Backward Compatibility Mechanics

Nested SegWit is a masterpiece of game theory and software engineering. It allowed Bitcoin to upgrade its entire signature system without forcing every user to update their software at the same time.

1. The "Anyone Can Spend" Illusion

To an old node (pre-2017), the Redeem Script 00 14 [Hash] looks like a script that simply pushes data to the stack and then terminates.

• In the old rules, if a script ends with data on the stack and no failing opcodes, it is considered Valid.

• This is why SegWit was a "Soft Fork." Old nodes thought these transactions were "Anyone Can Spend," so they didn't reject them.

2. The New Node Reality

Nodes that have upgraded to SegWit know that 00 14 is a special pattern.

• They don't just stop when the data is pushed.

• They pause the execution and go looking for a Witness block.

• If they don't find a valid signature in the Witness block, they reject the transaction, even though an old node would have accepted it.

3. Consensus Security

Because the majority of miners and nodes upgraded to SegWit, the new rules became the law of the network.

• An old node might think a transaction is valid without a signature.

• But if an old node tries to mine a block with an invalid SegWit spend, the upgraded majority will reject that block.

• Therefore, the old node's "opinion" no longer matters for the security of the chain.

4. The Bridge and the Wall

Nested SegWit acted as a "Bridge" over the "Wall" of incompatibility.

• The Bridge: Legacy addresses could send to the "3" address.

• The Wall: Legacy software could not verify the signatures, but the network protected the funds anyway using the upgraded miners.

Feature	Old Node View	New Node View
P2SH Hash Check	Required	Required
Signature Check	Ignored	Strictly Required
Witness Data	Hidden/Unknown	Verified

In the next section, we will look at the Fee Savings.