Blockchain Forking

Changes to the blockchain software and implementation are called forks.

Updating technology can be difficult at the best of times, such as when systems are centralised. When a system is comprised of many users, distributed around the world, and governed by the consensus of the users, it becomes greatly difficult. Changes to the blockchain software and implementation are called forks.

Soft Forks
A soft fork is a change to the technology that will not completely prevent users who do not adopt the change (e.g., an update to the latest version) from using the changed blockchain system. Since non-updated nodes will recognize the new blocks as valid, a soft fork can be backwards compatible, only requiring that a majority of nodes upgrade to enforce the new soft fork rules.

An example of a soft fork occurred on Bitcoin when a new consensus rule was added to support escrow and time-locked refunds. In 2014, a proposal was made to repurpose an operation code that performed no operation (OP_NOP2) to CHECKLOCKTIMEVERIFY, which allows a transaction output to be made unspendable at a point in the future. For future clients that implement this change, the blockchain interpreter will perform this new operation, but for clients that do not support the change, the script is still valid, and execution will continue as if a NOP had been executed.

Hard Forks
A hard fork is a change to the technology that will completely prevent users who do not adopt it from using the changed blockchain system. Under a hard fork, the blockchain protocol will change in a manner that requires users to either upgrade to stay with the developer’s “main fork” or to continue on the original path without the upgrades. Users on different hard forks cannot interact with one another. Any change to the block structure, such as the hashing algorithm choice, will require a hard fork.

A well-known example of a hard fork is from Ethereum. In 2016, a smart contract was constructed on Ethereum called the Decentralised Autonomous Organisation (DAO). Due to flaws in how the smart contract was constructed, an attacker extracted Ether, the cryptocurrency used by Ethereum, essentially allowing theft of $50 million. A hard fork proposal was voted on by Ether holders, and 89 percent agreed to hard fork and create a new version of the blockchain that returned the stolen funds.

With cryptocurrencies, if there is a hard fork and the blockchain splits, the coins each person has at the time of the split will be mirrored on each fork. If all the activity moves to the new chain, the old one will eventually not be used. In the case of the Ethereum hard fork, the vast majority of support moved to the new fork, and the old fork was renamed to Ethereum Classic, which has only a fraction of the original user base.

Cryptographic Changes and Forks
If flaws are found in the cryptographic technologies for a blockchain application, the only solution may be to create a hard fork, depending on the significance of the flaw. For example, if a flaw was found in the underlying algorithms, there could be a fork requiring all future clients to use a stronger algorithm. Until more than 50 percent of the network is on the new software version, the vulnerability could still exist. Switching to a new hashing algorithm could pose a significant practical problem because it could invalidate all existing specialised mining hardware.

Hypothetically, if SHA-256 were discovered to have a flaw, there would need to be a hard fork to migrate to a new hash algorithm. The block that switches over to the new hash algorithm would “lock” all previous blocks into SHA-256 (for verification), and all new blocks would need to employ the new hashing algorithm. For example, Bitcoin uses SHA-256 hashes, which is easy and fast to implement in hardware ASICs. Other cryptocurrencies such as Ethereum use Keccak857 (based on SHA-3), while Litecoin uses the scrypt hashing algorithm.

One possibility for the need to change cryptographic features present in a blockchain system would be the development of a practical quantum computer system, which would be capable of greatly weakening (and in some cases, rendering useless) existing cryptographic algorithms.

The cryptographic algorithms employed within most blockchain technologies for public/private key pairs will need to be replaced if a powerful quantum computer become a reality. This is because algorithms that rely on the computational complexity of integer factorisation (such as RSA) or work on solving discrete logarithms (such as DSA and Diffie-Hellman) are very susceptible to quantum computing. The hashing algorithms and Merkle trees that are the other basis for blockchains are much less susceptible to quantum computing attacks, but are still weakened when quantum computers become a reality.

Source: National Institute of Standards and Technology, U.S. Department of Commerce (Jan 2018)

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