Script Validation (P2PKH-lite)

Why this module exists

Bitcoin uses a stack-based scripting language to express spending rules. Even a minimal subset reveals the structure of real validation.

1) P2PKH flow

Pay-to-PubKey-Hash uses two scripts:

• scriptSig: pushes signature and pubkey
• scriptPubKey: verifies hash + signature

Execution steps:

1. Push sig + pubkey
2. Duplicate pubkey
3. Hash pubkey and compare to expected
4. Verify signature

2) Hashing and signatures

• We use HASH256 (double SHA-256) instead of HASH160
• ECDSA over P-256 (toy stand-in for secp256k1)
• Signature hash is simplified but deterministic

3) Failure modes

• Stack underflow or invalid opcodes
• Incorrect pubkey hash
• Signature mismatch

Any failure must make the script invalid.

What you will build

1. Script parsing (pushdata + opcodes)
2. Stack execution for P2PKH
3. ECDSA signature verification

Key takeaways

• Script execution is deterministic and strict.
• Small parsing errors lead to consensus splits.
• Validation is the core of trustlessness.