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I'm trying to create a custom script with if/else and a multisig operation in one branch. Currently, my script looks thus:

OP_IF
    <pub1>
    OP_CHECKSIG
OP_ELSE
    OP_2
    <pub2>
    <pub3>
    <pub4>
    OP_3
    OP_CHECKMULTISIG
OP_ENDIF

I have calculated a testnet address (2NF3yXy2MambEnJuY7eStLNcX92WSxytLvC) from this example script and sent testcoin there. Then, I tried spending the money with the following sig script:

OP_0 <sig2> <sig3> <redeemScript>

However, when trying to broadcast the transaction, I get an error:

PUSH TRANSACTION ERROR: 16: MANDATORY-SCRIPT-VERIFY-FLAG-FAILED (SIGNATURE MUST BE ZERO FOR FAILED CHECK(MULTI)SIG OPERATION)

For reference, here's my raw transaction:

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

Its sig script is this:

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

Which decodes to

0 3045022100f502ecfd2b7bbb3e7092b320c568abe0d68dd7e613ab3efcb9e479b2641a73a502202b416bb86bce3d242084f8e24cbed76e5a2c5a0795a5bbd7f3366bf6d086ae4f[ALL] 304402201022c7548b55f3fbe3ddf427ad2ae5c6a776e2a63ee25376dca123d53d0f940f022041aa7967208ee8aee1811f02d6c7db0ad2872304133d46dbdd117e9bc09d8c97[ALL] 632102bc050e51811c2c454bf3487129b19d60b87f7a51fb434e6073def19dc55cf4b3ac67522102a906b96ea2df77aff1f84cc9949550f8d6b8f3db5acd625d12d70b6da4b94b1e2102046e854e69a49dd210dbf554f1e420893935b47123667a1834def9fe5d44358a210325db22e2aef6f143026500522431d4f348f911562316350852e6382da6a048a753ae68

The last line is clearly visible as the redeem script, which in turn decodes to this:

OP_IF 02bc050e51811c2c454bf3487129b19d60b87f7a51fb434e6073def19dc55cf4b3 OP_CHECKSIG OP_ELSE 2 02a906b96ea2df77aff1f84cc9949550f8d6b8f3db5acd625d12d70b6da4b94b1e 02046e854e69a49dd210dbf554f1e420893935b47123667a1834def9fe5d44358a 0325db22e2aef6f143026500522431d4f348f911562316350852e6382da6a048a7 3 OP_CHECKMULTISIG OP_ENDIF

Is my sig script incorrect? Is the output script invalid? How can I fix this and make it work?

Thank you very much in advance!

EDIT: After taking into consideration Pieter's comment but placing the extra OP_0 right after the two signatures in the sig script, it worked. Here's the new decoded sig script:

0 3045022100f502ecfd2b7bbb3e7092b320c568abe0d68dd7e613ab3efcb9e479b2641a73a502202b416bb86bce3d242084f8e24cbed76e5a2c5a0795a5bbd7f3366bf6d086ae4f[ALL] 304402201022c7548b55f3fbe3ddf427ad2ae5c6a776e2a63ee25376dca123d53d0f940f022041aa7967208ee8aee1811f02d6c7db0ad2872304133d46dbdd117e9bc09d8c97[ALL] 0 632102bc050e51811c2c454bf3487129b19d60b87f7a51fb434e6073def19dc55cf4b3ac67522102a906b96ea2df77aff1f84cc9949550f8d6b8f3db5acd625d12d70b6da4b94b1e2102046e854e69a49dd210dbf554f1e420893935b47123667a1834def9fe5d44358a210325db22e2aef6f143026500522431d4f348f911562316350852e6382da6a048a753ae68

And here's the link to the transaction: https://testnet.smartbit.com.au/tx/1894cb91fde60432e59d60f30de503e8b6fbe5d1756bbf0ef7302b57bf31d195

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    I believe you may need two OP_0's at the start: one to be consumed by OP_IF, and one which is popped off by OP_CHECKMULTISIG. I'll turn this into a full answer once I've identified the issue. Mar 25, 2018 at 3:17
  • Thanks for your response! I just tried it, and I'm still getting the same error: PUSH TRANSACTION ERROR: 16: MANDATORY-SCRIPT-VERIFY-FLAG-FAILED (SIGNATURE MUST BE ZERO FOR FAILED CHECK(MULTI)SIG OPERATION)
    – arik
    Mar 25, 2018 at 3:33
  • ~30min in the Video Andreas explains the „IF“ handling of script. There must be a value before the IF statement. Either 1 for the If-branch or a 0 for the else branch. m.youtube.com/watch?v=yU3Sr07Qnxg Mar 25, 2018 at 8:33
  • @pebwindkraft: that should be an answer. ;)
    – Murch
    Mar 25, 2018 at 18:45
  • @murch: not chasing for credits anymore :-) Pieter and you have provided the answers, and this is just "a little bit" additional explanation, which might help the users to better understand the previous comments. Mar 26, 2018 at 8:19

1 Answer 1

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OP_IF consumes a number from the top of the stack.
The OP_IF will evaluate to true if the number is not 0, and to false if the number is 0.

H/T for this answer by David Schwartz, and to Pieter Wuille for getting me on the right track.

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