OP CODESEPARATOR

OP_CODESEPARATOR was originally designed to be inserted between the ScriptPubKey and ScriptSig by the script evaluation engine. When OP_CHECKSIG is called in the ScriptPubKey, it will remove everything before OP_CODESEPARATOR. In this case, the entire ScriptSig will be removed. This is because ScriptSig which contains the signature is not signed by the signature.

There is no rule to prevent users from inserting OP_CODESEPARATOR into the ScriptPubKey. However, OP_CODESEPARATOR will only be effective if it is read by OP_CHECKSIG in the same ScriptPubKey. OP_CHECKSIG will only read the closest OP_CODESEPARATOR just before it, and remove all contents before that.

Based on the understanding above, we can construct the following ScirptPubKey:

OP_CODESEPARATOR OP_CHECKSIGVERIFY OP_CODESEPARATOR OP_CHECKSIGVERIFY OP_CODESEPARATOR OP_CHECKSIG

To unlock this ScriptPubKey, we can use the following ScriptSig:

<Sig A> <PK A> <Sig B> <PK B> <Sig C> <PK C>

Note that the first OP_CHECKSIG will verify <Sig C> <PK C> on a message that including OP_CHECKSIGVERIFY OP_CODESEPARATOR OP_CHECKSIGVERIFY OP_CODESEPARATOR OP_CHECKSIG.

The second OP_CHECKSIG will verify <Sig B> <PK B> on a message that including OP_CHECKSIGVERIFY OP_CODESEPARATOR OP_CHECKSIG" and excluding the first OP_CHECKSIGVERIFY.

The last OP_CHECKSIG will verify <Sig A> <PK A> on a message that including OP_CHECKSIG and excluding the first two OP_CHECKSIGVERIFY.

If someone changes the order of the signature in the ScritSig, the verification of the signature will fail.