Understanding Bitcoin Transaction Requirements: Segwit Transaction Proofing

Bitcoin’s move to Segregated Witness (Segwit) has brought about significant changes in the way transactions are verified and authenticated on the network. One of the key features of Segwit is its ability to mix inputs, allowing for more complex transaction structures and reducing reliance on traditional payment processors.

However, when it comes to creating a raw Segwit transaction without Segwit inputs, one might wonder whether such a construction is feasible. In this article, we will delve into the requirements for exact witness (exact) transactions, examine how they can be created, and discuss whether it is possible to create a raw Segwit transaction without Segwit inputs.

What are exact witness transactions?

Exact witness transactions (EWT) are a type of Bitcoin transaction that includes exactly one input from an unconfirmed or partially confirmed sender. The remaining amount is completely spent on the destination address, making EWTs suitable for microtransactions and small payments.

In the context of Segwit, it is possible to construct precise witness transactions using inputs from both segwit and non-segwit addresses. This allows for greater flexibility in transaction design, allowing users to create complex payment flows that were not possible with traditional payment processors.

Building a raw Segwit transaction without Segwit inputs

To build a raw Segwit transaction without Segwit inputs, we need to consider the following:

• Segwit and non-segwit input requirements: An unmodified Segwit transaction must have at least one input from an unconfirmed or partially confirmed sender (segwit), and any amount spent on that destination address is considered valid.

• P2PKH and P2WPKH inputs: Our construction will also require P2PKH (public key hash, public key only) or P2WPKH (private key hash, private key only) inputs for the non-Segwit sender account.

• Exact witness requirements: The transaction must be exactly witnessed to ensure that all input amounts are spent on the destination address.

Given these constraints, one possible construction of a raw Segwit transaction without follow-through inputs could be:

Input 1: P2WPKH input from an unconfirmed sender (e.g. «0.0001 BTC» or «10 satoshi»)

Input 2:

Follow-through output from the same sender account with a value of zero (0) and no private key hash or public key only (i.e. 0x00...);

The resulting transaction would have two inputs:

• One P2WPKH input for an unconfirmed sender

• One segwit output for the same sender

Transaction Creation

Here is a simplified example of how this construction could be represented in raw Bitcoin transaction format:

0.0001 BTC 00… (unconfirmed sender)
+------+ +------+
| P2WPKH | | segwit output
+------+ +------+
|
| (10x satoshi spent on destination address)

Conclusion

While it is technically possible to create a raw Segwit transaction without segwit inputs, it would require careful consideration of the exact witness requirements and input formats. In practice, this approach may not be feasible due to limitations in the Bitcoin protocol and potential input validation issues.

In short, creating a raw Segwit transaction without Segwit inputs requires creative combinations of inputs that balance the need for accurate witness transactions with practicality and compliance with the Bitcoin protocol. As the network continues to evolve, it will be essential to refine our understanding of these requirements and explore new approaches to accommodate more complex transaction structures.

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