P2MS ScriptPubKey Structure

The P2MS locking script is a "Template" script. It lists every authorized public key and the threshold required to unlock them. It is one of the longest script types in the Bitcoin protocol.

1. The Locking Script (ScriptPubKey)

A P2MS script follows this format: OP_M [PubKey1] [PubKey2] ... [PubKeyN] OP_N OP_CHECKMULTISIG

• OP_M: The number of signatures required (e.g., OP_2).

• [PubKeys]: The raw bytes of the N authorized public keys.

• OP_N: The total number of public keys listed (e.g., OP_3).

• OP_CHECKMULTISIG: The opcode that evaluates the stack and verifies the signatures against the list of keys.

2. Threshold Values

The values M and N are represented by small integers (OP_1 through OP_16).

• Consensus rules allow N to be as high as 20.

• Standardness rules (what nodes will actually relay) usually limit N to 3 if it is a "Bare" P2MS script.

3. The Execution Logic

When the Bitcoin Virtual Machine hits OP_CHECKMULTISIG, it does the following:

1. It pops the value N from the stack.

2. It pops N public keys.

3. It pops the value M.

4. It pops M signatures.

5. It iterates through the keys to see if the signatures are valid.

4. Raw Hex Example (2-of-2)

A 2-of-2 multisig script would look like this: 52 [33-byte Key1] [33-byte Key2] 52 ae

• 52: Hex for OP_2.

• ae: Hex for OP_CHECKMULTISIG.

Component	Hex Code	Role
M (Threshold)	0x51 to 0x60	Required signatures
Public Keys	Variable	Authorized participants
N (Total Keys)	0x51 to 0x60	Total list size
Opcode	0xae	The verification logic

In the next section, we will discuss the famous Off-by-One Bug in multisig.