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https://github.com/breandan/crypto-koans

🔏 How to share a secret message: crypto katas for kids
https://github.com/breandan/crypto-koans

cipher crypto education encryption prime-numbers

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🔏 How to share a secret message: crypto katas for kids

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README 

        
## How to Share a Secret Message



### What’s a secret? Something meant to be kept unknown or unseen by others.



> ***se·cret /ˈsēkrit/***

>

> *adjective*

>

> 1. not known or seen or not meant to be known or seen by others. ("how did you guess I had a secret plan?")

>

> *noun*

>

> 1. something that is kept or meant to be kept unknown or unseen by others. ("a state secret")



### Let’s say you have a secret. How do you make sure your secret will stay secret?



* Do not write it down. (Even in your diary.)

* Do not talk about it. (Even in your sleep.)

* Do not record it. (Even on your smartphone.)



### What if you need to record your secret?



#### We need to turn the secret into a message.



* How do we write (or *encode*) the secret into the message?

* How do we read (or *decode*) the message into the secret?

* How do we keep our message safe?



---

### [Exercise #1](/src/main/kotlin/exercises/4_StringExercise.kt): Let’s encode a message!



*There are many ways to encode text.

Computers often use numbers to represent characters.

We can think of a message as a long list of numbers (or one really big number).*



---



### What if you need to share your secret?



#### We can share the message in person.



* How do you know you are alone?

* Is the other person who they say they are?

* What if your friend lives far away?



#### We can share the message in public.



* This is the safest way to share a secret. Requires that we be very careful.

* If you need to share a secret message, it’s best to assume everyone can read it.

* How do we prevent those who read our message from understanding the contents?



### How can you share a secret message in public without sharing the secret?



#### You can hide the secret in a secret place



  * How do we share the location of this place?

  * Can we be sure that no one is watching us?

  * What if someone finds out our hiding spot?



#### You can hide the secret in plain sight

  * This technique is called [steganography](https://en.wikipedia.org/wiki/Steganography).

  * What happens if someone is observant?

  * What if someone learns our technique?



#### We can use a language only we understand.

  * How can we be sure no one else listening can understand our language?

  * What if someone else can read our language? They will know our secrets!

  * What if someone else can write our language? They can change our messages!



#### We can scramble up the message somehow.

  * How do we scramble or (encrypt) the message?

  * How do we unscramble or (decrypt) the message?

  * How do we keep the secret safe?



---

### [Exercise #2](/src/main/kotlin/exercises/1_SimpleCiper.kt): Let’s write a cipher!



* What is a [substitution cipher](https://en.wikipedia.org/wiki/Substitution_cipher)?

  * Substitution ciphers are (weak) encryption schemes.

  * They work by replacing symbols and words (ie. Strings) with other symbols.

  * Ciphers must be reversible (when reversed they produce the original message)



* What are some substitution ciphers?

  * [Codepoint cipher](/src/main/kotlin/ciphers/4A_CodePointCipher.kt)

  * [Caesar cipher](/src/main/kotlin/ciphers/3A_CaesarCipher.kt)

  * [Isogram cipher](/src/main/kotlin/ciphers/3B_IsogramCipher.kt)



* What is the difference between encoding and encryption?

  * Substitution ciphers are just a fancy encoding mechanism.

  * Encryption is an encoding that is difficult to decode.



---



### How safe are substitution ciphers?



#### How do we communicate using ciphers?

* We need to agree on a common method (*protocol*).

* We need to agree on a common secret (*key*).

* How do we agree on a common secret? Back to square one.



#### What is a key?



* A key is a way to keep others from knowing your secret.

* A key is something you know, have or are.

  * Something you know: a password

  * Something you have: a physical key

  * Something you are: a fingerprint

* A key, like a message, can be encoded as a number.



#### What are the risks of using this kind of cipher?

* What happens if someone guesses our protocol? They still need to guess the key.

* What happens if someone deciphers a single message? They learn they key.

* What happens if someone learns our key? They can break every message.



---

### [Exercise #3](/src/main/kotlin/exercises/3_CipherBreaker.kt): Let's break a cipher!

#### Why are substitution ciphers dangerous?

If someone learns a plaintext-ciphertext pair they can break every message.



#### Small keys are easy to guess.



* What is the only way to guarantee an unbreakable cipher?

  * One-time pad



---



### How can we improve the safety of ciphers?



* Use keys with longer text

* Use keys that are difficult to guess



### What is a prime number?



* Prime numbers have only two factors.



### What is an algorithm?



* Some set of steps for a computer to take.



---

### [Exercise #4]: Computers are fast



Let's calculate some prime numbers. How many can you calculate in 5 seconds?



---



### What makes an algorithm hard?



* Numbers are easy to multiply

* Numbers are hard to factor



### What is a random number?



* Can we get computers to generate random numbers?



---

### [Exercise #5](/src/main/kotlin/exercises/6_RSA.kt): RSA



Turns out, we can share a secret without sharing a key.



---



---

### [Exercise #6]: What is a hash function? (optional)



Hash functions are cool.



---



## Building from the source



First ensure [JDK 1.8](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html) and [Git](https://git-scm.com/book/en/v2/Getting-Started-Installing-Git) are installed and run the following command in your terminal.



```

git clone https://github.com/breandan/crypto-exercises && cd crypto-exercises && ./gradlew runIde

```



After several minutes, a program called "IntelliJ IDEA" should start. If you receive an error, make sure JAVA_HOME is [correctly set](https://docs.oracle.com/cd/E19182-01/820-7851/inst_cli_jdk_javahome_t/). Once the program loads, right click on the file called *HelloCrypto.kt* and select "Run". You should see the following output.



```

Let's learn about cryptology!

```



## Running from the USB



### Mac OS X



Insert the provided USB drive and type or paste the following command from your terminal (⌘+Space, `🔍 Terminal`, Enter)



```

/Volumes/USB-STICK/idea.sh

```



This will launch an instance of IntelliJ IDEA with the project already configured.



If you see a message "Project SDK is not defined" on the top of your screen, click **Setup SDK | Configure... | + (Add new SDK)" | JDK**. Add the JDK which can be found here (under your Home directory): `~/jdk/`.



Once configured, you may need to select "Import Changes" and wait for indexing to complete.



### Windows



Insert the provided USB drive and open the folder named "Windows", then open the folder named "CryptoExercises", then double click on the icon "Launch_CryptoExercises.lnk".



### Sanity Check



You should now be able to run the `/src/main/kotlin/HelloCrypto.kt` file by right-clicking and selecting "Run".



If configured correctly, it should print the following output:



```

Let's learn about cryptology!

```



## Learn more



* [Intuitive Advanced Cryptography](https://github.com/cryptosubtlety/intuitive-advanced-cryptography/blob/master/advancedcrypto.pdf), Quan (2019)

* [Handbook of Applied Cryptography](https://cacr.uwaterloo.ca/hac/), Menezes et al. (2001)