CS120: Bitcoin for Developers I

A very common use for flow control in Bitcoin Script is to construct a redeem script that offers multiple execution paths, each a different way of redeeming the UTXO.

Let's look at a simple example, where we have two signers, Alice and Bob, and either one is able to redeem. With multisig, this would be expressed as a 1-of-2 multisig script. For the sake of demonstration, we will do the same thing with an IF clause:

IF
 <Alice's Pubkey> CHECKSIG
ELSE
 <Bob's Pubkey> CHECKSIG
ENDIF

Looking at this redeem script, you may be wondering: "Where is the condition? There is nothing preceding the IF clause!"

The condition is not part of the redeem script. Instead, the condition will be offered in the unlocking script, allowing Alice and Bob to "choose" which execution path they want.

Alice redeems this with the unlocking script:

<Alice's Sig> 1

The 1 at the end serves as the condition (TRUE) that will make the IF clause execute the first redemption path for which Alice has a signature.

For Bob to redeem this, he would have to choose the second execution path by giving a FALSE value to the IF clause:

<Bob's Sig> 0

Bob's unlocking script puts a 0 on the stack, causing the IF clause to execute the second (ELSE) script, which requires Bob's signature.

Since IF clauses can be nested, we can create a "maze" of execution paths. The unlocking script can provide a "map" selecting which execution path is actually executed:

IF
  script A
ELSE
  IF
    script B
  ELSE
    script C
  ENDIF
ENDIF

In this scenario, there are three execution paths (script A, script B, and script C). The unlocking script provides a path in the form of a sequence of TRUE or FALSE values. To select path script B, for example, the unlocking script must end in 1 0 (TRUE, FALSE). These values will be pushed onto the stack, so that the second value (FALSE) ends up at the top of the stack. The outer IF clause pops the FALSE value and executes the first ELSE clause. Then the TRUE value moves to the top of the stack and is evaluated by the inner (nested) IF, selecting the B execution path.

Using this construct, we can build redeem scripts with tens or hundreds of execution paths, each offering a different way to redeem the UTXO. To spend, we construct an unlocking script that navigates the execution path by putting the appropriate TRUE and FALSE values on the stack at each flow control point.