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Let's say I am a node. A transaction has been relayed to me, and one of the scriptSig fields is:

<sig> <pubKey>

  1. What is the easiest way for me to malleate this transaction?

    • For example, could I just add a "dummy" opcode to it, like this: OP_NOP <sig> <pubKey>?
  2. Also, are certain types of scriptSig easier to malleate than others?

Basically, I'm interested in the most practical way a third party might go about malleating a transaction.

  • I don't believe adding a dummy opcode would work. You can't change the payload of the transaction at all without invalidating the signature - that's a key property of any usable digital signature algorithm. Instead, you have to keep the payload the same, but modify the format of the signature itself. – Nate Eldredge Jun 13 '17 at 20:15
  • Adding a dummy opcode would actually be valid, however the result would be non-standard at least in Bitcoin Core. – Pieter Wuille Aug 12 '17 at 21:08
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The easiest way to malleate a transaction is to just change the signature from Low-S to High-S and vice versa.

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