Does Bitcoin script support nested IFs (OP_IF or OP_NOTIF inside one another)?

1

For example can we convert the following code to a script? 

if(bool_1)
{
    FOO1();
    if(!bool_2)
    {
        FOO2();
    }
    else
    {
        BAR1();
        if(bool_3)
        {
            BAR2();
        }
        BAR3();
    }
}
else
{
    FOOBAR();
}

<push> <push> <push> OP_IF OP_FOO1 OP_NOTIF OP_FOO2 OP_ELSE OP_BAR1 OP_IF OP_BAR2 OP_ENDIF OP_BAR3 OP_ENDIF OP_FOOBAR OP_ENDIF(correct?)

If yes then:
1. Is there any limit on how deep a nested IFs can go?
2. Is there any examples (raw bytes/hex) I could look at?

Coding Enthusiast

Posted 2019-10-08T05:20:49.710

Reputation: 488

Answers

1

Do bitcoin scripts support nested ifs (OP_IF OP_NOTIF inside one another)?

Yes, Bitcoin support nested IF/ELSE. In fact we use the nested IFs in Lightning Network all the time when adding HTLCs to forward a payment.

Is there any limit on how deep a nested if can go?

There is no explicit rules dictating how far deep the nested IFs can go but there are some constrains on the script itself:

  • Maximum number of bytes pushable to the stack should be less than 520 bytes
  • Maximum operations per script is limited to 201
  • Maximum size of the script is 10,000 bytes
  • Maximum number of values on script interpreter stack is 1,000 bytes

Is there any examples (raw bytes/hex) I could look at?

As I mentioned earlier, we use nested IFs all the time while adding HTLCs in the Lightning Network implementation. Below is an output script of offered HTLC outputs for a peer.

# To remote node with revocation key
OP_DUP OP_HASH160 <RIPEMD160(SHA256(revocationpubkey))> OP_EQUAL
OP_IF
    OP_CHECKSIG
OP_ELSE
    <remote_htlcpubkey> OP_SWAP OP_SIZE 32 OP_EQUAL
    OP_NOTIF
        # To local node via HTLC-timeout transaction (timelocked).
        OP_DROP 2 OP_SWAP <local_htlcpubkey> 2 OP_CHECKMULTISIG
    OP_ELSE
        # To remote node with preimage.
        OP_HASH160 <RIPEMD160(payment_hash)> OP_EQUALVERIFY
        OP_CHECKSIG
    OP_ENDIF
OP_ENDIF

Ugam Kamat

Posted 2019-10-08T05:20:49.710

Reputation: 5 180

IF/ELSE loops Is "loop" the correct term to use? – Coding Enthusiast – 2019-10-08T06:22:50.860

@CodingEnthusiast thanks for correcting. I wrote the answer from my phone. – Ugam Kamat – 2019-10-08T07:45:30.757

Asked: 2019-10-08T05:20:49.710

Viewed: 40 times

Active: 2019-10-08T07:46:54.523