SHA-256D Algorithm | cryptocurrencies, blockchains & live prices

SHA-256D (Double SHA-256) is a proof-of-work hashing method that applies the SHA-256 hash function twice in succession — the output of the first SHA-256 computation is fed directly as input into a second SHA-256 computation. This double-hashing approach was introduced by Satoshi Nakamoto in the original Bitcoin protocol and is used throughout Bitcoin's architecture, not only for proof-of-work mining but also for transaction hashing, block header hashing, and Merkle tree construction. The technical operation is straightforward: SHA-256D(x) = SHA-256(SHA-256(x)), producing a 256-bit output that is indistinguishable from random data.

The primary motivation for double-hashing with SHA-256D is to provide additional protection against length-extension attacks, a class of cryptographic vulnerabilities that affect single-pass SHA-256. In a length-extension attack, an attacker who knows the hash of a message can compute the hash of that message with additional data appended, without knowing the original message. By applying SHA-256 twice, this attack vector is completely eliminated. SHA-256D inherits all of SHA-256's properties regarding ASIC efficiency — it is extremely well-suited for hardware optimization, and the mining industry has developed highly specialized ASICs that make CPU and GPU mining economically unviable for SHA-256D coins.

SHA-256D is the de facto standard proof-of-work algorithm for Bitcoin and the vast majority of Bitcoin forks and derivative cryptocurrencies. Bitcoin, launched in January 2009, has always used SHA-256D for its proof-of-work consensus mechanism, and the algorithm underpins the security of the world's largest cryptocurrency by market capitalization. Major cryptocurrencies using SHA-256D include Bitcoin Cash, Bitcoin SV, Namecoin (which pioneered merged mining with Bitcoin), Peercoin, and dozens of other Bitcoin derivatives. The distinction between SHA-256 and SHA-256D is often blurred in casual usage, with many sources simply referring to Bitcoin's algorithm as "SHA-256," though the double-hashing implementation is technically the accurate description.