Serialization Flip-Flop

To master Bitcoin development, you must memorize which fields are flipped and which are not. Failing to reverse the bytes in the right place will result in invalid transactions or "Transaction Not Found" errors.

1. The "Flipped" List (Little Endian)

These values must be reversed when moving between a human-readable hex string and raw binary:

• Transaction Version: (e.g., 01 00 00 00 for version 1).

• Previous TXID: The pointer to the coin you are spending.

• Output Value: The amount in Satoshis.

• Locktime: The time or block height restriction.

• Block Hash: The identifier for a block.

2. The "Natural" List (Big Endian)

These values are copied "As-Is" and are not reversed:

• Public Keys: (e.g., 02... or 03...).

• Signatures: (e.g., DER encoded 30...).

• Script OpCodes: (e.g., OP_DUP is 76).

• P2P Magic Bytes: (e.g., F9 BE B4 D9).

3. The "Block ID" Trap

In Bitcoin, the "Block ID" is the Block Hash.

• If you calculate the SHA-256 hash of a block header, you get a 32-byte result.

• If you want to print that hash to the screen, you must reverse the bytes first.

• If you don't reverse them, your "Block ID" will be backward, and no explorer will recognize it.

4. Scripts and Endianness

Bitcoin Script has its own rules.

• When you put a number on the stack (like OP_1), it is stored as a "Little Endian" signed integer.

• However, the OP_CODE itself is a single byte, so endianness doesn't apply to it.

5. Identifying the Flip

A quick way to tell if a hex string is Little Endian:

1. Look at the trailing zeros.

2. If the number is small (like 100), and you see 64 00 00 00, it is Little Endian.

3. If you see 00 00 00 64, it is Big Endian.

Object	Part of Object	Endianness
Transaction	Input Sequence	Little
Transaction	Unlocking Script	Big (Natural)
Block Header	Merkle Root	Little

In the final section, we will build a Python Endianness Auditor.