# MATH 307 Cryptography with Matrices

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21 Νοε 2013 (πριν από 4 χρόνια και 5 μήνες)

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Dr. Neal, WKU

MATH 307

Cryptography

with Matrices

Goals
: (i) To take a message and encrypt it into a string of numbers via matrix
multiplication. (ii) Then to take the string of numbers and decrypt it back to a written
message. As an
initial example, We shall use the following message:

"HI THERE BUDDY"

Alpha
-
Numeric Substitution

First, we must assign each letter a numeric equivalent. Here, we shall reserve the
number 0 to represent a space and use the numbers 1

26 to represent
the 26 letters.
Rather than just making a direct substitution in order, we can permute the digits in
some way. For example:

A

B

C

D

E

F

G

H

I

J

K

L

M

2

1

4

3

6

5

8

7

10

9

12

11

14

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

13

16

15

18

17

20

19

22

21

24

23

26

25

Coding Matrix

Next, we choose an
invertible

€
n
×
€
n
matrix
A
, for some size
€
n
, which will be used to
encryp
t the message. Here we shall use

A
=
3
2
3

6

5

4
9
8
7








.

Encrypting The Message

We first convert the message into the numeric script, using the alpha
-
numeric
substitution above (H =
7, I = 10, Blank = 0, etc).

7 10 0 19 7 6 17 6 0 1 22 3 3 26

Then we convert this script into a
€
m

×
3 matrix (3 columns because our
A
is 3

×
3).
The number of rows
€
m
depends on the length of the message. We fill out the last row
with more 0's as necessary:

B
=
7
10
0
19
7
6
17
6
0
1
22
3
3
26
0












.

Here,
B
is 5
×
3. Now we multiply:
€
C
=
BA
.

Dr. Neal, WKU

C
=
BA
=
7
10
0
19
7
6
17
6
0
1
22
3
3
26
0













×
3
2
3

6

5

4
9
8
7








=

39

36

19
69
51
71
15
4
27

102

84

64

147

124

95












.

Finally, we convert matrix
€
C

into a string that becomes the encrypted message:

39

36

19

69

51

71

15

4

27

102

84

64

147

124

95

H

I

T

H

E

R

E

B

U

D

D

Y

Notice that “H” has been sent to two different values. The first H has been
encrypted as

39, a
nd the second H has been encrypted as 51. Likewise the three blanks
are assigned different values, as are the two “E”s and the two “D”s.

Decrypting the message

The second party receives only the encrypted message:

39

36

19

69

51

71

15

4

27

102

84

64

147

124

95

However the receiver knows the alpha
-
numeric code and the coding matrix
A
.
Thus, the encrypted script is converted back into an
€
m

×
3 matrix
€
C
and then
multiplied by
A

1
to undo the original matrix multiplication. Because
€
C
=
BA
, we
obtain the original matrix
B
by
€
B

CA

1
, and then we convert the string back to the
alphabet.

ToCharacterCode
Command

When using a software package to encode messages, we can use the built
-
in features
that will automatically convert alpha
betic symbols into numeric equivalents. With
Mathematica
, we can use the
ToCharacterCode
command.

For example, consider the following message:

Dear colleague, what is 4% of 120?

We now need alpha
-
numeric equivalents not only for the 26 le
tters, but also for the
symbols % and ?, and for numbers. Rather than trying to define our own alpha
-
numeric substitution, we can use the built
-
in
Mathematica
command that is also case
-
sensitive.

First, type the message enclosed in quotes. Here
we define the message to be the
term
script
. Then apply the
ToCharacterCode
command.

Dr. Neal, WKU

script="Dear colleague, what is 4% of 120?";

numeric=ToCharacterCode[script]

{68,101,97,114,32,99,111,108,108,101,97,103,117,101,44,32,119,

104,97,116,32,105,115,32
,52,37,32,111,102,32,49,50,48,63}

We observe that a “blank” is represented by 32 and that an “l” is assigned 108.

