What is Bitcoin Gold?
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๐ข The simple version
Plain English โ no jargon. Start here.
Bitcoin Gold is a hard fork of Bitcoin from October 2017 that replaced Bitcoin's SHA-256 proof-of-work algorithm with Equihash โ a GPU-friendly algorithm designed to prevent ASIC mining centralisation while keeping Bitcoin's transaction model, supply schedule, and 21 million coin cap intact.
Why Bitcoin Gold forked from Bitcoin
Bitcoin Gold (BTG) was created by a group of developers who believed Bitcoin's ASIC-dominated mining had become excessively centralised. By October 2017, a handful of Chinese ASIC manufacturers and mining pools controlled the majority of Bitcoin's hashrate. BTG's proponents argued this created unacceptable centralisation risk โ the same concern that motivated Vertcoin in 2014.
The solution: fork Bitcoin to change only the mining algorithm, keeping everything else โ the UTXO model, the 21 million supply cap, the halving schedule, the transaction format โ identical to Bitcoin. Anyone holding Bitcoin at the fork block height received an equal amount of BTG. Equihash was chosen as the replacement algorithm because Zcash had already demonstrated Equihash could be mined effectively by consumer GPUs.
The pre-mine controversy
Bitcoin Gold launched with a controversial 100,000 BTG pre-mine โ allocated to the development team for "development costs and bounties." This represented approximately 0.5% of total supply, but drew significant criticism from the community as inconsistent with Bitcoin's ethos of no developer allocation. The BTG team defended it as necessary for development funding; critics argued it undermined the "fair" fork narrative.
51% attacks โ documented history
Bitcoin Gold suffered two significant 51% attacks: May 2018 (~$18 million double-spent) and January 2020 (~$72,000 double-spent). Equihash ASICs had been developed by the time of the attacks, partly neutralising the ASIC-resistance rationale. BTG later upgraded to Equihash-BTG (a customised variant) to restore GPU mining.
Is it legal in India?
Yes. BTG is a VDA under Indian law. 30% tax and 1% TDS apply. See India regulation.
๐ก A bit more detail
For when you want to go a little deeper.
Equihash โ the memory-hard PoW
Equihash is a proof-of-work algorithm based on the generalised birthday problem โ finding multiple hash inputs (n inputs out of 2แต total) that XOR to zero. The algorithm has tunable parameters (n, k) that determine memory requirements. Bitcoin Gold uses Equihash 144,5 (also called "Equihash-BTG" after customisation) which requires approximately 3GB of GPU memory. This memory requirement makes Equihash harder to implement efficiently in custom ASICs, though specialised Equihash miners were eventually developed for the original Equihash parameters used by Zcash.
Bitcoin Gold is primarily used by GPU miners seeking to mine a Bitcoin-like coin without ASIC competition. The BTG ecosystem includes a wallet, block explorer, and listings on major exchanges. Its value proposition compared to Bitcoin is purely the ASIC-resistance argument โ the transaction model, security model (weaker due to lower hashrate), and monetary policy are otherwise similar to Bitcoin. Some users hold BTG as a speculative asset received from the 2017 Bitcoin fork claim.
Replay protection and fork mechanics
Bitcoin Gold implemented replay protection from the start โ a modification ensuring that BTG transactions cannot be "replayed" on the Bitcoin network and vice versa. This is done by changing the transaction format slightly (adding a different network version byte). Without replay protection, someone spending BTG could inadvertently spend Bitcoin with the same transaction โ a significant security risk. BTG's replay protection was correctly implemented, unlike some earlier Bitcoin forks that failed to include it properly.
Key BTG metrics: GPU mining profitability, hashrate security (lower than Bitcoin, making it vulnerable to 51% attacks), exchange trading volume, and development activity. BTG faces a fundamental challenge: its security comes from GPU miners, who allocate their hardware to whichever coin is most profitable to mine โ making BTG's security directly dependent on its price relative to other GPU-mineable coins. Live data: CoinGecko ยท BTG Explorer.
๐ฃ The full technical picture
For the technically curious.
The Equihash generalised birthday problem
Equihash (Wagner, 2002, adapted by Biryukov and Khovratovich, 2016) is based on the k-XOR problem: given 2โฟ random hash outputs of length n bits, find 2แต of them whose XOR is zero. The Wagner algorithm solves this problem in time and memory O(2^(n/(k+1))). By tuning n and k, designers control the time-memory trade-off. Equihash 144,5 (Bitcoin Gold's variant) requires approximately 3GB working memory for efficient solving โ making it impractical to implement multiple solvers on a single GPU (only one or two parallel solvers fit in a high-end GPU's memory). The birthday problem structure makes Equihash solutions easy to verify but computationally expensive to find in memory-constrained environments.
Sources: Biryukov, A. & Khovratovich, D. (2016). Equihash: Asymmetric Proof-of-Work Based on the Generalised Birthday Problem. iacr.org/2015/946 ยท Bitcoin Gold documentation: bitcoingold.org
The 2018 51% attack post-mortem
The May 2018 BTG attack is one of the most thoroughly documented 51% attacks in crypto history. The attacker rented Equihash hashrate and reorganised the chain by 22 blocks โ long enough to deposit BTG on exchanges, trade for Bitcoin, withdraw the Bitcoin, then broadcast the chain reorganisation that reversed the original BTG deposits. The attack exploited the "exchange confirmation window" โ exchanges require a certain number of confirmations before crediting deposits, and the attacker's chain reorganisation exceeded this window before exchanges detected it. The attack demonstrated that proof-of-work security is directly proportional to hashrate cost, and coins with lower market caps are proportionally cheaper to attack.
Bitcoin Gold's Equihash customisation (Equihash-BTG) changed the personalisation string parameter in the Equihash hash function โ a simple but effective change that made existing Equihash ASICs (designed for Zcash's personalisation string) incompatible with BTG. Miners would need to rebuild ASICs specifically for BTG's personalisation string. Since BTG's smaller market cap makes this commercially unviable for ASIC manufacturers, the personalisation change has effectively maintained GPU-only mining. This customisation approach has also been used by other Equihash coins (BitcoinZ, Komodo) to differentiate their mining from Zcash's.
Key facts
- Fork from: Bitcoin (block 491,407, October 2017)
- Algorithm: Equihash-BTG (144,5, GPU-friendly)
- Pre-mine: 100,000 BTG (developer allocation)
- Supply: 21 million BTG (same as Bitcoin)
- 51% attacks: May 2018 (~$18M) and January 2020 (~$72K)
- Replay protection: Yes (from launch)