Bases de la cryptographie

Durée estimée: 90 minutes

Text Version


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.


Part 1: Caesar Cipher

Activity: Caesar Cipher

  1. 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.

  2. Brute force search. A brute force search for a Caesar key would be to test every possible key. How many keys would you have to test?

    Use brute force search to crack the following message. What is the key?


Part 2: Simple Substitution Cipher

Activity: Simple Substitution Cipher

  1. 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.
  2. Decrypt: The following word, SIRTQSMTCKJ, was encrypted with the keyword simple. Can you decrypt it?
  3. 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

  1. 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

  1. By Hand: Use the Vigenere cipher to encrypt your name. Choose your own keyword.

  2. Decrypt the following message, which was encrypted using Vigenere cipher with the keyword zebras.

  3. 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


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)
texte simple
texte crypté
décryptage (ou décodage)
clef d'encryptage
algorithme d'encryptage
encryptage symétrique
cryptage de substitution
cryptage par transposition
attaque par force brute
analyse dela fréquence
substitution polyalphabétique
one time pad
problème d'échange de clefs
Not yet started
1 point
A cipher is _________________________.

Not yet started
1 point
Encrypt the word alphabet using a Caesar cipher with a shift of 3. Type your answer into the Textbox.

Not yet started
1 point
A Caesar cipher is an example of a ______________________.

Not yet started
1 point
The following word was encrypted using a Caesar cipher with a shift of 2: ecguct. What word is it? Type your answer into the text box.

Not yet started
1 point
One technique that can be used to break a Caesar cipher is called _________________________.

Not yet started
1 point
Suppose that the following word, EAIWSQI, 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.)

Not yet started
1 point
Caesar cipher, simple substitution cipher, and Vigener cipher are all examples of __________ ciphers.

Not yet started
1 point
A polyalphabetic cipher is one that ______________.

Not yet started
1 point
The 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 Cryptography Basics under the Reflections category of your portfolio and write brief answers to the following questions.
  1. Explain briefly how the following substitution ciphers can be broken: Caesar cipher, Simple substitution cipher, Vigenere cipher.
  2. Define symmetric encryption.