Encryption & decryption

Encryption scrambles a message so only the intended reader can unscramble it. It’s the most famous use of cryptography — and, surprisingly, the one Bitcoin uses least. Understanding it sharpens exactly what Bitcoin does and doesn’t hide.

The basic idea

Take readable text (“plaintext”), apply a key and an algorithm, and get scrambled nonsense (“ciphertext”). Anyone intercepting the ciphertext learns nothing. The intended recipient, holding the right key, reverses the process and reads it. From Caesar shifting letters by three, to the Enigma machine, to today’s AES protecting your bank login — it’s the same shape, just harder to break.

Two flavours

• Symmetric — the same secret key locks and unlocks. Fast and simple, but both sides must somehow already share the secret. This is AES, used everywhere to protect data at rest and in transit.
• Asymmetric (public-key) — lock with the recipient’s PUBLIC key, and only their PRIVATE key can unlock. This solves the key-sharing problem: you can send a secret to someone you’ve never met, using a key they published openly. It’s the same key pair from two lessons ago, used in its “encryption” direction.

Why the distinction matters

People often assume “crypto” means “encrypted money”. It doesn’t. The “crypto” in cryptocurrency refers to cryptographic proof — hashes and signatures — not to hidden messages. Bitcoin’s security comes from everyone being able to check everything, not from keeping things secret.

One thread runs through hashes, keys and signatures alike: a calculation that’s easy forwards and impossible backwards. For Bitcoin’s keys and signatures, that trapdoor is built from a beautiful piece of geometry — elliptic curves. Let’s finally look at it, and play with it.