BA-06 · LESSON 04

P2PKH and P2SH

KEY DEFINITIONS

Terms used in this lesson

P2PKH: Pay to Public Key Hash, a legacy Bitcoin output template that commits to the hash of a public key.
P2SH: Pay to Script Hash, a legacy Bitcoin output template that commits to a redeem script.
Hash160: SHA-256 followed by RIPEMD-160, producing the 20-byte hash used by several Bitcoin spending conditions.
Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
CHECKSIG: The Bitcoin Script operation that checks whether a signature is valid for the required public key and transaction message.
OP_DUP: A Bitcoin Script operation that copies the top stack item.
OP_HASH160: A Bitcoin Script operation that replaces the top stack item with its Hash160 digest.
OP_EQUALVERIFY: A Bitcoin Script operation that compares two values and immediately fails if they differ.

Legacy templates separate address-friendly commitments from revealed spending scripts.

65 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 · script templateP2PKH and P2SH commit to different thingsSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered validation pipeline

01
OUTPUT TYPEP2PKH
The output commits to HASH160 of the public key that may authorize the spend.
02
LOCKING SCRIPTDUP HASH160 <key hash> EQUALVERIFY CHECKSIG
The scriptPubKey defines the byte-exact condition stored in the previous output.
03
SPEND DATASignature and public key
The public key is hashed and the signature is checked against the transaction commitment.
04
FIRST CHECKPublic key hash matches
A mismatch fails before the committed authorization policy can succeed.
05
RESULTCHECKSIG returns true
A matching commitment is necessary but the revealed signature or script policy must still pass.

Rule shown: P2PKH commits to a public-key hash; P2SH commits to a redeem-script hash and then executes the revealed script.

STAGE 1 OF 5OUTPUT TYPE: P2PKH

The output commits to HASH160 of the public key that may authorize the spend.

Governing relationship: P2PKH commits to a public-key hash; P2SH commits to a redeem-script hash and then executes the revealed script. Complete path: 1. OUTPUT TYPE, P2PKH. The output commits to HASH160 of the public key that may authorize the spend. 2. LOCKING SCRIPT, DUP HASH160 <key hash> EQUALVERIFY CHECKSIG. The scriptPubKey defines the byte-exact condition stored in the previous output. 3. SPEND DATA, Signature and public key. The public key is hashed and the signature is checked against the transaction commitment. 4. FIRST CHECK, Public key hash matches. A mismatch fails before the committed authorization policy can succeed. 5. RESULT, CHECKSIG returns true. A matching commitment is necessary but the revealed signature or script policy must still pass.

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

BEFORE

The output script is OP_DUP OP_HASH160 <AliceKeyHash> OP_EQUALVERIFY OP_CHECKSIG. The scriptSig pushes <AliceSig> <AlicePubKey>.

MECHANISM

The output script duplicates and hashes the supplied public key, compares it with a committed 20-byte key hash, then checks a signature.

AFTER

The hash check identifies the permitted public key; the signature check proves authorization for this transaction.

Text equivalent: Starting from The output script is OP_DUP OP_HASH160 <AliceKeyHash> OP_EQUALVERIFY OP_CHECKSIG. The scriptSig pushes <AliceSig> <AlicePubKey>. The mechanism begins when The output script duplicates and hashes the supplied public key, compares it with a committed 20-byte key hash, then checks a signature. The resulting lesson is The hash check identifies the permitted public key; the signature check proves authorization for this transaction.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Create a P2PKH commitment

The output script duplicates and hashes the supplied public key, compares it with a committed 20-byte key hash, then checks a signature.

Complete path: 1. Create a P2PKH commitment: The output script duplicates and hashes the supplied public key, compares it with a committed 20-byte key hash, then checks a signature. 2. Spend P2PKH: The scriptSig pushes a signature and public key. Combined execution checks the key hash before using that key in CHECKSIG. 3. Create a P2SH commitment: The output script requires a redeem script whose HASH160 equals the committed 20-byte script hash. 4. Spend P2SH: The scriptSig's final pushed item is the redeem script. Nodes verify its hash, then execute the redeem script with the remaining stack items. 5. Understand who chooses complexity: P2SH lets a recipient give senders a short address while revealing the complex condition only at spend time. The spender bears the redeem-script bytes.

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

ACCEPTED PATH

P2PKH verifies the revealed key hash and signature; P2SH verifies HASH160 of the exact redeem script before executing that script with the remaining stack.

CHANGED CONDITION

A redeem-script hash mismatch fails before its policy runs, while a matching script still fails if its threshold or signatures are unsatisfied.

Important distinction: P2SH makes Bitcoin consensus understand the business meaning of the redeem script. Consensus verifies the script's bytecode behavior. Labels such as escrow or recovery are interpretations used by wallets and people.

Text equivalent: The accepted path is enforced by P2PKH verifies the revealed key hash and signature; P2SH verifies HASH160 of the exact redeem script before executing that script with the remaining stack. If the required condition changes, A redeem-script hash mismatch fails before its policy runs, while a matching script still fails if its threshold or signatures are unsatisfied.

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Learn the idea

P2PKH means Pay to Public Key Hash. Its output commits to HASH160 of a public key and a spend reveals the public key plus signature. P2SH means Pay to Script Hash. Its output commits to HASH160 of a redeem script and a spend reveals that script plus data that satisfies it. Both are legacy templates whose scriptSig participates in the transaction's non-witness serialization.

GUIDED EXPLANATION1/5 ideas inspected

Create a P2PKH commitment

The output script duplicates and hashes the supplied public key, compares it with a committed 20-byte key hash, then checks a signature.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“P2SH makes Bitcoin consensus understand the business meaning of the redeem script.”

Consensus verifies the script's bytecode behavior. Labels such as escrow or recovery are interpretations used by wallets and people.

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