Differences from Bitcoin Layer 1
This document outlines the key differences between OP_CAT and Bitcoin's layer 1 implementation.
Overview
OP_CAT introduces several modifications to the standard Bitcoin protocol to enhance functionality and support advanced smart contract capabilities.
Key Differences
| Component | OP_CAT | Bitcoin |
|---|---|---|
| Transaction Structure | Extended with data field for each transaction output and witness is removed | Legacy/witness transaction format |
| Transaction Hash | Hash of TxHashPreimage | Hash of raw transaction |
| SigHashPreimage | Enhanced sighash preimage for smart contract support | Traditional sighash format |
| BVM (Bitcoin Virtual Machine) | Restore all opcodes and support bigint | Disable lots of opcodes |
Detailed Explanations
Transaction Structure Modifications
OP_CAT implements significant changes to the transaction structure:
Witness Removal: The witness structure is completely removed, simplifying the transaction format and reducing complexity compared to Bitcoin's witness-based approach.
Output Extension: Each transaction output is extended with a
datafield, enabling:- Additional data storage capabilities
- Enhanced smart contract functionality
- More flexible transaction output handling
Transaction Structure Field Comparison
The following table shows which fields are present in OP_CAT vs Bitcoin transaction structures:
| Field | OP_CAT | Bitcoin | Description |
|---|---|---|---|
| Version | ✓ | ✓ | Transaction version number |
| SegWit Marker | ✗ | ✓ | Segregated Witness marker |
| SegWit Flag | ✗ | ✓ | Segregated Witness flag |
| Input Count | ✓ | ✓ | Number of transaction inputs |
| Input[0].prevout | ✓ | ✓ | Previous output reference |
| Input[0].scriptSig | ✓ | ✓ | Input locking script (script signature) |
| Input[1]... | ✓ | ✓ | Additional inputs (if any) |
| Output Count | ✓ | ✓ | Number of transaction outputs |
| Output[0].amount | ✓ | ✓ | Output value in satoshis |
| Output[0].scriptPubKey | ✓ | ✓ | Output script (script public key) |
| Output[0].data | ✓ | ✗ | Additional data field (OP_CAT only) |
| Output[1]... | ✓ | ✓ | Additional outputs (if any) |
| Witness | ✗ | ✓ | Witness data for SegWit |
| LockTime | ✓ | ✓ | Transaction lock time |
Legend: ✓ = Present, ✗ = Not present
These modifications streamline transaction processing while providing enhanced functionality for smart contract operations.
Transaction Hashing
- OP_CAT: Uses TxHashPreimage for transaction hashing, which provides more flexibility in hash computation and enables advanced smart contract features
- Bitcoin: Uses the raw transaction data directly for hash calculation, following the traditional Bitcoin protocol
TxHashPreimage Structure
The TxHashPreimage contains the following fields for transaction hashing. All hash values are computed using SHA256:
| Field | Description |
|---|---|
| version | Transaction version number |
| input count | Number of transaction inputs |
| input[0].prevout | Previous output reference (hash + index) |
| input[0].unlockScriptHash | SHA256 hash of the unlocking script |
| input[0].sequence | Input sequence number |
| input[1]... | Additional inputs (if any) |
| output count | Number of transaction outputs |
| output[0].satoshis | Output value in satoshis |
| output[0].scriptHash | SHA256 hash of the output script |
| output[0].dataHash | SHA256 hash of the output data field |
| output[1]... | Additional outputs (if any) |
| locktime | Transaction lock time |
SigHashPreimage Enhancement
OP_CAT implements an improved sighash preimage structure specifically designed to support smart contract functionality. This enhancement provides better control over signature verification and enables more sophisticated contract interactions.
SigHashPreimage Structure (when sighash type is SIGHASH_ALL)
| Field | Description |
|---|---|
| nVersion | Transaction version |
| hashPrevouts | Double SHA256 of the serialization of all input outpoints |
| spentScriptHash | SHA256 of scriptPubKey of the previous output spent by this input |
| spentDataHash | SHA256 of the data field of the previous output spent by this input |
| value | Value of the previous output spent by this input |
| nSequence | Sequence number of this input |
| hashSpentAmounts | Double SHA256 of the serialization of all spent output amounts |
| hashSpentScriptHashes | Double SHA256 of all spent outputs' script hashes |
| hashSpentDataHashes | Double SHA256 of all spent outputs' data hashes |
| hashSequences | Double SHA256 of the serialization of all input sequences |
| hashOutputs | Double SHA256 of the serialization of all outputs |
| inputIndex | Index of the current input being signed |
| nLocktime | Transaction lock time |
| sighashType | Signature hash type |
Bitcoin Virtual Machine (BVM) Enhancements
OP_CAT significantly enhances the Bitcoin Virtual Machine by restoring all opcodes that were previously disabled in Bitcoin:
- OP_CAT: Restores all opcodes to enable full smart contract functionality
- Bitcoin: Disables many opcodes for security and stability reasons
This restoration enables:
- Advanced smart contract development
- Complex cryptographic operations
- Enhanced data manipulation capabilities
- More flexible scripting possibilities
Technical Implications
These architectural differences provide OP_CAT with several advantages:
- Simplified Transaction Structure: The removal of witness data reduces transaction complexity and processing overhead
- Enhanced Flexibility: TxHashPreimage allows for more sophisticated hash computation methods
- Extended Functionality: The additional
datafield enables advanced smart contract features and data storage - Improved Smart Contract Support: Enhanced sighash preimage provides better contract interaction capabilities
- Reduced Transaction Size: Elimination of SegWit components results in smaller transaction sizes
- Optimized Hashing: TxHashPreimage structure enables more efficient transaction hash computation using SHA256
- Consistent Hash Algorithm: All hash computations use SHA256, ensuring consistency and security
- Advanced Signature Verification: Enhanced SigHashPreimage structure provides more sophisticated signature verification capabilities
- Full Opcode Support: BVM restoration enables complete smart contract functionality
Compatibility Considerations
While OP_CAT maintains compatibility with Bitcoin's core principles, these modifications may affect:
- Transaction validation processes
- Block structure and size calculations
- Smart contract deployment and execution
- Signature verification mechanisms
- Legacy transaction format compatibility
- SegWit transaction handling
- Hash computation methods and validation
- Opcode availability and script execution