739a0c061b892aefb3b2d93f9bca72c564ece339bf221946bddbd4c5b64390100000000ffffffff01bced0600000000001600146f01b2a5c8028a29f9631b6d494535fbd907e37a000000000001042200204b29bdce823c1afde68cc26012e49a913fc005a629748afbea7285d7ac281b7c010547522102dbbedff721dcdc7737c0e931c0473db3aadf4690f01dd5fc6da65188e21ecffd2102f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b85660595252ae0000  

After signing it with the first party's electrum wallet, i get

70736274ff010052010000000118c739a0c061b892aefb3b2d93f9bca72c564ece339bf221946bddbd4c5b64390100000000ffffffff01bced0600000000001600146f01b2a5c8028a29f9631b6d494535fbd907e37a0000000000220202f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b85660595247304402207bdd58e7e837818b1bac12ee6615e48775c0354e3e0d22dd8ccf2d8afa0280bd022038f94ad49b245145e676a69d0af0233f0188b2b4bc88faec310de51f42cc18670101042200204b29bdce823c1afde68cc26012e49a913fc005a629748afbea7285d7ac281b7c010547522102dbbedff721dcdc7737c0e931c0473db3aadf4690f01dd5fc6da65188e21ecffd2102f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b85660595252ae0000

Finally after signing it with the second party

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

When I attempt to broadcast this transaction, I get "sendrawtransaction RPC error: {"code":-26,"message":"non-mandatory-script-verify-flag (Witness program hash mismatch)"}". I've double checked to make sure I am signing with the correct keys and the amounts are correct so I'm lost on how to fix this. Any help is appreciated :)

Answer 1

In signing with the second party, the multisig script that is your witness script got messed up.

In both the initial PSBT and the first signing, we see that the witness script is

522102dbbedff721dcdc7737c0e931c0473db3aadf4690f01dd5fc6da65188e21ecffd2102f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b85660595252ae

which is decoded as

2 02dbbedff721dcdc7737c0e931c0473db3aadf4690f01dd5fc6da65188e21ecffd 02f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b856605952 2 OP_CHECKMULTISIG

This script has a sha256 of 4b29bdce823c1afde68cc26012e49a913fc005a629748afbea7285d7ac281b7c which matches the hash given in the redeem script of the input, and the redeem script has a hash of 4d3a0d4465e19bb387df92f5e052f24370718117 which matches the hash in the output script of the output being spent. This is all correct.

But when we look at the final transaction, the script for some reason is changed to

522102f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b8566059522102dbbedff721dcdc7737c0e931c0473db3aadf4690f01dd5fc6da65188e21ecffd52ae

which is obviously different from the witness script originally. It's sha256 is 64b235d9fd37f2d68cb6edf19a25cc7765ef9c7299e3b448c86196cecd60feb0 which doesn't match the hash in the redeem script, so the script interpreter fails here.

This script is decoded as

2 02f4e91f49be47c24571e100c501cc50e9c499e744a25e51e2153d26b856605952 02dbbedff721dcdc7737c0e931c0473db3aadf4690f01dd5fc6da65188e21ecffd 2 OP_CHECKMULTISIG

Looking at these two scripts, we can see the second one has the keys flipped around. For some reason, the software you used to finalize the PSBT changed the positioning of the keys in the script, which has resulted in the final transaction failing.