Goal

:

Write

a P

ython program

plaifair

.py

as follows:

def

playfair(key, plaintext):

#

your code here

if __name__ == "__main__":

# your code here

The

conditional statement above

allows

the program

to be

ran

directly.

STEP

A

-

C

ompute

and print the encryption tab

le

1

-

Create

a 5x5 table as a list of lists

.

2

D

efine a function called

init

_table()

which

initializes the table

with stars

. This function does not return anything

.

3

Print out the table. The output should look like this:

* * * * *

* * * * *

* * * * *

* * * * *

* * * * *

4

Define a function called

table_has

(

letter

)

which checks if a

letter already exists in the table

. This function should return true

or false depending on

whether

the letter exists.

5

Define a function

clean_key(key)

which changes

the secret key

to

uppercase and replace I by J, then returns the clean key

.

6

Define a function

set_cell(letter)

whic

h sets a table cell to a

specific letter.

7

Define a function

create

_table(key)

to

populate

the encryp

tion

table given the secret key.

This function should:

-

I

nitialize the table

.

-

C

lean the secret key

.

-

Build

the table using the set_cell

()

and table_has

()

function

s

.

8

Print

out the encryption table.

STEP

B

Encrypt

your

secret message

1

D

efine

a functio

n

find_letter(letter)

which

returns

the row and

column of the letter in the table.

2

-

D

efine a

function

encode_pair(a,

b)

which takes

a pair of letters

a and b,

runs t

he encoding rules

and returns the encoded pair as

follows:

-

AB

-

> LF (sample)

-

F

unction should print an error message if a equals b

and stop

program execution.

Suggested approach

:

-

C

ompute

a_row, a_col, b_row, b_col

using the find_letter

function

.

-

U

se an if

-

elif

-

else statement to compute a_new_row, a_new_col,

b_new_row

, b_new_col

based on the encryption rules

.

-

Optional: To make your code cleaner, y

ou may want to create

separate

encoding

functions for the 3 different

cases:

same

row

(

with

wrapping

)

, same column

(

with

wrapping

)

, and rectangle

.

3

D

efine a function

cleanup(plaintext)

which

return

s

the cleaned up

message as follows

:

-

Change

s

it to uppercase

.

-

Remove

s

spaces

.

-

Replaces J by I

.

-

Places a letter X between 2

consecutive repeating letters

as

follows:

o

EE

-

> EXE

-

I

f the repeated

letter

happen to be an

X, insert the letter Q

in between as follows:

o

XX

-

> XQX

-

If the length of the resulting text is an

add

number, add the

letter Z

at the end as follows:

o

..G

-

> ..GZ

-

If the last letter happens to be a Z

, add

the letter Q at the

end as follows:

o

..Z

-

> ..ZQ

Su

ggested appro

ach

:

Note that fixing a problem may lead to an new

one

. E.g. insert

ing

an

X may lead to

a

plaintext with an odd

length. Therefore, you should

c

r

eate a while loop which will

repeatedly

call a function to

fix

issue

s

and return a

new

plaintext until no more issues are found.

STEP C

Encrypt your secret message

1

Define

a function

encrypt

(

plaintext

)

which encode

s

the message

plaintext

and

returns the cipher

text.

Make

sure

to insert a space

every 5 letters

in the resulting ciphertext

(e.g. KLDGD NXSLO CWGQH

KUINF ACWER)

2

Execute and print the ciphertext. The execution phase should:

a.

Prompt for the secret key

.

b.

Prints the secret key as

entered.

c.

Prompt for the secret message (plaintext)

.

d.

Prints the secret message as

entered.

e.

Computes and p

rints the encrypted message (ciphertext)

.

f.

Asks if

user

want

s

to encrypt another message

o

If yes, program goes back to step a

.

o

If no, program stops

ex

ecution.

STEP D

Extra

-

Credit

Create a

function

decrypt

(key, ciphertext)

which takes two parameters,

the secret and the ciphertext and returns the plaintext.

THE SUBMISSION

Submit a playfair.txt

or py

file as follows:

-

Program should be structure as

described in this project.

-

A

ll functions

should have the same names as indicated above. T

hey

may be tested separately for partial credit in case your program

fails.

-

All functions have a comment above them explaining their purpose.

-

Submission should be free

from debugging code or test code.

TESTIN

G YOUR CODE

Not to include with you

r

submission

Create a Python program called test_playfair.py in and place it in the

same directory as your main program. The code should import your

encryption code and then

run some tests as follows:

import playfair

def

test(key, plain, expected):

cipher = playfair.playfair(key, plain)

if

cipher != expected:

print("%s : %s != %s" % (plain, cipher, expected))

# execute test:

test(mysecretkey, this is my secret message,

WQIUW QOKSG ADAJH

AOIQU WIQUW

)