Question

1. Following are two problems using a rail fence cipher. Prove both of your answers by showing the steps involved in encryption/decryption. As [art of this problem you must also show the steps involved in reversing what you are being asked for. (20 points)

Using a rail fence cipher with a depth of 2 encrypt the following message:

The bird is a raven

[ your answer goes here ]

Reverse the steps to decrypt your answer and explain the steps.

[ your answer goes here ]

Using a rail fence cipher with zig zag pattern and a depth of 3 decrypt the following message:

      MAEKN   ETTHPRAOEM ETATP

          

[ your answer goes here ]

Reverse the steps to encrypt your answer and explain the steps.

[ your answer goes here ]

Answer 1

1) Encrypting "THE BIRD IS A RAVEN":

Length of Plaintext = 15

Key = 2

At first, we use a grid of size 2x15. The plaintext is written diagonally in a zig-zag fashion.

T

E

I

D

S

R

V

N

H

B

R

I

A

A

E

The individual rows are read one-by-one to get the ciphertext.

Ciphertext is "TEIDSRVNHBRIAAE"

Next -

Decrypting the ciphertext "TEIDSRVNHBRIAAE"

Length of Ciphertext = 15

Key = 2

We start by making a grid with as many rows as the key is, and as many columns as the length of the ciphertext ,i.e, a 2x15 grid. Next, we place "T" in the first square. We then dash the diagonal down spaces until we get back to the top row, and place the "E" here. We continue filling the first row. Finally, we get this -

T

E

I

D

S

R

V

N

-

-

-

-

-

-

-

Continuing this for the next row, we get the following:

T

E

I

D

S

R

V

N

H

B

R

I

A

A

E

Finally, we can read off diagonally in a zig-zag fashion to get back the plaintext.

Plaintext is "THEBIRDISARAVEN".

2) Decrypting "MAEKN   ETTHPRAOEM ETATP":

Length of Ciphertext = 20

Key = 3

We start by making a grid of size 3x20. Next, we place "M" in the first square. We then dash the diagonal down spaces until we get back to the top row, and place the "A" here. We continue filling the first row. Finally, we get this -

M

A

E

K

N

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Continuing this for the second row, we get the following:

M

A

E

K

N

E

T

T

H

P

R

A

O

E

M

-

-

-

-

-

And for the third row,

M

A

E

K

N

E

T

T

H

P

R

A

O

E

M

E

T

A

T

P

We can read off diagonally in a zig-zag fashion to get back the plaintext.

Plaintext is "MEETATTHEPARKATONEPM".

This can be deciphered as "MEET ME AT THE PARK AT ONE PM"

Decrypting the plaintext "MEETATTHEPARKATONEPM":

Length of Plaintext = 20

Key = 3

At first, we use a grid of size 3x20. The plaintext is written diagonally in a zig-zag fashion.

M

A

E

K

N

E

T

T

H

P

R

A

O

E

M

E

T

A

T

P

The individual rows are read one-by-one to get the ciphertext.

Ciphertext is "MAEKNETTHPRAOEMETATP"