BA-01 · LESSON 07

CompactSize and Bitcoin byte fields

KEY DEFINITIONS

Terms used in this lesson

Outpoint: A transaction ID plus output number that identifies one exact prior output.

Parse variable-length counts, fixed-width integers, hashes, scripts, and length-prefixed vectors without confusing display order with serialized order.

55 min Difficulty 1/5 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 · compactsize parserCompactSize sets exact typed byte boundariesSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
PREFIXFd
Values 0 through 252 use one byte; fd selects a two-byte little-endian payload.
02
PAYLOADFd 00
fd 00 decodes to 253, the smallest value allowed in this form.
03
COUNT253 records
The result bounds how many complete records follow.
04
VECTORAll bytes present
Each nested length must fit inside the remaining outer object.
05
PARSEExact consumption
Noncanonical, truncated, oversized, or trailing data fails closed.

Rule shown: A parser must know the field type, use the shortest allowed count encoding, and consume exactly the declared bytes.

STAGE 1 OF 5PREFIX: Fd

Values 0 through 252 use one byte; fd selects a two-byte little-endian payload.

Governing relationship: A parser must know the field type, use the shortest allowed count encoding, and consume exactly the declared bytes. Complete path: 1. PREFIX, Fd. Values 0 through 252 use one byte; fd selects a two-byte little-endian payload. 2. PAYLOAD, Fd 00. fd 00 decodes to 253, the smallest value allowed in this form. 3. COUNT, 253 records. The result bounds how many complete records follow. 4. VECTOR, All bytes present. Each nested length must fit inside the remaining outer object. 5. PARSE, Exact consumption. Noncanonical, truncated, oversized, or trailing data fails closed.

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

BEFORE

The bytes are fd fd 00 followed by 253 records.

MECHANISM

Determine whether the next field is a fixed-width integer, 32-byte hash, CompactSize count, length-prefixed byte vector, Script number, or raw byte sequence. The surrounding…

AFTER

Bytes become Bitcoin fields only through a typed, bounded, canonical parser.

Text equivalent: Starting from The bytes are fd fd 00 followed by 253 records. The mechanism begins when Determine whether the next field is a fixed-width integer, 32-byte hash, CompactSize count, length-prefixed byte vector, Script number, or raw byte sequence. The surrounding structure supplies meaning. The resulting lesson is Bytes become Bitcoin fields only through a typed, bounded, canonical parser.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Read the field specification

Determine whether the next field is a fixed-width integer, 32-byte hash, CompactSize count, length-prefixed byte vector, Script number, or raw byte sequence. The surrounding structure supplies meaning.

Complete path: 1. Read the field specification: Determine whether the next field is a fixed-width integer, 32-byte hash, CompactSize count, length-prefixed byte vector, Script number, or raw byte sequence. The surrounding structure supplies meaning. 2. Decode CompactSize canonically: Values 0 through 252 use one byte. Prefix fd adds a two-byte little-endian value, fe adds four bytes, and ff adds eight bytes. A longer form for a smaller value is noncanonical. 3. Use counts to bound parsing: A transaction input count determines how many input records follow. Each ScriptSig length determines exactly how many script bytes belong to that input before sequence begins. 4. Keep hash display and serialization separate: Outpoint transaction identifiers are commonly displayed with reversed digest bytes but serialized in internal byte order. This convention does not permit reversing every hash or field blindly. 5. Fail on truncation or leftovers: A parser rejects a declared vector longer than available bytes, an impossible count, a noncanonical prefix where required, or unconsumed trailing bytes when the outer object requires exact consumption.

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

ACCEPTED PATH

The parser applies the expected field type, canonical prefix, byte order, bounds, and exact consumption before exposing a structured result.

CHANGED CONDITION

fd fc 00 encodes 252 using an unnecessarily long form and must fail where canonical CompactSize is required. fd fd with one missing byte is truncated.

Important distinction: CompactSize means any Bitcoin number can be shortened by removing leading zero bytes. CompactSize is a specific unsigned count and length encoding. Other fields keep their own fixed width or Script-number rules.

Text equivalent: The accepted path is enforced by The parser applies the expected field type, canonical prefix, byte order, bounds, and exact consumption before exposing a structured result. If the required condition changes, fd fc 00 encodes 252 using an unnecessarily long form and must fail where canonical CompactSize is required. fd fd with one missing byte is truncated.

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Learn the idea

Bitcoin serialization combines fixed-width fields with CompactSize-prefixed vectors. CompactSize is a variable-length unsigned-integer encoding used for counts and lengths. It is not a general-purpose compressed number and it is not Bitcoin Script's signed-number encoding. A correct parser knows the field type before reading bytes.

GUIDED EXPLANATION1/5 ideas inspected

Read the field specification

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“CompactSize means any Bitcoin number can be shortened by removing leading zero bytes.”

CompactSize is a specific unsigned count and length encoding. Other fields keep their own fixed width or Script-number rules.

FINISH LEARNING FIRST

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