Conditional Flows

Conditional operators allows us to write scripts that have two ways of being unlocked, depending on whether the condition evluates to TRUE or FALSE.

Conditionals can be nested indefinitely, only limited by the maximum size of a script.

Operators used to write conditionals are: IF, ELSE, ENDIF, NOTIF

Note: In a stack based language like Bitcoin Script, the condition comes before the IF. e.g.

condition
IF
  code to run when condition is true
ELSE
  code to run when condition is false
ENDIF
code to run in either case

VERIFY opcodes

Verify means that if a condition is not TRUE, then the evaluation is stopped immediately.

Thus VERIFY suffixes act like guard clauses, continuing only if the pre-condition is met.

e.g. redeem script with an EQUALVERIFY guard clause.

HASH160 <expected hash> EQUALVERIFY <Bob's Pubkey> CHECKSIG

This can also be written using an IF instead:

HASH160 <expected hash> EQUAL
IF
   <Bob's Pubkey> CHECKSIG
ENDIF

Note

  • the VERIFY construction is more efficient, it uses two fewer opcodes.
  • VERIFY opcodes do not leave anything on the stack, unlike say an EQUAL opcode.

Example script using conditionals

Following is an example of a script with multiple outcomes:

01  IF
02    IF
03      2
04    ELSE
05      <30 days> CHECKSEQUENCEVERIFY DROP
06      <A's Pubkey> CHECKSIGVERIFY
07      1
08    ENDIF
09    <M's Pubkey> <S's Pubkey> <Z's Pubkey> 3 CHECKMULTISIG
10  ELSE
11    <90 days> CHECKSEQUENCEVERIFY DROP
12    <A's Pubkey> CHECKSIG
13  ENDIF

We can evaluate the script in 3 ways:

1. two of three multisig

We can unlock for 2 of 3 multisig using:

0 <M's Sig> <Z's Sig> TRUE TRUE

Here TRUE TRUE force evaluation of the nested if’s.

2. one of three multisig + A’s Pubkey

We can use 1 of 3, along with A’s Pubkey using:

0 <A's Sig> <S/M/Z's Sig> FALSE TRUE

3. For A’s SIG only path

<A's Sig> FALSE