BA-08 · LESSON 13

Block height and BIP34 coinbase commitments

KEY DEFINITIONS

Terms used in this lesson

BIP34: The consensus rule requiring a block's coinbase transaction to begin its ScriptSig with the block height.
Coinbase transaction: The special first transaction in a Bitcoin block, which claims permitted subsidy and fees and has no ordinary prior-output input.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.

Encode the block height as the first minimally encoded Script number in the coinbase ScriptSig and keep it distinct from header fields.

35 min Difficulty 3/5 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 · bip34 heightBIP34 commits derived height inside coinbase ScriptSigSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
PARENTHeight 839999
Height is derived from the validated parent relationship.
02
HEIGHT840000
The candidate connects one position after the parent.
03
SCRIPT NUMBER40d10c
Positive magnitude is written little-endian.
04
FIRST PUSH0340d10c
The length byte precedes the exact height bytes.
05
COMMITCoinbase txid to Merkle root
Height is not an invented seventh block-header field.

Rule shown: The validated parent gives candidate height; its minimally encoded Script number is the coinbase ScriptSig first push and therefore enters the Merkle root.

STAGE 1 OF 5PARENT: Height 839999

Height is derived from the validated parent relationship.

Governing relationship: The validated parent gives candidate height; its minimally encoded Script number is the coinbase ScriptSig first push and therefore enters the Merkle root. Complete path: 1. PARENT, Height 839999. Height is derived from the validated parent relationship. 2. HEIGHT, 840000. The candidate connects one position after the parent. 3. SCRIPT NUMBER, 40d10c. Positive magnitude is written little-endian. 4. FIRST PUSH, 0340d10c. The length byte precedes the exact height bytes. 5. COMMIT, Coinbase txid to Merkle root. Height is not an invented seventh block-header field.

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

BEFORE

A valid parent is at height 839,999.

MECHANISM

A candidate whose parent is at height h can connect at height h plus one. An unknown or invalid parent cannot provide trustworthy height context.

AFTER

Height comes from chain position and BIP34 commits it inside coinbase transaction bytes.

Text equivalent: Starting from A valid parent is at height 839,999. The mechanism begins when A candidate whose parent is at height h can connect at height h plus one. An unknown or invalid parent cannot provide trustworthy height context. The resulting lesson is Height comes from chain position and BIP34 commits it inside coinbase transaction bytes.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Derive height from the parent

A candidate whose parent is at height h can connect at height h plus one. An unknown or invalid parent cannot provide trustworthy height context.

Complete path: 1. Derive height from the parent: A candidate whose parent is at height h can connect at height h plus one. An unknown or invalid parent cannot provide trustworthy height context. 2. Encode a Script number: Serialize the positive integer little-endian by magnitude. Append a zero sign byte when the highest data byte would otherwise set the sign bit. 3. Prefix the byte length: The coinbase ScriptSig begins with a push of those height bytes. The first opcode states their length; remaining coinbase ScriptSig bytes may carry extranonce or pool data. 4. Require minimal encoding: Redundant high zero bytes, negative encodings, truncation and a value different from the derived height fail the BIP34 commitment check. 5. Commit through the Merkle root: Changing height bytes changes the coinbase txid, ordinary transaction Merkle root and header. Miners also change coinbase extranonce to create more header search space.

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

ACCEPTED PATH

The node derives candidate height from the validated parent, parses the first coinbase ScriptSig push as a minimal Script number and requires exact equality.

CHANGED CONDITION

Putting height in an invented header field, using display-endian integer bytes, or adding a redundant sign byte creates a different or nonminimal commitment.

Important distinction: The block header contains the block number. The header contains version, previous hash, Merkle root, time, nBits and nonce. Height is derived from parent links and committed in modern coinbase ScriptSig bytes.

Text equivalent: The accepted path is enforced by The node derives candidate height from the validated parent, parses the first coinbase ScriptSig push as a minimal Script number and requires exact equality. If the required condition changes, Putting height in an invented header field, using display-endian integer bytes, or adding a redundant sign byte creates a different or nonminimal commitment.

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Learn the idea

Block height is a position derived by following validated parent links from genesis. It is not one of the six header fields. BIP34 requires the coinbase transaction's ScriptSig to begin with the current height encoded as a minimally encoded Script number, binding that derived position into the transaction list and therefore the block's Merkle root.

GUIDED EXPLANATION1/5 ideas inspected

Derive height from the parent

A candidate whose parent is at height h can connect at height h plus one. An unknown or invalid parent cannot provide trustworthy height context.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“The block header contains the block number.”

The header contains version, previous hash, Merkle root, time, nBits and nonce. Height is derived from parent links and committed in modern coinbase ScriptSig bytes.

FINISH LEARNING FIRST

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Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.