diff --git a/7_3_Scripting_a_Pay_to_Public_Key_Hash.md b/7_3_Scripting_a_Pay_to_Public_Key_Hash.md new file mode 100644 index 0000000..744fe63 --- /dev/null +++ b/7_3_Scripting_a_Pay_to_Public_Key_Hash.md @@ -0,0 +1,67 @@ +# 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 ` `. + +## 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 ``. + +## Run a P2PKH Script + +When you run a P2PKH unlocking script with the P2PKH in the UTXO, the following is (effectively) generated: + +``` +Script: OP_DUP OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG +``` +First, the script puts the initial constants on the stack and makes a duplicate of the pubKey: +``` +Script: OP_DUP OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG +Stack: [ ] + +Script: OP_DUP OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG +Stack: [ ] + +Script: OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG +Stack: [ ] +``` +Why the duplicate? Because that's what's required by the script! + +Next `OP_HASH160` pops the `` off the stack, hashes it, and puts the result back on the stack. +``` +Script: OP_EQUALVERIFY OP_CHECKSIG +Stack: [ ] +``` +The next constant follows it: +``` +Script: OP_EQUALVERIFY OP_CHECKSIG +Stack: [ ] +``` +`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 `es` are equal, we now have the following situation: +``` +Script: OP_CHECKSIG +Stack: [ ] +``` +At this point we've proven that the `` 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. + +