🔐 What Is Cryptography?
The art of keeping secrets — and proving you kept them⏱ ~3 min
Imagine writing a letter, putting it in a locked box, and mailing it. Only the person with the right key can open the box and read it. Even if someone intercepts the box along the way, they see nothing useful. Cryptography does this digitally — it transforms readable data into scrambled noise that only authorized parties can unscramble.
A 4,000-Year-Old Problem
Cryptography predates computers by millennia. Ancient Egyptians used substitution in hieroglyphs around 1900 BCE. Julius Caesar shifted letters by 3 to send military orders. World War II was shaped by the breaking of Germany's Enigma cipher. Today cryptography secures every HTTPS website, every text message, and every bank transfer — invisibly, automatically, billions of times per second.
The Three Goals of Cryptography
| Goal | What It Means | Example |
|---|---|---|
| Confidentiality | Only authorized parties can read the data | Your WhatsApp messages are unreadable to anyone but the recipient |
| Integrity | Data has not been altered in transit | A downloaded file's hash matches the publisher's hash — it wasn't tampered with |
| Authenticity | The sender is who they claim to be | A digital signature on an email proves it came from that person's key |
Core Vocabulary
| Term | Definition |
|---|---|
| Plaintext | The original readable message (before encryption) |
| Ciphertext | The scrambled output after encryption — unreadable without the key |
| Encryption | The process of converting plaintext → ciphertext using an algorithm and key |
| Decryption | The reverse — converting ciphertext → plaintext using the correct key |
| Key | A secret value that controls how the algorithm scrambles the data |
| Algorithm / Cipher | The mathematical recipe for encryption and decryption |
| Cryptanalysis | The science of breaking ciphers without the key |
Kerckhoffs's Principle — The Most Important Rule in Cryptography
According to Kerckhoffs's principle, what must stay secret?