# Raw Transaction Format

## Abstract

This BRC specifies the format used for raw hex Bitcoin transactions, which are a widely-used way of representing Bitcoin transactions on the network. The specification includes details on the layout and various fields within Bitcoin transactions.

## Motivation

Bitcoin transactions are the mechanism for transferring custody of bitcoin tokens from one party to another. It is crucial to have a clear and unambiguous specification for their format. The raw hex format is widely-used and understanding its structure is important for developers and other stakeholders in the Bitcoin ecosystem.

## Specification

A Bitcoin transaction consists of a version number, a locktime value, a list of inputs, and a list of outputs. The format for a raw hex Bitcoin transaction is as follows:

* Version: 4-byte integer (little-endian)
* Input Count: variable-length integer
* Inputs: a list of input objects, where each input object has the following fields:
  * Previous Transaction Hash: 32-byte hash (little-endian)
  * Previous Transaction Output Index: 4-byte integer (little-endian)
  * Script Length: variable-length integer
  * Unlocking Script: variable-length script
  * Sequence Number: 4-byte integer (little-endian)
* Output Count: variable-length integer
* Outputs: a list of output objects, where each output object has the following fields:
  * Value: 8-byte integer (little-endian)
  * Script Length: variable-length integer
  * Locking Script: variable-length script
* Locktime: 4-byte integer (little-endian)

### Variable Integers

The variable-length integer is a compact representation of an integer value. The first byte of the integer determines the format of the integer:

* If the first byte is less than 0xfd, then the integer is that byte value.
* If the first byte is 0xfd, then the integer is the next two bytes in little-endian format.
* If the first byte is 0xfe, then the integer is the next four bytes in little-endian format.
* If the first byte is 0xff, then the integer is the next eight bytes in little-endian format.

The script fields in the input and output objects are interpreted as bytecode for a [Bitcoin Script](https://bsv.brc.dev/scripts/0014), which is a stack-based language used to define spending conditions for bitcoin.

The transaction hash (referred to as the "TXID") is calculated by taking the double-SHA256 hash of the entire transaction. This hash is used as a unique identifier for the transaction on the Bitcoin network.