[sbierwagen]

>There is no centralized party that owns Handshake.

Historically, blockchains haven't been terribly firm on the "irreversibility" point. Ethereum reversed the DAO transactions. The BTC rollbacks in 2010 and 2013. Etc etc.

In practice, whoever owns the client update server and whoever has 51% of the hashing power can make arbitrary changes to the chain. For most cryptocurrencies, a meeting of these stakeholders could fit into a phone booth.

[troquerre]

You're right that Handshake can suffer from a 51% attack like other proof-of-work chains. Importantly, the security of DNS records on Handshake is strong even with the possibility of a 51% attack.

51% attacks on blockchains used as a store of value are bad because an attacker can spend their coins (ie BTC) on an exchange, withdraw their profits, then perform a 51% attack on the previous block to take back their spent coins.

On Handshake, that same attack exists for HNS, but DNS updates on chain take 36 blocks (about 6 hours worth of transactions) to propagate, which is significantly more expensive and unlikely than pulling off a normal 51% attack on a single block (it gets exponentially more unlikely). Furthermore this wouldn't even be an attack per se. An attacker may be able to undo a DNS update, but they wouldn't be able to falsify DNS records because only the owner of the name who controls the private key would be able to submit valid UPDATE transactioins.