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According to the bitcoin wiki, the most common form of transaction (pay-to-pubkey-hash) looks like this:

scriptPubKey: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
scriptSig: <sig> <pubKey>

I am curious why OP_CHECKSIG comes at the end. Couldn't we check the signature even before doing the hash?

More precisely, couldn't it instead do:

scriptPubKey: OP_CHECKSIGVERIFY OP_DUP OP_HASH160 <pubKeyHash> OP_EQUAL
scriptSig: <pubKey> <sig>

This way you don't have to go through all the hashing if the pubkey and sig don't match to begin with.

I'm sure there's a good reason for this or i'm completely misunderstanding something, please enlighten me. Thank you!

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Couldn't we check the signature even before doing the hash?

We certainly could, but that would be silly. The hash is about 1000x faster than checking a signature. Doing the signature check when the hash fails would be a waste of time

More precisely, couldn't it instead do:

scriptPubKey: OP_CHECKSIGVERIFY OP_DUP OP_HASH160 OP_EQUAL

No, that certainly wouldn't work. The OP_CHECKSIGVERIFY would remove the public key from the stack, so there is nothing to compare it with. The OP_DUP would always fail, as there is nothing to duplicate anymore.

  • aha i see. thanks for the answer. Just to make sure i understood completely, so if we do scriptPubKey: OP_DUP OP_CHECKSIGVERIFY OP_HASH160 <pubKeyHash> OP_EQUAL and scriptSig: <sig> <pubKey>, this would: 1. work; 2. but not efficient therefore not used because hashing is much faster than checksig? – Vlad Oct 8 '17 at 7:08
  • Not quite. After OP_DUP, your stack will be <sig> <pubkey> <pubkey>. Invoking OP_CHECKSIGVERIFY at that point will do a verification with <pubkey> as signature. – Pieter Wuille Oct 8 '17 at 7:50
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    What would work (but be inefficient): scriptSig={<pubkey> <sig>}, scriptPubKey={OP_OVER OP_CHECKSIGVERIFY OP_HASH160 <pubkeyhash> OP_EQUAL}. – Pieter Wuille Oct 8 '17 at 8:07
  • Actually, the scriptSig would be "<sig> <pubkey>" to use OP_OVER. If it was already "<pubkey> <sig>" the suggestion in the first comment would work, but it would require a change in the scriptSig, not only the scriptPubKey. – diego nunes Apr 23 '18 at 16:39

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