BA-05 · LESSON 03

Serialization and txids

KEY DEFINITIONS

Terms used in this lesson

Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
Witness transaction ID: An identifier that commits to a transaction including its witness data.
SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
SHA-256: A cryptographic hash function used throughout Bitcoin to produce fixed-size 256-bit digests.

Version, vectors, scripts, sequence, witness, and locktime serialize deterministically; txid and wtxid commit to different data.

55 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · transaction serializationField order becomes the transaction identifierSelect any state to inspect its exact role.

RELATIONSHIP MAPInputs, rule, and outputs

01
VERSION02000000
Multi-byte integers use little-endian byte order in transaction serialization.
02
INPUTS01 | prev txid | vout | script | sequence
CompactSize counts and fixed field order make the byte stream parseable.
03
OUTPUTS01 output
Each output contains an 8-byte satoshi value and length-prefixed script.
04
WITNESSNone
SegWit serialization moves authorization data into a witness section.
05
IDENTIFIERTxid equals wtxid
Both identifiers reverse the displayed byte order after double-SHA256.

Rule shown: Bitcoin hashes canonical serialized bytes; txid excludes SegWit witness data while wtxid includes the marker, flag, and witnesses.

STAGE 1 OF 5VERSION: 02000000

Multi-byte integers use little-endian byte order in transaction serialization.

Governing relationship: Bitcoin hashes canonical serialized bytes; txid excludes SegWit witness data while wtxid includes the marker, flag, and witnesses. Complete path: 1. VERSION, 02000000. Multi-byte integers use little-endian byte order in transaction serialization. 2. INPUTS, 01 | prev txid | vout | script | sequence. CompactSize counts and fixed field order make the byte stream parseable. 3. OUTPUTS, 01 output. Each output contains an 8-byte satoshi value and length-prefixed script. 4. WITNESS, None. SegWit serialization moves authorization data into a witness section. 5. IDENTIFIER, Txid equals wtxid. Both identifiers reverse the displayed byte order after double-SHA256.

1. State transitionWhat exists, what changes, and what results

BEFORE

A transaction begins 02000000 01, where spaces separate fields for this example.

MECHANISM

A transaction starts with a four-byte little-endian version and ends with a four-byte little-endian locktime.

AFTER

Identifiers commit to precisely defined byte sequences, not to a wallet's visual summary.

Text equivalent: Starting from A transaction begins 02000000 01, where spaces separate fields for this example. The mechanism begins when A transaction starts with a four-byte little-endian version and ends with a four-byte little-endian locktime. The resulting lesson is Identifiers commit to precisely defined byte sequences, not to a wallet's visual summary.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Write fixed fields

A transaction starts with a four-byte little-endian version and ends with a four-byte little-endian locktime.

Complete path: 1. Write fixed fields: A transaction starts with a four-byte little-endian version and ends with a four-byte little-endian locktime. 2. Serialize input and output vectors: Compact-size counts precede inputs and outputs. Each input names a 32-byte prior transaction hash, four-byte output index, scriptSig bytes, and sequence. Each output contains an eight-byte value and script bytes. 3. Place witness separately: A SegWit serialization adds marker 00 and nonzero flag, then places each input's witness stack after outputs and before locktime. 4. Compute identifiers: Double-SHA-256 of non-witness serialization yields the internal txid hash. Double-SHA-256 of witness serialization yields wtxid when witness is present. 5. Display conventional byte order: Explorers usually reverse the 32 digest bytes when displaying txid or wtxid. This display convention does not alter the transaction bytes.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The parser must consume every canonical byte, field width, CompactSize prefix, and witness vector before independently reproducing displayed txid and wtxid.

CHANGED CONDITION

Truncation or noncanonical length fails parsing; a witness-only mutation changes wtxid, while an output mutation changes both identifiers.

Important distinction: The txid always commits to every signature and witness byte. For SegWit transactions, txid excludes witness data while wtxid includes it. Legacy scriptSig data remains part of txid serialization.

Text equivalent: The accepted path is enforced by The parser must consume every canonical byte, field width, CompactSize prefix, and witness vector before independently reproducing displayed txid and wtxid. If the required condition changes, Truncation or noncanonical length fails parsing; a witness-only mutation changes wtxid, while an output mutation changes both identifiers.

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Learn the idea

Serialization turns transaction fields into one exact byte sequence. A compact-size integer prefixes variable-length vectors. The txid is the displayed byte-reversed double-SHA-256 digest of the legacy serialization, which excludes SegWit witness data. The wtxid applies the same digest convention to the serialization that includes marker, flag, and witness.

GUIDED EXPLANATION1/5 ideas inspected

Write fixed fields

A transaction starts with a four-byte little-endian version and ends with a four-byte little-endian locktime.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“The txid always commits to every signature and witness byte.”

For SegWit transactions, txid excludes witness data while wtxid includes it. Legacy scriptSig data remains part of txid serialization.

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