Where to go next
You have tried everything on this site and you want to go further. That is where it gets awkward: there is an enormous amount of material about cryptography, and most of it is either a cartoon about Caesar or a university course that loses you on page two. What follows sits in between.
This list was not made up. In September 2026 we put the question "what actually gets a bright fifteen-year-old hooked?" to r/cryptography, where people do this for a living. Almost everything below was named by someone who said it had worked for them. Where several people named the same thing, it says so.
Breaking in yourself
This came up most often, and by a distance. Not reading about cryptography but getting something that is broken and prising it open yourself. Exactly what you did in chapter 9.5, but hundreds of times over and steadily harder.
| Where | What |
|---|---|
| Cryptopals | Eight sets of exercises, from "convert this to base64" to attacks that were in the news last year. You program everything yourself, in whatever language you like. The most recommended one, and according to several people the most underrated. Free, no account. |
| MysteryTwister | A few hundred challenges at rising levels of difficulty. The easiest you do with pen and paper; the hardest category holds problems nobody has got to the bottom of. If you do not program (yet), start here. |
| CryptoHack | The same idea with a game around it: points, levels, a leaderboard. More mathematics than Cryptopals, and it builds up neatly. |
| picoCTF | Made for secondary school pupils, by Carnegie Mellon University. There is a crypto section in it, and the rest of the problems are worth your time too. |
| OverTheWire | You get a real server you are allowed on. Start with Bandit: it teaches you along the way how a computer works from the inside, and you need that for all the rest. |
| Cyber Security Challenge Belgium | A Belgian competition, every year, in teams of four. Exactly who may enter differs per edition, so check that on their site. This is where you meet people who enjoy the same things. |
Three books
One book was named far more often than all the others put together.
| Book | What |
|---|---|
| Simon Singh, The Code Book (1999) | From the Egyptians to the quantum computer, written as a story rather than a textbook. There is also an edition for younger readers, The Code Book for Young People. |
| David Kahn, The Codebreakers (1967, expanded in 1996) | The standard work, and thick with it. Someone in that thread recounted finding it in the library as a teenager and building a whole career on it. |
| Jean-Philippe Aumasson, Serious Cryptography (2nd edition 2024) | This is the book for when it has clicked and you want to know how it really fits together. Heavier than the rest of this list, but still written to be understood. |
Watching
| Where | What |
|---|---|
| Numberphile | one idea per video, explained by people who do the subject themselves, with a marker on brown paper. Search for RSA, Diffie-Hellman or elliptic curves. |
| CrypTool | not a video but a free program in which you can take classical and modern ciphers apart, with every intermediate step on screen. There is a version that just runs in your browser. |
| Bletchley Park | the estate where Enigma was broken is now a museum with the machines in it. If you are ever in England. |
Build something
One answer in that thread came from somebody who started university at fifteen and ended up with a doctorate in cryptography. Their advice was not a book and not a website: build a website with a login on it. Then the questions come by themselves. How do you store those passwords? What is a session? What does HTTPS actually give you? Where does that key come from?
You already know the answer to all four — they are in chapter 4, chapter 8.4, chapter 8.2 and chapter 2. The difference is that you will have asked them yourself, and that is what decides whether something sticks.
And if you want to study this
Which course to pick for that and where it takes you is in About this site. In short: it is mathematics or computing, and of the two, mathematics is the door that opens further.
Almost everything above is in English. That is not a choice of ours: in this field practically everything happens in English. Treat it as the first tool you need. Anyone who can read a challenge on Cryptopals can later read the manual of a cryptography library, and that is where the real work is.