Understanding Bitcoin Script: The Foundation of Transaction Locking and Unlocking

Bitcoin's scripting language enables basic program execution on the blockchain, forming the backbone of transaction security. This guide explores Bitcoin Script's core principles, focusing on its stack-based architecture and the interplay between locking (scriptPubKey) and unlocking (scriptSig) scripts.

Key Characteristics of Bitcoin Script

Turing-Incomplete Design
- Lacks complex functionalities like loops to minimize attack vectors
- Prevents resource-exhaustion risks (e.g., denial-of-service via infinite loops)
- Prioritizes security and deterministic execution
Stack-Based Execution Model
- Operations process data in Last-In-First-Out (LIFO) order
- Uses opcodes (operation codes) like OP_ADD and OP_EQUAL
  Example Execution Flow:
```
PUSH 2 → PUSH 3 → OP_ADD → PUSH 5 → OP_EQUAL
Stack evolution: [2] → [2,3] → [5] → [5,5] → [true]
```

Transaction Anatomy: Locking and Unlocking Mechanisms

Locking Script (`scriptPubKey`)

Appears in transaction outputs (vout) to specify spending conditions. Common P2PKH (Pay-to-Public-Key-Hash) structure:

OP_DUP OP_HASH160 <PublicKeyHash> OP_EQUALVERIFY OP_CHECKSIG

Unlocking Script (`scriptSig`)

Provides credentials to satisfy scriptPubKey conditions. Contains:

Digital signature (proves ownership)
Public key (validates signature)

Step-by-Step Validation Process

Public Key Verification
OP_DUP duplicates the public key → OP_HASH160 computes:
RIPEMD160(SHA256(public_key))
Matches against the scriptPubKey hash
Signature Verification
OP_CHECKSIG confirms the digital signature corresponds to the provided public key

Critical Note: The entire process must complete successfully for funds to be spendable.

Operational Codes (Opcodes) Breakdown

Opcode	Function	Stack Effect
`OP_DUP`	Duplicates top stack item	[a] → [a,a]
`OP_HASH160`	Double-hashes (SHA-256 + RIPEMD160)	[x] → [hash160(x)]
`OP_EQUALVERIFY`	Compares two items; aborts if unequal	[a,b] → [] (continues if a==b)
`OP_CHECKSIG`	Validates ECDSA signature	[sig, pubkey] → [true/false]

Security Implications

No Third-Party Trust Required: Validation occurs via cryptographic proof
Deterministic Outcomes: Same inputs always produce identical results
Anti-Fraud Design: Invalid signatures automatically fail verification

Frequently Asked Questions

Why is Bitcoin Script intentionally limited?

The constraints prevent:

Undecidable program states
Resource exhaustion attacks
Non-deterministic outcomes that could split the blockchain

Can Bitcoin Script create smart contracts?

Yes, but with fundamental limitations:

Supports basic conditions (multisig, timelocks)
Lacks state persistence between transactions
No native looping/recursion support

How does Script differ from Ethereum's EVM?

Bitcoin: Focused solely on transaction validation
Ethereum: Turing-complete environment for general computation

What happens if a Script execution fails?

The entire transaction becomes invalid, preventing any fund movement.

Real-World Implementation Example

👉 See Bitcoin Script in action on live transactions

"scriptPubKey": "OP_DUP OP_HASH160 ab68025513c3dbd2f7b92a94e0581f5d50f654e7 OP_EQUALVERIFY OP_CHECKSIG"

This locks funds to the specified public key hash until proper signature provided.

👉 Explore advanced scripting techniques

Conclusion

Bitcoin Script's elegant simplicity enables secure value transfer without centralized intermediaries. By mastering its stack-based execution model and cryptographic verification processes, users gain deeper insight into Bitcoin's trustless transaction system.

Remember: Every Bitcoin transaction ultimately depends on these script validation rules to ensure only authorized spending occurs.