Skip to content

19.1: Understanding MAST

MAST stands for Merkelized Abstract Syntax Tree, an idea first suggested in 2016 as BIP 114. Though that BIP has been Closed, it lives on in the Taproot Update.

Understand the Merkle Tree

A Merkle Tree is also known as a hash tree. That's because it's a tree of hashes. Data is hashed to form the leaves at the bottom level of the tree. Then hashes are combined to form a node: usually a Merkle tree is a binary hash tree, which means that every pair of hashes are combined.

📖 What is a hash? A hash is the output of a hash function. It's a short, fixed length representation of a longer variable length input. Hash functions are designed to be collision resistant, which means that each hash uniquely represents each input, and one way, which means that you can derive a hash from the input, but you can't derive the input from the hash.

As you ascend in a hash tree, more hashes are combined, resulting in fewer and fewer nodes. Ultimately, you get root hash that represents all of the individual hashes of the tree, and so all of the individual data input into the tree. A hash tree is usually represented just by its root hash.

Individual leaves of a hash tree can be revealed without revealing the content as a whole: a user simply reveals the leaf and then each of the hashes that it contributes to, up to the root hash.

Understand the MAST

MAST is a simple application of the Merkle Tree to Bitcoin Scripts: each leaf represents a Bitcoin Script (or rather, the slightly varied Tapscript) that can be used to unlock the funds at a Taproot address.

  • If there is no Script, then the Taproot address is tweaked with an unspendable script path.
  • If there is a Merkle Tree, then the Taproot address is tweaked with the root hash of the tree.

To spend from a Taproot address using a key just requires a signature from the private key and the inclusion of the public key (just like with traditional P2PKH and P2WPKH addresses).

To spend from a script in the Merkle tree requires that the spender supply the script and the inputs that satisfy the script (just as with traditional P2SH and P2WSH addresses), but also the path in the Merkle Tree that runs from the leaf script being satisfied to the root hash that was encoded in the P2TR address.

(The term MAST is not used in BIP-341, but is referenced here for its historic inclusion.)

Summary: Understanding MAST

Merkle Trees, or MASTs, are included in Taproot. They're a way to simply and efficiently support a whole collection of scripts, any of which can unlock an address, and which can be used as alternatives to typical key path spending.

🔥 What is the power of MAST? MAST is a way to record a whole collection of scripts as a single root hash that Taproot then applies as a tweak to a Taproot address. This methodology ensures that no one knows what the scripts or, or even that there are scripts, before they're actually used. So besides the versatility implicit in being able to unlock funds in a variety of ways, you also get the privacy of what those ways are.

What's Next?

Continue "Using Tapscript" with §19.2: Designing Tapscripts.