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Learning-Bitcoin-from-the-C.../10_1_Using_Script_Conditionals.md
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10.1: Using Script Conditionals

NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.

There's one more aspect of Bitcoin Scripting that's crucial to unlocking its true power: conditionals allow you create various paths of execution.

Understand Verify

You've already seen one conditional in scripts: OP_VERIFY (0x69). It pops the top item on the stack and sees if it's true, and if it's not it ends execution of the script.

Verify is usually incorporated into other opcodes. You've already seen OP_EQUALVERIFY (0xad), OP_CHECKLOCKTIMEVERIFY (0xb1), and OP_CHECKSEQUENCEVERIFY (0xb2). Each of these opcodes does its core action (equal, checklocktime, or checksequence) and then does a verify afterward. The other verify opcodes that you haven't seen are: OP_NUMEQUALVERIFY (0x9d), OP_CHECKSIGVERIFY (0xad), and OP_CHECKMULTISIGVERIFY (0xaf).

So how is verify a conditional? It's the most powerful sort of conditional. Using OP_VERIFY, if a condition is true, the Script continues executing, else the Script exits. This is how you check conditions that are absolutely required for a Script to succeed. For example, the P2PKH script (OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG) has two required conditions: (1) that the supplied public key match the public-key hash; and (2) that the supplied signature match that public key. An OP_EQUALVERIFY is used for the check of the public key and the public-key hash because it's an absolutely required condition. You don't want the script to continue on.

You may notice there's no OP_VERIFY at the end of this (or most any) script, despite the final condition being required as well. That's because Bitcoin effectively does an OP_VERIFY at the very end of each Script, to ensure that the final stack result is true.

Understand If/Then

The other major conditional in Bitcoin Script is the classic OP_IF (0x63) / OP_ELSE (0x67) / OP_ENDIF (0x68). This is typical flow control: if OP_IF detects a true statement, it executes the block under it; otherwise, if there's an OP_ELSE, it executes that; and OP_ENDIF marks the end of the final block.

WARNING: These conditionals are technically opcodes too, but as with small numbers, we're going to leave the OP_ prefix off for brevity and clarity.

  • Verify * If * DOn't know inputs (in unlocking!)