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7.3: Scripting a Pay to Public Key Hash
NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
With basic understanding of Bitcoin Scripting in hand, you can now easily analyze the functioning of a standard P2PKH script.
Understand the Unlocking Script
We've long said that when funds are sent to a Bitcoin address, they're locked to the private address associated with the address. This is all managed through scriptPubKey in a P2PKH transaction, which is set up so that it requires two secret bits of information: the private key and the public key associated with the Bitcoin address, which is itself a public key hash.
The example in §6.2 has a scriptSig unlocking script that is 3045022100c4ef5b531061a184404e84ab46beee94e51e8ae15ce98d2f3e10ae7774772ffd02203c546c399c4dc1d6eea692f73bb3fff490ea2e98fe300ac6a11840c7d52b6166[ALL] 0319cd3f2485e3d47552617b03c693b7f92916ac374644e22b07420c8812501cfb. This is just <signature> <pubKey>.
Understand the Locking Script
The scriptPubKey locking script from §6.2 is OP_DUP OP_HASH160 371c20fb2e9899338ce5e99908e64fd30b789313 OP_EQUALVERIFY OP_CHECKSIG, which is the standard methodology for a P2PKH address. That long chain string in the middle is a <pubKeyHash>.
Run a P2PKH Script
When you run a P2PKH unlocking script with the P2PKH in the UTXO, the following is (effectively) generated:
Script: <signature> <pubKey> OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
First, the script puts the initial constants on the stack and makes a duplicate of the pubKey:
Script: <pubKey> OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Stack: [ <signature> ]
Script: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Stack: [ <signature> <pubKey> ]
Script: OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Stack: [ <signature> <pubKey> <pubKey> ]
Why the duplicate? Because that's what's required by the script!
Next OP_HASH160 pops the <pubKey> off the stack, hashes it, and puts the result back on the stack.
Script: <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
Stack: [ <signature> <pubKey> <pubKeyHash> ]
The next constant follows it:
Script: OP_EQUALVERIFY OP_CHECKSIG
Stack: [ <signature> <pubKey> <pubKeyHash> <pubKeyHash> ]
OP_EQUALVERIFY is really two opcodes: OP_EQUAL, which pops the two two elements and pushes true or false depending on if they're equal; and verify which pops the result and immediately marks the transaction as invalid if it's false.
Assuming the two <pubKeyHash>es are equal, we now have the following situation:
Script: OP_CHECKSIG
Stack: [ <signature> <pubKey> ]
At this point we've proven that the <pubKey> supplied in the scriptSig matches the one associated with the Bitcoin address.
Finally, OP_CHECKSIG verifies that the unlocking script's signature proves ownership of the private key associated with that private key:
Script:
Stack: [ True ]
At this point, the Script ends and the transaction is allowed to respend the UTXO in question.
Summary: Scripting a Pay to Public Key Hash
Sending to a P2PKH address was relativel easy when you were just using bitcoin-cli. Examining the Bitcoin Script underlying it lays bare the cryptographic functions that were implicit in funding that transaction: how UTXOs were unlocked with a signature and a public key; and how the new transaction output was in turn locked with a new public-key hash.