Vigenère Substitution

A cryptographic system. Vigenère substitution makes use of a table of shifting alphabets, each successive row of the table shifting one letter ahead of the preceeding row:

THE VIGENÈRE TABLE:

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

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

A

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

E

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

E

F

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

E

F

G

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

E

F

G

H

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

D

E

F

G

H

I

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

A

B

C

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E

F

G

H

I

J

L

M

N

O

P

Q

R

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T

U

V

W

X

Y

Z

A

B

C

D

E

F

G

H

I

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K

M

N

O

P

Q

R

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U

V

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Y

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A

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Q

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G

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Q

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U

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Y

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A

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F

G

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K

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N

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U

V

W

X

Y

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A

B

C

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F

G

H

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J

K

L

M

N

O

P

R

S

T

U

V

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X

Y

Z

A

B

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D

E

F

G

H

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J

K

L

M

N

O

P

Q

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U

V

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A

B

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D

E

F

G

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J

K

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N

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P

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U

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Y



To encode a message, one must use a key word in combination with the table. To demonstrate how a message would be decoded with this method, take the encrypted message “QPVEOTXJZCAZ” which can be decoded with the key word “KEYWORD” .

First, write out the encoded message on a piece of paper:

QPVEOTXJZCAZ

Now write the key word above the coded message, one letter of the keyword corresponding to one letter of the code. Repeat the key word until you reach the end of the code:

k e y w o r d k e y w o
Q P V E O T X J Z C A Z

The letters of the encoded message are changed using the paired key letters written above. The first letter, “Q” , is changed by the key letter “k” . Find the “Q” running across the top row of the Vigenere table and find the “K” in the leftmost column. Trace the “K” row inwards until you intersect with the “Q” column. The “K” row and “Q” column intersect at “A” . So “A” is the first letter of the decoded message! The next letter of the code, “P” , intersects with “E,” the next key letter, at “T” . So “T” is the next code letter. “V” and “Y” , the next pair, also intersect at “T” , so the next letter of the decoded message is another “T” . The first three letters of the decoded message are therefore “ATT” . Decode the rest yourself!

As you can see, it would be impossible to decode the message without knowing the keyword; the Vigenère method creates codes of random letters, making it impossible to analyze the encoded message based on letter frequency or double letters.