Code 54 : Alberti Cipher

Alberti Cipher

Background

Ciphering wheels were one of the first devices that were developed to aid people in encoding messages. The Italian Leon Battista Alberti (* 14. February 1404 in Genua; † end of April 1472 in Rome) is assumed to be the inventor of the ciphering wheel.

Fig. 1: Ciphering wheel from the American civil war.¹

As can be seen in figure 1, there is an outer and an inner wheel with different alphabets. Encoding is achieved by turning the wheels. The inner wheel shows the plaintext characters and the outer wheel shows the ciphertext characters. In fig. 1 the R currently corresponds to the 1, the 7 to the 11, the U to the 18 etc. The key that was used in fig. 1 can be phrased as for example C=88.

Principle

The original ”Alberti cipher“ had only two arbitrarily chosen ciphertext alphabets, which were used alternately.²

Plaintext alphabet:

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

Ciphertext alphabet 1:

Q

W

E

R

T

Z

U

I

O

P

A

S

D

F

G

H

J

K

L

M

N

B

V

C

X

Y

Ciphertext alphabet 2:

P

L

M

K

O

I

J

N

B

H

U

Z

G

V

C

F

T

R

D

X

Y

S

E

A

W

Q

If, for example, the text ”Geheimnis“ should be encoded, the first character G would be mapped with a Caesar cipher and the key U, which yields the character E. A Caesar cipher with the key O would be applied to the second character of the message and a Caesar cipher with the key H to the third character. Completely encoding the message like this yields the ciphertext: ”UOIOOGFBL“.

Security

If the ciphering wheel is set up once, and then used for the whole message, using the ciphering wheel corresponds to a monoalphabetic substitution. This would not be much more difficult than to break the Caesar cipher.
A possible key could be: ”First C=88: After encoding one character, turn the inner wheel one position to the right. After the next encoded character, turn the inner wheel two positions to the right, for the next one three positions and so on.“.  This results in a polyalphabetic substitution, similar to the Vigenère cipher.

Weblinks

http://en.wikipedia.org/wiki/Alberti_cipher

References

¹ http://de.wikipedia.org/w/index.php?title=Datei:UnionCipherDisk.nsa.jpg
² Kryptographiespielplatz