How to Share a Secret Message
What’s a secret? Something meant to be kept unknown or unseen by others.
se·cret /ˈsēkrit/
adjective
- not known or seen or not meant to be known or seen by others. ("how did you guess I had a secret plan?")
noun
- 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: 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.
- 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: Let’s write a cipher!
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What is a 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)
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What are some substitution ciphers?
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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: 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: 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 and 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. 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, Quan (2019)
- Handbook of Applied Cryptography, Menezes et al. (2001)