# Cryptography Activity

AI and Robotics

Nov 21, 2013 (4 years and 7 months ago)

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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

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

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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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:

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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.

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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:

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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