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10.3: Running a Bitcoin Script with P2SH

Now that you know the theory and practice behind P2SH addresses, you're ready to turn a non-standard Bitcoin Script into an actual transaction. We'll be reusing the simple locking script from §9.2: Running a Bitcoin Script, OP_ADD 99 OP_EQUAL.

Create a P2SH Transaction

To lock a transaction with this Script, do the following:

  1. Serialize OP_ADD 99 OP_EQUAL:
    1. OP_ADD = 0x93 — a simple opcode translation
    2. 99 = 0x01, 0x63 — this opcode pushes one byte onto the stack, 99 (hex: 0x63)
      • No worries about endian conversion because it's only one byte
    3. OP_EQUAL = 0x87 — a simple opcode translation
    4. <serialized99Equal> = "93016387"
$ btcc OP_ADD 99 OP_EQUAL
93016387
  1. Save <serialized99Equal> for future reference as the redeemScript.
    1. <redeemScript> = "93016387"
  2. SHA-256 and RIPEMD-160 hash the serialized script.
    1. <hashed99Equal> = "3f58b4f7b14847a9083694b9b3b52a4cea2569ed"
  3. Produce a P2SH locking script that includes the <hashed99Equal>.
    1. scriptPubKey = "a9143f58b4f7b14847a9083694b9b3b52a4cea2569ed87"

You can then create a transaction using this scriptPubKey, probably via an API.

Unlock the P2SH Transaction

To unlock this transaction requires that the recipient produce a scriptSig that prepends two constants totalling ninety-nine to the serialized script: 1 98 <serialized99Equal>.

Run the First Round of Validation

The process of unlocking the P2SH transaction then begins with a first round of validation, which checks that the redeem script matches the hashed value in the locking script.

Concatenate scriptSig and scriptPubKey and execute them, as normal:

Script: 1 98 <serialized99Equal> OP_HASH160 <hashed99Equal> OP_EQUAL
Stack: []

Script: 98 <serialized99Equal> OP_HASH160 <hashed99Equal> OP_EQUAL
Stack: [ 1 ]

Script: <serialized99Equal> OP_HASH160 <hashed99Equal> OP_EQUAL
Stack: [ 1 98 ]

Script: OP_HASH160 <hashed99Equal> OP_EQUAL
Stack: [ 1 98 <serialized99Equal> ]

Script: <hashed99Equal> OP_EQUAL
Running: <serialized99Equal> OP_HASH160
Stack: [ 1 98 <hashed99Equal> ]

Script: OP_EQUAL
Stack: [ 1 98 <hashed99Equal> <hashed99Equal> ]

Script: 
Running: <hashed99Equal> <hashed99Equal> OP_EQUAL
Stack: [ 1 98 True ]

The Script ends with a True on top of the stack, and so it succeeds ... even though there's other cruft below it.

However, because this was a P2SH script, the execution isn't done.

Run the Second Round of Validation

For the second round of validation, verify that the values in the unlocking script satisfy the redeemScript: deserialize the redeemScript ("93016387" = "OP_ADD 99 OP_EQUAL"), then execute it using the items in the scriptSig prior to the serialized script:

Script: 1 98 OP_ADD 99 OP_EQUAL
Stack: [ ]

Script: 98 OP_ADD 99 OP_EQUAL
Stack: [ 1 ]

Script: OP_ADD 99 OP_EQUAL
Stack: [ 1 98 ]

Script: 99 OP_EQUAL
Running: 1 98 OP_ADD
Stack: [ 99 ]

Script: OP_EQUAL
Stack: [ 99 99 ]

Script: 
Running: 99 99 OP_EQUAL
Stack: [ True ]

With that second validation also true, the UTXO can now be spent!

Summary: Building a Bitcoin Script with P2SH

Once you know the technique of building P2SHes, any Script can be embedded in a Bitcoin transaction; and once you understand the technique of validating P2SHes, it's easy to run the scripts in two rounds.

What's Next?

Continue "Embedding Bitcoin Scripts" with §10.4: Scripting a Multisig.