Scripting

Design Goals

1. Built for Bitcoin (inspired by Forth)

2. Simple, compact

3. Support for cryptography

4. Stack-based

5. Limits on time/memory

6. No looping

Script Executable Example

<sig> <pubKey> OP_DUP OP_HASH160 <pubKeyHash?> OP_EQUALVERIFY OP_CHECKSIG

OP_DUP : Take values on the top of the stack, pop it off, write two copies back to the stack.

HASH160 : Take the top value and convert into a hash

OP_EQUALVERIFY : Verifying the top values of the stack are equal. If error, exec will stop.

OP_CHECKSIG : Checks the signature from both the ends.

Script Instructions

1. 256 opcodes total [1 byte each] (15 disabled, 75 reserved)

   1. Arithmetic
   2. if/then
   3. Logic/Data Handling
   4. Crypto

      1. Hashes

      2. Signature Verification

      3. Multi-Signature Verification

   OP_CHECKMULTISIG lets you check multiple signatures at once - Has built in support for joint signatures - Specify n public keys and parameter t - Verification requires t signatures

   Sender’s Script (Has to send the complete hash)

   OP_HASH160
   <hash of redemption script>
   OP_EQUAL

   Receiver’s Script (Has to verify the whole hash sender sent)

   <signature>
   <<pubkey> OP_CHECKSIG>

   Applications of Scripting

   1. Escrow Transactions : Involving 3rd parties in the transaction pipe with MULTISIG

   2. Green Addresses : Involving a bank in the transaction. (Not used much in practice)

   • Efficient Micro-Payments : All of the transactions could be double spend.