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I'm working with Partially Signed Bitcoin Transactions (PSBTs) and have a question about the capabilities of secondary signers in the signing process. I understand that in a multi-signature scenario, each participant is expected to sign the transaction. However, I'm curious about whether a second (or subsequent) signer can also append additional data to the transaction, aside from their signature.

Specifically, I'd like to know:

Can additional metadata or inputs/outputs be added by a second signer? Does the PSBT format support modifications by signers beyond merely adding their signatures? Any insights or references to documentation would be greatly appreciated!

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PSBT is just an interchange format for whatever information signers may need.

It does contain an unsigned transaction as "the thing being signed". Nothing prevents participants from modifying that transaction data, but doing so will generally invalidate the existing signatures, so it's not expected that participants do this after agreeing on what the transaction being signed is.

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  • so isnt there way to add data as second signer but without invalidating the psbt ?
    – Nerses Asaturyan
    Commented Jun 28 at 10:27
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    Normally, no, because the signatures cover the outputs; so if the outputs change, the signatures are invalidated (this too is unrelated to PSBT, it's just how signatures in Bitcoin work). It's possible to avoid this if the signatures do not cover the outputs (using the SIGHASH_NONE or SIGHASH_SINGLE flags), but there are many pitfalls in that case.
    – Pieter Wuille
    Commented Jun 28 at 11:52
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    @NersesAsaturyan: It might help if you take a look at the PSBT format specification in BIP 174.
    – Murch ♦
    Commented Jun 28 at 14:46

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