A blockchain runs on hashes and signatures, not encryption — every transaction is public by design. Knowing which primitive does what is the difference between reading a security claim and being sold one.

Every crypto loss is a key problem: stolen, lost, or signed away. Hardware wallets, multisig, MPC and social recovery each relocate the single point of failure — none of them deletes it.

Deployed contracts are public, usually immutable, and hold real money — so every bug is a bounty for whoever finds it first. Tests, audits, formal verification and bug bounties each cover a different slice.

A public ledger makes pseudonymous the opposite of private. Mixers, stealth addresses and zero-knowledge proofs claw some privacy back — and run straight into sanctions and money-transmission law.