Bitcoin: Can a Second Signer Add Data to a Bitcoin PSBT Transaction?

Can a second signer add data to a Bitcoin PSBT transaction?

With partially signed Bitcoin transactions (PSBTs), there can be multiple parties involved in the signing and verification process. In a traditional multi-signature wallet setup, two or more signatures are required for authorization, with at least one signature verified by the other signers. But what if we want to allow secondary signs to contribute data to the transaction? Can a second signer add data to a Bitcoin PSBT transaction?

Basics of Partially Signed Bitcoin Transactions

In Bitcoin, PSBTs are used to represent a signed partial transaction that includes only a subset of the final transactions that will be included in the master block. Each PSBT contains a list of public keys and a set of transactions (or “data”) that can be added or removed.

Signatures and Verification

When a sender adds data to a PSBT, they create a new transaction. This transaction is then signed by one or more secondary signers (i.e. parties not included in the initial two-signature setup). The secondary signers verify the transaction using their own private keys.

The key concept is that each signature is verified independently, and the verification process relies on the cryptographic algorithms used to create the signatures. In Bitcoin, these algorithms are designed to be secure and tamper-resistant.

Can a second signer add data to a Bitcoin PSBT transaction?

Can a second signer now add data to a Bitcoin PSBT transaction?

The answer is no, in general. When adding data to a PSBT, secondary signers must verify their signatures separately from the initial two-signature setup. This means that each secondary signer must create and sign a new transaction, which may be different from any other transactions added to the PSBT.

However, there are some special cases where a second signer may be able to add data to a Bitcoin PSBT transaction:

• Public Key Signatures: If the sender used public key signatures (PKS) to sign the initial two-signature setup, then the secondary signer can use their private key to create and sign a new PSBT that contains only the data they want to add.

• Digital Signing: Bitcoin allows transactions to be digitally signed using non-standard formats, such as PEM-encoded strings. In some cases, a secondary signer may be able to digitally sign additional data as part of an existing transaction.

Conclusion

In conclusion, while it is theoretically possible for a second signer to contribute data to a Bitcoin PSBT transaction, this would require significant changes to the standard signing and verification process. Using public key signatures and digital signing allows for greater flexibility, but these mechanisms can also introduce additional complexity and security risks.

When working with partially signed transactions, it is essential to carefully consider the implications of secondary signers and ensure that any modifications are consistent with existing cryptographic standards and Bitcoin best practices.

Additional Reading

For more information on PSBT and Bitcoin signatures, we recommend reading:

• “The Bitcoin Wallet Protocol” by Satoshi Nakamoto (open source documentation)

• “PSBT Tutorial” by the Bitcoin Developer Network

Note: This article is for informational purposes only and should not be construed as investment advice. Always consult a qualified professional before making any decisions regarding cryptocurrency or other financial matters.