LearnITWithMrC

Set your Learning Objectives to red.
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To download the PowerPoint by clicking on the image below

Save your PowerPoint as 'Lesson 4'

Frequency Analysis

Go to the slide in you powerpoint like the image below and try and complete the fact list.

Do a little research and see what you can find out.

Frequency Analysis

In cryptography, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking substitution ciphers

Frequency analysis consists of counting the occurrence of each letter in a text. Frequency analysis is based on the fact that, in any given piece of text, certain letters and combinations of letters occur with varying frequencies. For instance, given a section of English language, letters E, T, A and O are the most common, while letters Z, Q and X are not as frequently used.

The chart below shows the frequency of each letter of the alphabet for the English language:

We can assume that most samples of text written in English would have a similar distribution of letters. However this is only true if the sample of text is long enough. A very short text may lead to a significantly different distribution.

When trying to decrypt a cipher text based on a substitution cipher, we can use a frequency analysis to help identify the most recurring letters in a cipher text and hence make hypothesis of what these letters have been encoded as (e.g. E, T, A, O, etc). This will help us decrypt some of the letters in the text. We can then recognise patterns/words in the partly decoded text to identify more substitutions.

Click on the button below to go to the frequency analysis task.

Decode as many of the encoded messages as you can

Go to the slide in you powerpoint like the image below and try and complete the fact list.

Do a little research and see what you can find out.

The Vigenère Cipher

The Vigenère cipher, was invented by a Frenchman, Blaise de Vigenère in the 16th century. It is a polyalphabetic cipher because it uses two or more cipher alphabets to encrypt the data. In other words, the letters in the Vigenère cipher are shifted by different amounts, normally done using a word or phrase as the encryption key .

Unlike the monoalphabetic ciphers, polyalphabetic ciphers are not susceptible to frequency analysis, as more than one letter in the plaintext can be represented by a single letter in the encryption.

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The Vigenère cipher uses this table in conjunction with a key to encipher a message.

So, if we were to encode a message using the key COUNTON, we write it as many times as necessary above our message. To find the encryption, we take the letter from the intersection of the Key letter row, and the Plaintext letter column.

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Complete the slides below in your powerpoint

Summing it all up

Lets look at the learning outcomes and decide which one best describes our current level of understanding :

Tick the one you feel is closest to your level

Learning Outcomes I need to learn how to collaborate with fellow code breakers to crack messages

I have a basic understanding of how I can collaborate with fellow code breakers to crack messages with a little help from my teacher
I can show my teacher that I can collaborate with fellow code breakers to crack messages without their help.
I can collaborate with fellow code breakers to crack messages independently and I can also explain it to others and can complete any extension tasks I am given.

Understanding Ciphers

What am I Learning today?

Knowledge Organiser

Lesson

Task 1 - Getting Organised - PowerPoint Click to see more

To download the PowerPoint by clicking on the image below

Task 2 - Starter Task Click to see more

Frequency Analysis

Task 3 - Frequency analysis Click to see more

Frequency Analysis

Task 4 - Extension Click to see more

Task 5 - Understanding Vigenère Click to see more

Task 6 - The Vigenère Cipher Click to see more

The Vigenère Cipher

Task 7 - Lesson review Click to see more

Summing it all up

Learning Outcomes I need to learn how to collaborate with fellow code breakers to crack messages

🠜 Now update your learning objectivesClick on the Assessment image

My Notes: cryptography

Task Notes/Comments - Add here Click to see more

Comments/Notes