Trezor One cannot sign tx that has external pre-signed inputs

[SomberNight]

Consider two standard (singlesig) wallets trying to do a coinjoin.
If the last signer is a Trezor One, it raises RuntimeError('ProcessError: Unverifiable external input.').

Example steps for manual coinjoin:

1. wallet1 creates unsigned tx1, paying itself (e.g. 1 input, 2 outputs)
2. in wallet1, go to tx Preview. Share>"Include SLIP-19 ownership proofs"
   • this step is optional, might be needed depending on the requirements of wallet2 (namely if it is a Trezor)
3. in wallet1, tx dialog, Share>"Copy to clipboard"
   • let's call the exported tx "psbt1"
4. wallet2 creates unsigned tx2, paying itself (e.g. 1 input, 2 outputs)
5. in wallet2, go to tx Preview. Combine>"Join inputs/outputs", paste psbt1.
   • now the tx preview dialog shows a new tx, "tx3", which has 2 inputs and 4 outputs
6. in wallet2 still, tx dialog, now you can click "Sign"
   • the inputs of tx3 that are owned by wallet2 will be signed now
7. in wallet2 still, tx dialog, click Share>"Copy to clipboard"
   • let's call the exported tx "psbt2"
8. in wallet1, menubar>Tools>"Load transaction">"From text". paste psbt2.
9. in wallet1, tx dialog, now you can click "Sign"
   • the inputs of tx3 that are owned by wallet1 will be signed now
10. tx3 is now fully signed and you can broadcast it

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• If wallet1 uses a Trezor One (fw 1.12.1), e.g. with p2wpkh scripts, it will raise on step 9: RuntimeError('ProcessError: Unverifiable external input.').
  • wallet2 could be a software wallet with p2wpkh scripts, or a Trezor T, does not matter
• If wallet1 uses a Trezor T, and wallet2 uses a Trezor One, no errors
• If wallet1 uses a Trezor Safe 3, and wallet2 uses a Trezor T, no errors
• If wallet1 uses a Trezor T, and wallet2 uses a Trezor Safe 3, no errors

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It looks like the Trezor One is refusing to sign a tx with external pre-signed inputs. The other trezor models work however.
@matejcik any idea?

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originally reported at #8871 (comment) by @hynek-jina
related #8324

[matejcik]

Trezor One has limited support for EXTERNAL features. Only ownership proofs are accepted and only for segwit/taproot inputs. Pre-signed inputs are not supported.

cc also @andrewkozlik

[SomberNight]

I see. Thanks.
So the trick is that wallet2 needs to also add ownership proofs besides signing their own inputs.
To do this though, 85ea2df is needed. (EDIT: or actually, if you add the proofs before signing, the commit is not needed)

---

Modified steps from OP:

1. wallet1 creates unsigned tx1, paying itself (e.g. 1 input, 2 outputs)
2. in wallet1, go to tx Preview. Share>"Include SLIP-19 ownership proofs"
   • this step is optional, might be needed depending on the requirements of wallet2 (namely if it is a Trezor)
3. in wallet1, tx dialog, Share>"Copy to clipboard"
   • let's call the exported tx "psbt1"
4. wallet2 creates unsigned tx2, paying itself (e.g. 1 input, 2 outputs)
5. in wallet2, go to tx Preview. Combine>"Join inputs/outputs", paste psbt1.
   • now the tx preview dialog shows a new tx, "tx3", which has 2 inputs and 4 outputs
6. [[[NEW]]] in wallet2 still, tx dialog. Share>"Include SLIP-19 ownership proofs"
   • this step is optional, might be needed depending on the requirements of wallet1 (namely if it is a Trezor model One)
   • note: before 85ea2df, the proofs should be added before signing the tx. After that commit, the order does not matter.
7. in wallet2 still, tx dialog, now you can click "Sign"
   • the inputs of tx3 that are owned by wallet2 will be signed now
8. in wallet2 still, tx dialog, click Share>"Copy to clipboard"
   • let's call the exported tx "psbt2"
9. in wallet1, menubar>Tools>"Load transaction">"From text". paste psbt2.
10. in wallet1, tx dialog, now you can click "Sign"
    • the inputs of tx3 that are owned by wallet1 will be signed now
11. tx3 is now fully signed and you can broadcast it

[hynek-jina]

Now it works even for Trezor One, thank you! 🙏