Mathematica Codes

Now we need a short program that will allow us to input our
€
n
×
€
n
coding matrix
A
,
convert
numeric

into an
€
m
×
€
n
matrix
B
and fill out
the last row with “Blanks,”
multiply
B
A
to get
€
C
, and then convert the matrix
€
C
into just a string of numbers.
This string of numbers wil
l be the encrypted message which is sent. The following
commands perform each of these tasks:

script="Dear colleague, what is 4% of 120?";

A={{3,2,3},{
-
6,
-
5,
-
4},{9,8,7}};size=n=3;Det[A]

numeric=ToCharacterCode[script];

While[Mod[Length[numeric],n]!=0,
AppendTo[numeric,32]]

Do[Do[b[i,j]=numeric[[n(i
-
1)+j]],{j,1,n}],

{i,1,Ceiling[Length[numeric]/n]}];

B=Table[b[i,j],{i,1,Ceiling[Length[numeric]/n]},{j,1,n}];

MatrixForm[B]

Z=B.A;MatrixForm[Z]

encrypt=Flatten[Z]

{471,407,479,1041,860,907,657,546,6
57,648,541,636,141,81,255,

318,301,348,
-
117,
-
130,51,
-
87,
-
109,79,222,175,232,9,
-
32,149,

279,232,283,285,222,285}

We see common letters are assigned to different numerical values. One “l” has been
assigned 546 and the other is given 657.
Now the code cannot be broken by studying
the frequency of numeric occurrences. In addition, different letters may be assigned the
same numerical value. The first 657 in
encrypt
came from the letter “o”, while the
second 657 came from “l”. Likewise, o
ne 285 is from “?” and the other 285 is actually a
“blank.”

Decryption

The person receiving the encrypted message must also know the encoding matrix
A
.
The received
string must first be re
-
converted into a matrix
Y
, that is then multiplied on
the right by
A

1
. The resulting product is put back into a string, and then converted
into its charac
ter code giving the plaintext message. We illustrate with the encrypted
message
encrypt
created above.

Dr. Neal, WKU

encrypt={471,407,479,1041,860,907,657,546,657,648,541,636,141,

81,255,318,301,348,
-
117,
-
130,51,
-
87,
-
109,79,222,175,232,9,

-
32,149,279,232,283,285,222
,285};

A={{3,2,3},{
-
6,
-
5,
-
4},{9,8,7}};size=n=3;

Do[Do[c[i,j]=encrypt[[n(i
-
1)+j]],

{j,1,n}],{i,1,Length[encrypt]/n}];

Y=Table[c[i,j],{i,1,Length[encrypt]/n},{j,1,n}];

V=Y.Inverse[A];MatrixForm[V]

decrypt=Flatten[V]

FromCharacterCode[decrypt]

De
ar colleague, what is 4% of 120?

Assignment

1. Pick a partner. Together decide on a square invertible coding matrix
A
. Adjust the
programs accordi
ngly with this matrix
A
.

2. Each student creates an encrypted message to send to the other. Print out the
program and outputs used to create your encrypted message. On it write "Message
from 'your name' to 'par
tner's name'."

3. Email your partner only your encrypted message. (Send the numbers enclosed in
braces and separated by commas.)

4. Check your email to obtain the encrypted message from your partner. Print out this
email.

5. Decrypt your rece
ived message. Print out the codes and outputs used to decrypt.
On it write, "Message from 'partner's name' decoded by 'your name'."

6. Turn in both print
-
outs along with the received email message.

Note
: If doing this in a computer lab, you can
first create your encrypted message with
Mathematica
. Then highlight and copy the final encrypted message output. Then use
email on the same computer to send it by simply pasting into the body of the message.
Likewise, you can check your email in the
lab, highlight and copy the received message,
then open your
Mathematica
file and paste it into the decryption codes. Otherwise, you
will have to type out the numbers when you send the email, and type them out again
when you decrypt.