How can we get rid of the tell tale fingerprints of the charactertistics frequencies?

Blaise de Vigenere

Idea: Use different shifts for different characters in the message: The amount you shift depends on the relative position of the character in the message
 
 
 
 

keyword: VERMONT
 

V

E

R

M

O

N

T

V

E

R

M

O

N

T

V

E

R

M

I

D

D

L

E

B

U

R

Y

C

O

L

L

E

G

E

 
 

V	E	R	M	O	N	T	V	E	R	M	O	N	T	V	E	R
22	5	18	13	15	14	20	22	5	18	13	15	14	20	22	5	18
13	9	4	4	12	5	2	21	18	25	3	15	12	12	5	7	5
M	I	D	D	L	E	B	U	R	Y	C	O	L	L	E	G	E

22	5	18	13	15	14	20	22	5	18	13	15	14	20	22	5	18
13	9	4	4	12	5	2	21	18	25	3	15	12	12	5	7	5
35	14	22	17	27	19	22	43	23	43	16	30	26	32	27	12	23
9	14	22	21	1	19	22	17	23	17	16	4	26	6	1	12	23
I	N	V	U	A	S	V	Q	W	Q	P	D	Z	F	A	L	W

M

I

D

D

L

E

B

U

R

Y

C

O

L

L

E

G

E

I

N

V

U

A

S

V

Q

W

Q

P

D

Z

F

A

L

W

OBSERVE:

D has been encrypted as V and as U

L has been encrypted as Z and as F

E has been encrypted as Aand as W

Ciphertext A came from L and E

Ciphertext W came from R and E

The resulting frequency distribution is much flatter.

Vigenere.html

1881. La Jangada or Eight Hundred Leagues on the Amazon

SECURITY OF VIGENERE CIPHER

Once Thought Unbreakable.

French: "Le Chiffre Indechiffrable"
 

Used by Confederacy in Civil War

"Manchester Bluff"

"Come Retribution"

"Complete Victory"
 

Basis of German Enigma Machine
 
 

Vigenere Cipher cracked by

Charles Babage (1854)

Friedrich W. Kasiski (1805 - 1881) (1863)

How to crack a Vigenere Cipher:

Determine length of keyword

Use frequency analysis on subsets of the cipher text consisting of text enciphered with the same shift.

Generate a sequence of random shifts (between 1 and 26) on a pad of paper and use them to encipher your plaintext.

Shift each character by the next shift on the pad. When you're done, mark your location in the pad. Start next message at the new location. Use the pad only once.

The One-Time Pad is a Vigenere cipher with an infinitely long randomly chosen keyword.

Example:

Random stream is 9, 26, 1, 7, 23, 15, 21, 14, 11, 11, 2, 8, 9, 21,...
 

S	E	N	D	M	O	R	E	M	O	N	E	Y
19	5	14	4	13	15	18	5	13	15	14	5	25
9	26	1	7	23	15	21	14	11	11	2	8	9
2	5	15	11	10	4	13	19	24	26	16	13	8
B	E	O	K	J	D	M	S	X	Z	P	M	H

In Theory: Completely Secure
 

Practical Problems:

Pad must be randomly generated.

You and your friend need identical pads.

You must stay synchronized.

You must use the pad only once.

Creation, Distribution, and Storage of pads are a problem.

How many pads would the State Department require?
 

Part 4