Double-SHA256 Linkage

Bitcoin doesn't just use SHA-256 for its block links; it uses Double-SHA256 (also known as hash256). This means the Block Header is hashed, and then the resulting hash is hashed again.

1. The Algorithm

Mathematically, the link between blocks is calculated as: $$Block\ Hash = SHA256(SHA256(Block\ Header))$$

2. Why Hash Twice?

Satoshi Nakamoto never explicitly stated why he chose double hashing, but there are two primary technical theories:

A. Length Extension Attacks

Standard SHA-256 is vulnerable to a "length extension attack," where an attacker who knows $Hash(Message)$ can potentially calculate $Hash(Message + ExtraData)$ without knowing the original message. While this attack doesn't directly break block security, hashing twice ($Hash(Hash(M))$) completely eliminates this class of vulnerability.

B. Birthday Attack Hardening

Double-hashing provides an extra layer of protection against future cryptographic breakthroughs. If a partial weakness is found in a single round of SHA-256, the second round acts as a safety buffer.

3. Computational Efficiency

While double-hashing sounds like it would take twice as long, it is actually very efficient for SPV Nodes. Since the headers are only 80 bytes, calculating two rounds of SHA-256 takes only a fraction of a millisecond on modern hardware.

4. The Resulting Hash

The final output is a 256-bit number. For a block to be valid, this number must be lower than the current Target.

Property	Value
Input size	80 Bytes
Output size	32 Bytes (256 bits)
Common Format	Hexadecimal (Big-Endian)

Next, we will explore the Immutability Mechanics that result from this linked structure.