# Bases de la cryptographie

### Durée estimée: 90 minutes

## Introduction

*Cryptography* means *secret writing*. It is the art and science of
sending secret messages and it has been used by generals and governments and
everyday people practically since the invention of written language.

As we will see in the next lesson, modern cryptographic techniques are essential to guaranteeing the security of our transactions on the Internet. Cryptography plays a role whenever you make an online purchase at Amazon or provide your password to Google. If we couldn't rely on those transactions being secure -- i.e., being encrypted using some cryptographic algorithm -- we really wouldn't have the Internet as we know it today.

This lesson focuses on some of the classical ciphers, going all the way back to
one of the first ciphers, the Caesar cipher, which Julius Caesar used in 34 B.C. Like the
other ciphers we study in this lesson, it is what is known as a *symmetric cipher*,
which means that the same *key* is used both for *encrypting* and
*decrypting* messages. As we will also see, all ciphers consist of two parts,
the *key* and their *algorithm*. And it is the key, not the algorithm,
that allows the cipher to create secret messages. In fact, in modern cryptography the
algorithms are all based on *open standards* that are created by teams of
experts, discussed openly and adopted and maintained by standards organizations.

So, let's take a look at some of the classical ciphers and some of the basic principles of cryptography. There are several hands-on activities in this lesson, where you'll have a chance to practice encrypting and decrypting messages and analyzing ciphers.

(Slides)

## Part 1: Caesar Cipher

## Activity: Caesar Cipher

**By Hand:**Use the Caesar cipher to encrypt your name. Use a key of 3 on the first try. Then use the script above to check your answer.**Brute force search**. Asearch for a Caesar key would be to test every possible key. How many keys would you have to test?**brute force**Use brute force search to

the following message. What is the key?*crack*YTNP HZCV JZF OTO TE!

## Part 2: Simple Substitution Cipher

## Activity: Simple Substitution Cipher

**By Hand:**Use the Simple Substitution cipher to encrypt your name. Choose your own keyword to create a cipher alphabet. Then use the script to check your result.**Decrypt:**The following word,, was encrypted with the keyword*SIRTQSMTCKJ*. Can you decrypt it?*simple***Brute force attack.**How many keys (arrangements of the alphabet) would you have to try to perform a brute force attack?

## Part 3: Frequency Analysis

## Activity: Frequency Analysis

**Frequency analysis:**One of the following messages was encrypted using a*substitution cipher*and the other with a*transposition cipher*. Can you identify which is which? Paste the messages into the frequency analyzer tool (above) and observe their frequency histograms?.**Text 1.**nybfx ymjgj xytky nrjxn ybfxy mjbtw xytky nrjx nybfx ymjfl jtkbn xitrn ybfxy mjflj tkktt qnxms jxxn ybfxy mjjut hmtkg jqnjk nybfx ymjju thmtk nshwj izqny dnyb fxymj xjfxt stkqn lmyny bfxym jxjfx tstki fwpsj xxny bfxym jxuwn sltkm tujny bfxym jbnsy jwtki jxufn wbjm fijaj wdymn slgjk twjzx bjmfi stymn slgjk twjzx bjbj wjfqq ltnsl inwjh yytmj fajsb jbjwj fqqlt nslin wjhy ymjty mjwbf dnsxm twyym jujwn tibfx xtkfw qnpjy mjuwj xjsy ujwnt iymfy xtrjt knyxs tnxnj xyfzy mtwny njxns xnxyj itsny xgjn slwjh jnaji ktwlt titwk twjan qnsym jxzuj wqfyn ajijl wjjt khtru fwnxt stsqd

