The hacker behind the biggest cryptocurrency heist of all time has finally handed access to the final tranche of stolen funds. 

Poly Network, a platform in the decentralized finance or "DeFi" area, was hacked last month, with the hacker or hackers acquiring almost $600 million in digital tokens. The criminal took advantage of Poly Network's software flaw to move the cash to their own accounts. 

In an unexpected twist, the Poly Network hacker did not flee with the funds. Instead, they initiated contact with the targeted organization, offering to return all funds. Last week, the hacker returned all of the funds virtually, except $33 million in tether, or USDT, a dollar-pegged bitcoin that was locked by its issuers. 

However, there was a problem, more than $200 million in assets were locked up in an account that needed both Poly Network and the hacker to enter passwords. The hacker has been refusing to provide out their password for several days, only stating that they would do it when "everyone is ready." 

Poly Network appealed to the hacker, dubbed "Mr. White Hat," to refund the money. The company guaranteed the anonymous person a $500,000 reward for assisting in identifying a security weakness in its systems, as well as a post as "chief security advisor." 

Poly Network now has access to the final batch of stolen cash. According to a blog post published Monday, hacker Mr. White Hat provided the so-called private key needed to restore control of the remaining assets. 

“At this point, all the user assets that were transferred out during the incident have been fully recovered,” Poly Network stated. 

“We are in the process of returning full asset control to users as swiftly as possible.” 

Last week, the Japanese cryptocurrency exchange Liquid announced that it had been the target of a cyberattack in which hackers obtained $97 million worth of cryptocurrencies. However, in the case of Poly Network, the hacker kept an open dialogue going with their victim, eventually reclaiming the assets they had stolen. 

Security experts believe the attacker recognized it would be impossible to launder the money and cash because all transactions are recorded on the blockchain, the public ledgers that underlie most major digital currencies. 

An unidentified individual claiming to be the hacker stated they were “(quitting) the show” in a message embedded in a digital currency transaction. 

“My actions, which may be considered weird, are my efforts to contribute to the security of the Poly project in my personal style,” the unidentified person stated. 

“The consensus was reached in a painful and obscure way, but it works. Some people even suspect that the whole story is a PR stunt.” 

Poly Network's team verified that the private key is authentic, according to the organization.

“As of now, Poly Network has regained control of the $610 million (not including the frozen $33 million USDT) in assets that were overall affected in this attack. Once again, we would like to thank Mr. White Hat for keeping his promise, as well as the community, partners, and the multiple security agencies for their assistance.”