P.S. In fact, I was going to send that book to my friend's son who is about same age, and who got some sudden interest in cryptography recently. So it is safe to say that I am now aware of at least two 10 year olds who have interest in Cryptography. I feel a bit surprised, but in a good way :)
He started with inventing his own alphabet and language as he was learning to read and write, which eventually evolved into reinventing the Secret Decoder Ring with increasing complexity, so we want to encourage him in his adventure.
Something like that would be practical.
The Code Book by Simon Singh
The Codebreakers by David Kahn
The Mathematics of Secrets by Joshua Holden
It does assume a fair bit of historical context knowledge, though, much of which probably exceeds 5th grade. Probably 9th graders would have it easier as far as, e.g., familiarity with WWII, etc.
In the same vein, is the "The Adventure of the Dancing Men" [1]
[1] https://en.wikipedia.org/wiki/The_Adventure_of_the_Dancing_M...
ISBN-10 : 1681195143 ISBN-13 : 978-1681195148
Title: Top Secret: A Handbook of Codes, Ciphers and Secret Writing
ISBN-10 : 0763629723 ISBN-13 : 978-0763629724
Mine are out of the house, but I had a few who might have really liked doing something like this.. could make a nice Christmas present, the first mysterious letter..
https://web.archive.org/web/20100128072801/http://www.nsa.go...
(They went offline a few administrations ago, hence the archive link)
You can also jump around the timeline in the archive to see different versions.
"I think most children can grasp difficult material if an adult that's good at the meta-skill of teaching is helping them with it via individual attention" That's how we feel too and they appreciate being given that chance if we don't push.
(used copies still available)
U WJZI VH KZJBY QYIM HUZFGBZ.
Key: TRENCH
The Giant Raft or 800 Leagues On The Amazon
Mathias Sandorf
A Journey To The Centre of the Earth
After reading those, I played with the ciphers described there, i.e. I made and used the transposition grid from Mathias Sandorf and I used various Vigenère ciphers, like that from "800 Leagues On The Amazon".
Due to the interest generated by those stories, a few years later I read various textbooks about modern cryptography.
The book "Applied Cryptography" by Bruce Schneier is now somewhat old, so it is no longer up-to-date, but it is written in a style more accessible for newbies than most textbooks about cryptography, so I think that it would be adequate for a teenager who is good at mathematics.
The funny thing is that I got that book as a prize from a previous year's programming fair.
However, I was a kid in the seventies, when PCs were still in the future, so I had to practice cryptography with pen and paper.
While in high school, I wrote my first programs, but at that time this was done by writing the program (e.g. in FORTRAN) on a special printed form, on a grid where each character was accurately positioned, then the form was given to an operator who transferred the written program on 80-column punched cards, and then the next day you could retrieve the printed results from running your program on an IBM mainframe (printed with a 132-column line printer), together with the deck of punched cards, to be used if you wanted to rerun the program (which you almost always wanted to do, even if the program had been intended to be run only once, because your first attempt of running might have printed only compiler error messages).
There has been a long journey from that until being able to use VB6 or Visual C++.
Was one of the best books I read back in the day, It wasn't only about the algorithms themselves, also about how many verification schemes worked, in an easy to understand manner.
Today I'd say the knowledge of protocols is still valid, just that the algorithms first themselves need to be post-quantum secure.
It is not missing most of modern cryptography, it misses only a few, albeit very important things, like post-quantum public-key algorithms, memory-hard functions for password-based algorithms, and integrity-verification algorithms with intrinsic encryption (for the latter 2 classes of algorithms I am not aware of any modern cryptography book that includes a correct and complete presentation, despite their practical importance; the information about them is spread over a large number of research papers published during the last quarter of century, and one must search and read all those papers, to be well informed).
Most of the principles of modern cryptography have barely changed after 1980, even if the specific algorithms that implement them have greatly evolved, so that almost no algorithm older than AES is still useful.
For an introduction to the principles of modern cryptography the old book of Schneier is still fine, even if the specific algorithms are no longer useful.
There are much better books for someone who is already familiar with cryptography or with abstract algebra and wants to learn something that can be used in practice, but most of those are too dense for someone without good prior knowledge of mathematics or cryptography.
https://sockpuppet.org/blog/2013/07/22/applied-practical-cry...
https://www.amazon.com/Cryptanalysis-Study-Ciphers-Their-Sol...
Helen Gaines Cryptanalysis: A Study of Ciphers and Their Solution
It’s a slim volume that focuses on historical and fictional ciphers. It stars with simple shift ciphers like Caesar’s cipher, progresses to more complex substitution ciphers with examples from Sherlock Holmes and Edgar Allen Poe, then covers polyalphabetic ciphers like Vigenère and Playfair. It covers basic cryptanalysis and has practice riddles and cryptograms throughout. It even has a chapter on a variety of ways to make invisible ink, which is often an object of fascination for children around that age!
- _Codes & Secret Writing_ by Herbert S. Zim
- _Codes and Ciphers: Secret Writing Through the Ages_ by John Laffin
- _Codes, ciphers, and secret writing_ by Martin Gardner
There was another book on alphabets/letterforms and ciphers --- pretty sure it had a cover designed by Oscar Ogg, but blanking on the name and can't find it.
While searching online for that found:
Secret Writing: An Introduction to Cryptograms, Ciphers, and Codes by Henry Lysing and John L. Nanovic
Cryptography adjacent (it's about the difficulty of deciphering a dead language), but H. Beam Piper's novella "Omnilingual" really should be part of the grade school canon --- lightly updated version at:
[0]: https://sherlock-holm.es/stories/pdf/letter/1-sided/danc.pdf
https://usborne.com/au/official-spy-s-handbook-9781409584384
One angle from which it's fun is just learning operations you can do on text (and text represented as numbers). Empirically it turns out just Shannon's "diffusion and confusion" and lots of rounds gets you far in symmetric cryptography, which we can see from a lot of simple RAX primitives working fine. But it's nice for reasons outside cryptography to get a feel for the zoo of other things you can do, what offers diffusion and nonlinearity, how you can ensure a function's reversible, etc.
If you did one topic from public-key land, I think Diffie-Hellman is easier to understand than RSA without much build-up: it makes sense from the basic properties of exponents (ones modular arithmetic doesn't change), and modular arithmetic connects to fun stuff about the primes. Merkle hash signatures also make sense with just symmetric cryptography, and are post-quantum-resistant!
I hadn't really thought about how you'd even approach PQ encryption, but maybe ECC-based McEliece is cool? You can get a feel for it even without detail on the hard decoding problem, and error correcting codes are their own interesting topic.