**Text 2.**ttbti swhot istta osmwh gflhs tsecf liaho ondia henit ahena nwtpnf ewtie fpree rhbou hnhbo uerli deovw rlode oeasr hrdsa itrei ttein ittie ntote gceoo rrits etegc psoya hsfmt sesfm iahew dtseo oiewh pheet tecir uytss sohts ssoks isero oisen oeawa vtnee watne ewagn rtenw egnit htwih tpiao reeet eoaoo sieuo tiiei ieidg dfvih pliee omrol setet wtese iotao siaoo fwphe lwtof wtofs tsipt wtsid egfed gfweo gtaea grehn oeofl psrdm fssri sdbnv foone avefi nweoi arowg fiaef nsteb isefc tieag ieare ahgha hrdhy irsoi rseli ceeli ctryt ewskh nphst oahss nsrer oelur droan

## Part 4: Vigenere Cipher

## Activity: Vigenere Cipher

**By Hand:**Use the Vigenere cipher to encrypt your name. Choose your own keyword.**Decrypt**the following message, which was encrypted using Vigenere cipher with the keyword*zebras*.SLJJ IK OSMPADOLBSELHG

**Frequency analysis:**Use the*Frequency Analyzer*tool (above) to count the letter frequencies in the following text, which is the same text that was encrypted in an earlier exercise. In this case it was encrypted using Vigenere cipher. What differences do you observe from the histograms you used in the previous exercise.sabjt zdffj tgexj dekhx xrslg ixfrk ssgki edwj kwsrx ivayd sgnik csnzt ozwuy esfip wfgnp jjhfd wtzt ozwuy ewosd yoxai mzexh xxrsl gifgo ugsgz nuqie llasc jkws rxivs wzwpe oxhki kilve tkhwr ibjof njbik fdwt ztozw uyeko vjegg elpge asabj tzdaj etwqs gueko ejiw wgeev vwqcu yifff fwojd ytnez zhoft zhrhs exnvf lsod afies kphfi ffhji eusxp vandr xvwwq ibcly nmoxd aqidk tzds uyejv ezznk gsskt zdtfi igcab jsgee scicd xivpj dwfet hdvj fdlge ujoed sgztk msjji wrxbl tznvj kiwrm ojiks iefna swcv iffvf teaui ewojf spuoj essvv akmok hwryq vrdzx jmevd ksve gegpd psqmt fngmp z

## Part 5: Perfect Secrecy and the Key Exchange Problem

## Self-Check

Here is a table of some of the technical terms discussed in this lesson. Hover over the terms to review the definitions.

cipher (système de cryptage)

cryptographie

encryptage

texte simple

texte cryptédécryptage (ou décodage)

clef d'encryptage

algorithme d'encryptage

encryptage symétrique

cryptage de substitutioncryptage par transposition

attaque par force brute

analyse dela fréquence

substitution polyalphabétique

one time pad

problème d'échange de clefs

*1 point*

*1 point*

*using a Caesar cipher with a shift of 3. Type your answer into the Textbox.*

**alphabet***1 point*

*1 point*

**ecguct**. What word is it? Type your answer into the text box.

*1 point*

*1 point*

*, was encrypted with a Caesar cipher and when you do a frequency analysis you learn that the most frequent letter was 'i'. What is the secret word? Type your answer into the Textbox. (Make sure there are no extra spaces in your answer.)*

**EAIWSQI***1 point*

*Caesar*cipher,

*simple substitution*cipher, and

*Vigener*cipher are all examples of __________ ciphers.

*1 point*

*polyalphabetic*cipher is one that ______________.

*1 point*

*key exchange problem*is ____

## Still Curious

Ever want to incorporate encryption into one of your App Inventor apps? Here's a Caesar cipher tutorial that shows you how to do that.

After implementing the Caesar cipher, if you want an additional challenge, try
creating an app that does *Simple Substitution*. Or try incorporating an
encryption scheme into one of your own apps.

## Reflection: For Your Portfolio

Create a new page named*under the*

**Cryptography Basics***Reflections*category of your portfolio and write brief answers to the following questions.

- Explain briefly how the following substitution ciphers can be broken: Caesar cipher, Simple substitution cipher, Vigenere cipher.
- Define symmetric encryption.