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Arkade Script

Arkade Script consists of a set of Tapscript opcodes that extend Bitcoin Script’s functionality to enable powerful smart contracts. These opcodes are designed to work within the VTXO (Virtual Transaction Output) paradigm.

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The VTXO Paradigm

The VTXO paradigm provides two execution paths for every contract:

1. Cooperative Path: Requires the Arkade server’s co-signature for immediate execution with instant finality
2. Unilateral Path: A timelock-protected fallback path that can be executed on-chain without the server

This dual-path approach ensures that contracts can execute efficiently off-chain with server cooperation, while maintaining the security guarantee that users can always execute on-chain after a timelock if the server becomes unavailable.

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Tapscript Opcodes

Arkade Script leverages a set of new Tapscript opcodes that extend Bitcoin Script’s functionality. These opcodes are implemented using the OP_SUCCESS mechanism introduced in Taproot, which allows for cleaner opcode additions than the previous OP_NOP approach.

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Resource Limits in Taproot

Taproot already increases many resource limitations compared to previous segwit versions:

• No script size limit: The 10,000 byte maximum no longer applies (bounded only by block weight)
• No non-push opcode limit: The 201 non-push opcode limit no longer applies
• Per-script sigops budget: Instead of a block-wide limit, each script has a budget of 50 + witness size in bytes
• Stack limits: The 1,000 element stack+altstack limit remains, now also applying to the initial stack
• Stack element size: The 520 byte maximum per stack element remains during execution

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Streaming Hash Opcodes

These opcodes overcome the 520-byte limitation for hashing operations while maintaining security against resource exhaustion:

• OP_SHA256INITIALIZE (OP_SUCCESS196): Pops a bytestring and pushes a SHA256 context initialized with that data
• OP_SHA256UPDATE (OP_SUCCESS197): Pops a bytestring and a SHA256 context, then pushes an updated context
• OP_SHA256FINALIZE (OP_SUCCESS198): Pops a bytestring and a SHA256 context, then pushes the final hash value

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Transaction Introspection Opcodes

These opcodes provide efficient access to transaction data, enabling covenants and complex state transitions:

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Input Introspection

• OP_INSPECTINPUTOUTPOINT (OP_SUCCESS199): Examines the outpoint of an input
• OP_INSPECTINPUTASSET (OP_SUCCESS200): Examines the asset of an input
• OP_INSPECTINPUTVALUE (OP_SUCCESS201): Examines the value of an input
• OP_INSPECTINPUTSCRIPTPUBKEY (OP_SUCCESS202): Examines the scriptPubKey of an input
• OP_INSPECTINPUTSEQUENCE (OP_SUCCESS203): Examines the sequence number of an input
• OP_INSPECTINPUTISSUANCE (OP_SUCCESS204): Examines the issuance information of an input
• OP_PUSHCURRENTINPUTINDEX (OP_SUCCESS205): Pushes the current input index

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Output Introspection

• OP_INSPECTOUTPUTASSET (OP_SUCCESS206): Examines the asset of an output
• OP_INSPECTOUTPUTVALUE (OP_SUCCESS207): Examines the value of an output
• OP_INSPECTOUTPUTNONCE (OP_SUCCESS208): Examines the nonce of an output
• OP_INSPECTOUTPUTSCRIPTPUBKEY (OP_SUCCESS209): Examines the scriptPubKey of an output

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Transaction Introspection

• OP_INSPECTVERSION (OP_SUCCESS210): Examines the version of the transaction
• OP_INSPECTLOCKTIME (OP_SUCCESS211): Examines the locktime of the transaction
• OP_INSPECTNUMINPUTS (OP_SUCCESS212): Examines the number of inputs in the transaction
• OP_INSPECTNUMOUTPUTS (OP_SUCCESS213): Examines the number of outputs in the transaction
• OP_TXWEIGHT (OP_SUCCESS214): Examines the weight of the transaction

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64-bit Arithmetic Opcodes

These opcodes provide signed 64-bit arithmetic operations with overflow detection, overcoming the limitations of the 4-byte CScriptNum:

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Basic Operations

• OP_ADD64 (OP_SUCCESS215): 64-bit addition with overflow detection
• OP_SUB64 (OP_SUCCESS216): 64-bit subtraction with overflow detection
• OP_MUL64 (OP_SUCCESS217): 64-bit multiplication with overflow detection
• OP_DIV64 (OP_SUCCESS218): 64-bit division with overflow detection
• OP_NEG64 (OP_SUCCESS219): 64-bit negation with overflow detection

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Comparison Operations

• OP_LESSTHAN64 (OP_SUCCESS220): 64-bit less than comparison
• OP_LESSTHANOREQUAL64 (OP_SUCCESS221): 64-bit less than or equal comparison
• OP_GREATERTHAN64 (OP_SUCCESS222): 64-bit greater than comparison
• OP_GREATERTHANOREQUAL64 (OP_SUCCESS223): 64-bit greater than or equal comparison

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Conversion Opcodes

These opcodes facilitate conversion between different numeric formats:

• OP_SCRIPTNUMTOLE64 (OP_SUCCESS224): Converts a CScriptNum to an 8-byte little-endian value
• OP_LE64TOSCRIPTNUM (OP_SUCCESS225): Converts an 8-byte little-endian value to a CScriptNum
• OP_LE32TOLE64 (OP_SUCCESS226): Converts a 4-byte unsigned little-endian value to an 8-byte signed little-endian value

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Crypto Opcodes

These opcodes enable more complex cryptographic operations:

• OP_ECMULSCALARVERIFY (OP_SUCCESS227): Verifies scalar multiplication of EC points (Q = k*P)
• OP_TWEAKVERIFY (OP_SUCCESS228): Verifies Taproot key tweaking (Q = P + k*G)

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Enhanced Existing Opcodes

• OP_CHECKSIGFROMSTACK and OP_CHECKSIGFROMSTACKVERIFY: Enhanced to follow BIP340 semantics for Schnorr signatures

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Usage Tips

• Use OP_PUSHCURRENTINPUTINDEX with the input introspection opcodes to examine the current input
• The input nonce field cannot be introspected as it’s not consistently stored in the UTXO database
• Use conversion opcodes to convert between CScriptNum, 4-byte LE, and 8-byte LE formats for arithmetic operations