Tapscript & BIP342 Updates

While BIP341 defines the structure of the address, BIP342 (Tapscript) defines the rules for the scripts hidden inside the Script Path. It is a cleaned-up, modernized version of the original Bitcoin Script.

1. Schnorr Support

The most obvious change is that all signature-checking opcodes (like OP_CHECKSIG) now expect a Schnorr Signature and a 32-byte public key, rather than ECDSA.

2. Removing Old Limits

Original Bitcoin Script had several "Legacy Limits" that made complex contracts difficult:

• Signature Limit: A transaction could only have a maximum of 201 signatures.

• Script Size: Individual script elements were limited to 520 bytes. Tapscript removes or significantly increases these limits, allowing for much more powerful smart contracts (like those used in BitVM).

3. OP_CHECKSIGADD (New Opcodes)

Tapscript replaced the old, inefficient OP_CHECKMULTISIG with a new opcode called OP_CHECKSIGADD.

• Old Way: You had to provide a list of all keys and a number of required signatures.

• New Way: You go through the keys one by one and "add" to a counter if the signature is valid.

• Benefit: This is much more flexible and compatible with Batch Verification.

4. Resource Accounting (SigOps)

Tapscript introduces a better way to account for "expensive" operations (SigOps).

• Instead of having a flat limit per block, SigOps are now counted based on the Weight of the transaction. This ensures that miners are fairly compensated for the CPU time they spend verifying your script.

5. Future-Proofing (OP_SUCCESS)

Satoshi originally disabled many opcodes (the "Disabled Opcodes") to prevent security vulnerabilities. Tapscript replaces these with OP_SUCCESS opcodes.

• The Rule: Any script containing an OP_SUCCESS opcode is automatically valid.

• The Reason: This allows Bitcoin to add new features in the future (via another soft-fork) by simply redefining what an OP_SUCCESS opcode does. It makes future upgrades much easier to implement.

Change	Legacy Script	Tapscript
Sig Algorithm	ECDSA	Schnorr
Multisig Op	OP_CHECKMULTISIG	OP_CHECKSIGADD
Upgrade Path	Difficult	Easy (OP_SUCCESS)

In the final section, we will build a Python Taproot Auditor.