MAST: Merklized Alternative Script Trees

Before Taproot, if you had a complex script with multiple spending conditions (e.g., "Either Alice and Bob sign OR Alice signs after 1 year"), you had to reveal the Whole Script on the blockchain when you spent the money. MAST changes this.

1. Hiding the Branches

MAST uses the power of Merkle Trees to organize script conditions.

• Each possible spending condition (Branch) is hashed.

• These hashes are combined into a single Merkle Root.

• This Root is then "committed" to the Taproot address.

2. Reveal Only What You Use

If you spend using the "Alice and Bob" branch:

1. You provide the signature for that branch.

2. You provide the Merkle Proof (the path to the root).

3. The Result: You NEVER reveal the "1 year" branch. It stays secret forever.

3. Privacy Benefits

Because unused branches are never revealed, observers cannot know:

• How many other ways the money could have been spent.

• If there were any Timelocks or multisig requirements involved.

• Which participants were part of the "backup" spending conditions.

4. Scalability Benefits

If a script has 1,000 different conditions, a traditional script would be massive. With MAST, the data needed to spend is proportional to the Logarithm of the number of conditions.

• 1 condition = 1 hash.

• 1,000 conditions = ~10 hashes. This allows for incredibly complex smart contracts to be executed with minimal on-chain fees.

5. Security against "Script Analysis"

In the past, chain analysis firms could flag addresses that used specific types of complex scripts (often associated with high-security vaults or Lightning channels). MAST makes this type of analysis much harder because the "Complex" part is only revealed if the participants disagree and have to use a backup path.

Aspect	Pre-MAST	With MAST
Visibility	Full Script Revealed	Only Used Branch Revealed
Size	Linear ($O(N)$)	Logarithmic ($O(\log N)$)
Privacy	Low	High

In the next section, we will discuss Key Path vs. Script Path.