Help with understanding Bitcoin script.

[evanlinjin]

I'm going through the book Mastering Bitcoin, and I have a query regarding the section on "Using Flow Control in Scripts".

So, there is an example of a redeem script as follows:

Code:

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

I'm wondering why it is structured in this manner, instead of the following:

Code:

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

Would the latter work? And if so, would it be the better version as it saves space by one opcode?

Thank you all in advance.

[pooya87]

yes the second one works and essentially does the same thing as the first one but in a more efficient way. this example is probably like this to show how OP_IFs work otherwise there is a better way of defining this particular script which is using multi-sig.

Code:

1 <Alice's PK> <Bob's PK> 2 CHECMULTIKSIG

usually the conditional scripts like this are more complex and each expression could contain multiple pubkeys with a multi-sig OP or sometimes locktimes or other complications which has to be defined like the first script.
examples:

Code:

IF
  <Alice's PK> <Joe's PK> CHECMULTIKSIG
ELSE
  <Bob's PK> CHECKSIG
ENDIF

Code:

IF
  <somedatetimevalue> CheckLocktimeVerify DROP <Alice's PK> CHECKSIG
ELSE
  2 <Alice's PK> <Bob's PK> 2 CHECKMULTISIG
ENDIF

[evanlinjin]

Awesome! Thank you for your help with clarifying