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, 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.