Security

To add a new block, miners have to solve a computational task that requires real electricity and hardware. Finding the solution is hard, but checking it is easy for everyone else. Whoever succeeds is rewarded with new bitcoin and fees.

That the work costs something is the whole point. It ties the digital history to real energy, and makes cheating expensive to attempt.

The combined computing power that secures the network is called the hash rate, and it has grown enormously over the years. To rewrite the history, an attacker would need more power than the rest of the world's miners combined, while paying for huge amounts of electricity.

The more the network grows, the more unreasonable such an attack becomes. Energy is thus continuously converted into security.

The network can be as secure as you like, but your bitcoin are only as safe as your keys. Whoever has the secret recovery phrase controls the money. So it should be written down offline, never shared and never entered at the prompting of someone who reaches out to you.

Precisely so that money can have a single true owner, the responsibility lies with you. That's the price of no intermediary being able to freeze, take or make disappear what you own.