Key Path vs. Script Path

A Taproot output can be spent in two fundamentally different ways. This "Dual Path" architecture is what allows it to be both highly efficient and incredibly flexible.

1. The Key Path (The "Main" Way)

The Key Path allows an address to be spent with a single Schnorr Signature.

• Use Case: When all participants in a script agree on where the money should go.

• The Internal Key ($P$mount): This is a standard public key.

• Efficiency: It looks exactly like a normal payment. No scripts are revealed. It takes up the minimum possible space in a block.

2. The Script Path (The "Backup" Way)

The Script Path is used when the "Main" key cannot be used (e.g., one participant is offline or refuses to sign).

1. The spender reveals the hidden MAST tree.

2. They prove that a specific script branch exists within that tree.

3. They fulfill the requirements of that specific script (e.g., providing a signature + waiting for a timelock).

3. The Tweak (The Mathematical Glue)

How can one address have two different paths? It uses a process called Public Key Tweaking. $$P_{output} = P_{internal} + Hash(P_{internal} || MerkleRoot) \times G$$

• To spend via Key Path: You just need to know the private key for $P_{internal}$. You "tweak" your signature to match the $P_{output}$.

• To spend via Script Path: You reveal $P_{internal}$ and the MerkleRoot. The node verifies the math and then executes the script.

4. Why this is revolutionary for Multisig

In old multisig, you always revealed the participants. In Taproot multisig (using MuSig2):

• Alice and Bob create a joint "Internal Key."

• If they agree, they spend via Key Path. The world sees one signature.

• If Bob vanishes, Alice spends via Script Path (revealing the backup condition).

5. Economic Incentives

Taproot creates a strong economic incentive for privacy. The Key Path (private) is significantly cheaper than the Script Path (public). This encourages developers to design systems where "cooperation" is the default state.

Path	Spend Method	On-Chain Appearance	Fee Cost
Key Path	Schnorr Signature	Standard Single-Sig	Lowest
Script Path	MAST Branch	Complex Revealed Script	Higher

In the next section, we will analyze Tapscript & BIP342 Updates.