Cryptography Activity

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IDEAS & VALUES
:


Cryptography

Frequency distribution/ tally





Problem

solving

Percent





Logic




Teamwork


PRE

䅃T䥖䥔I
:


Content


Students will decipher an encoded text using letter frequencies.



Cryptography
(from Gree
k
kryptos,
“hidden”, and
graphein,
“to write”)





Process


Review activity objectives and guidelines. Prepare participants to participate in discussion, self assess
and set goals.

Each student should have a copy of the cryptogram, letter freq
uencies, instructions, and the tally sheet.
(Included in this document below)


Pre

Activity questions:


1) What is the most

used letter? …the least

used letter?


2) How many
t
’s would you expect to find in a paragraph of 100 letters? In a paragraph of 50
0 letters?


3) As a group, vowels account for what percentage of letters used?


4) Which two letters have the most unusual percentages?


5) Are most letters used rarely or frequently?


ACTIVITY
:


Problem

獯汶sn朠慣瑩v楴y㨠

0) Students should work in pairs
or small groups for the decoding. Each group should let the others
know when they have decoded a letter. One or two students with calculators can figure the percents for
the group.


1) Using the coded story, fill in tally marks for the number of times each

letter is used. This gives the
frequency for each letter.


2) Find the
percent

frequency for each letter.
%
Freq
SingleLett
erFrequenc
y
SumOfAllLe
tterFreque
ncies


100

3) Compare the letters with the highest frequencies to the known frequencies for the English language.
This should gi
ve you some letters you can use to start decoding.


4) Remember:



It is also useful to think about short words: a single

letter word is usually
a

or
I
; a double

letter
word is commonly , to , in, as, an and so on.



Think about the two letters that can appea
r next to each other in different places in a word, and
about the single

letter word that would be capitalized in the middle of a sentence.


5) Decipher the cryptogram. Whenever you can deduce a letter, fill in all the other blanks that use the
same coded
letter. You can write the letters in the blank line above the coded message.





shoulderslyrical_099afef0
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55c72a7b6bcd.doc

11/21/2013





Opening questions:


Round robin: What was the most difficult part of decoding the message? …or the easiest? Explain.



Core Questions:


Have you ever written or received a

coded message? Explain.


If you did
not

know about letter frequencies, what plan would you use to decode this text?


What is a “Caesar cipher”? Does this coded message use a Caesar cipher? Justify your answer.


If you had to select a strategy to use when
putting an important message into code, would you use the



Caesar cipher? Explain.



Closing:

Can you think of a different way to encode a text that would make it more difficult to decode?





POST

䅃T䥖䥔I
:


Process


(Assess individual and group p
articipation in seminar discussion)


Content


(Extend application of textual and discussion ideas; continuation of pre

avtivity.)


Find out if any kind of code or cryptography is used today and for what purpose.


Writing:







shoulderslyrical_099afef0
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Name ______________________________



Cryptogram




Qeshh pho vhsh rkqqkof anvo cq c


qcdih. Noh rcka,“K ikjh ecp.”, coa


noh rcka, “K ikjh b
ehhrh.” Qeh qeksa


noh innjha cq qeh qkph, coa ahbkaha


qn ihcwh qehp qn qehks dskiikcoq


bnowhsrcqkno. “Kg K an onq ihcwh onv,


K vkii ecwh qn ehs c inq pnsh


dcinohl.



















shoulderslyrical_099afef0
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5104
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55c72a7b6bcd.doc

11/21/2013

Name _________________________ Date ________




Ciphertext
Letter

Tally of Letters used in Cryptogram

Letter
Frequency

Percent
Frequency

Plaintext

Letter

A





B





C





D





E





F





G





H





I





J





K





L





M





N





O





P





Q





R





S





T





U





V





W





X





Y





Z











Total:




shoulderslyrical_099afef0
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5104
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55c72a7b6bcd.doc

11/21/2013




Letter Frequencies for Standard English



Letter

Percent

A

8.2

B

1.4

C

2.8

D

3.8

E

13

F

3

G

2

H

5.3

I

6.5

J

0.1

K

0.4

L

3.4

M

2.5

N

7

O

8

P

2

Q

0.1

R

6.8

S

6

T

10.5

U

2.5

V

0.9

W

1.5

X

0.2

Y

2

Z

0.07



TOTAL

99.97