Code Cracking

A presentation about the basic skills used in cracking secret codes.

From the collections of The National Archives (United Kingdom)

Learning Question

What are the tricks for cracking other people’s secret codes?

Here are some simple codesyou might be able to crack already…

Crack The Code

Kool ta RUOY rehcaetDNA ELIMS

Alpha Bravo Charlie Delta Echo Foxtrot Golf Hotel India

JuliettKilo Lima Mike November

Oscar Papa Quebec Romeo

Romeo Alpha India Sierra Echo Bravo Oscar Tango Hotel Hotel Alpha

November Delta Sierra Sierra Tango Uniform Victor Whiskey X-ray Yankee Zulu

The Caesar cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet.

Plaintext

Ciphertext

Caesar Cipher

What clues can you find in the coded text above?

What might the single letter words be?

What might the two letter words be?

What might the three letter word be?

Two letters, an apostrophe then a third letter. What could it be?

Did you notice the question mark? What

might the first word of a question be?

What might the word before a quotation be?

What might the double letters be?

A 9 B 6C 1D 0E 4F 5G 11H 3

I 1J 2K 1L 4M 0N 10O 3P 5

Q 6R 12S 0T 3U 2V 8W 1X 1Y 3Z 1

The letters G,N and R occur many times.

What letters might they represent?

Take a section of text from a book, newspaper or webpage and count the number of times each letter occurs.Use pen and paper or ...

Use the ‘Find’ [Ctrl F] facility in Word to highlight all of the instances of a letter

Take a section of text from a book, newspaper or webpage and count the number of times each letter occurs.Use technology to help you.

Letter Frequency Count

Each person should find some ‘typical’ text and do a letter count. You may choose a story or a newspaper article and it should contain around 500 words.

When you have finished you will be able to combine your results with those of the rest of the class using an online form:

Is this ‘typical’ text?

I wish you wd tell me how u.r. when u. write. My smmr hols wr CWOT. B4, we used 2go2 NY 2C my bro, his GF & thr 3 :-@ kids FTF. ILNY, it's a gr8 plc.

Is this ‘typical’ text?

This morning I saw a Zebra Waxbill. A Waxbill is a small bird and the one I saw is often confused with the Gold-breasted Waxbill, the Yellow-breasted Waxbill or the Box-breasted Waxbill. The feature that makes the Zebra Waxbill stand out is its stripes. All Waxbills have some stripes but the Zebra Waxbill has extra large extended stripes, more than any other Waxbill I know…

Is this ‘typical’ text?

Je me demande si la fréquence des lettres est la même dans toutes les langues. J'en doute. Si je suis en train de décoder un texte en anglais, je ne voudrais pas utiliser les statistiques sur la fréquence des lettres françaises. Ce ne serait pas très utile serait-il? Assurez-vous que le texte que vous utilisez est écrit en anglais standard.

Action Plan

1. Find some ‘typical’ text and do a letter count.

2. Combine your results with those of the rest of the class using a Google online shared spreadsheet

Graph the combined results.

Based on an analysis of 9,481 English books from Project Gutenberg

Top Ten letters

E T A O I N S H R D L

Can you remember what clues we found in the

ciphertext above?

Single letter words are either I or a.

The most common two letter words are of, to,

in, it & is

The most common three letter words are the &

and.

Two letters, an apostrophe then a third letter could be he’s or

it’s

Did you notice the question mark? What

might the first word of a question be?

What might the word before a quotation be?

The most common double letters in order of

frequency are:ss ee tt ff ll mm oo

www.Codecracking.info

Now it is time for you to practice your code cracking skillsby trying this online activity and earning virtual trophies: