What Is the Role of Private Keys in Preventing Coin Theft during a 51% Attack?

Private keys are the cryptographic secret that proves ownership of coins and is required to authorize any spending transaction. A 51% attack, while capable of manipulating the order of transactions, cannot generate a valid signature for a transaction without the corresponding private key.

Therefore, an attacker cannot steal coins from wallets they do not own. The fundamental security of individual coin ownership, which is based on cryptography, remains intact even during a successful 51% attack.