Bitcoin gold (BTG), an offshoot cryptocurrency from bitcoin, was hit by 51 percent attacks on Thursday and Friday. 

The malicious mining attack removed roughly 29 blocks via two deep blockchain reorganizations (reorgs), resulting in double spends of 1,900 BTG and 5,267 BTG, respectively.

Blockchain reorgs

A blockchain reorganization attack occurs when a mining group with 51 percent of the hash power (computing power) removes previously confirmed blocks from the blockchain.

For instance, a mining group could pick block number 100 as the point of attack and start mining privately, leaving behind the miners on the public blockchain. So, when malicious miners are on block number 110, the public chain is on 109.

When this happens, there are two separate versions of every block. Essentially, the network gets forked, which needs to be reorganized.

In such cases, the original 10 blocks mined from 100 to 110 are orphaned and replaced with the new blocks mined by the attackers. This process allows individual nodes across the network to agree on the same version of the blockchain.

Note that transactions in the orphaned blocks become invalid as they are no longer the part of the longest chain. Such attacks are often used to double spend crypto, erasing transactions from the record and spending the same funds a second time.

Not the first attack

Bitcoin gold was attacked in May 2018, when the equivalent of roughly $18 million on the blockchain was double spent. 

The forked cryptocurrency utilizes a variant of the Equihash algorithm (Equihash 144, 5 or “Zhash”) which can be mined with a GPU. Its parent blockchain, however, uses SHA256 consensus. 

Impact on bitcoin gold's price

The cryptocurrency picked up a strong bid after the news of the attack hit the wires. Bitcoin gold rose from $10.42 to $11.60 in a few hours and was last seen trading at $12.14, representing an 11 percent gain on a 24-hour basis.