Codes And Ciphers

#13 Bifid Cipher

#Cryptography

LECTURE #13: BIFID CIPHER

[INTRODUCTION]
Bifid is a cipher is a combined Polybius square and transposition cipher, it uses fractionation to achieve diffusion. The cipher was invented by Felix Delastelle. Delastelle was a Frenchman who invented several ciphers including the bifid, trifid, and four-square ciphers. The first presentation of the bifid cipher appeared in the French Revue du Génie civil in 1895 under the name of cryptographie nouvelle. Though never used militarily, it is popular among classical cryptographers because it is simple to operate manually yet reasonably secure. Bifid cipher uses a Polybius square to substitute a digit-pair for each plain-text letter, then transposition causes fractionation of the digit pairs, creating diffusion, and finally the transposed digits are reformed into cipher-text letters. An example is shown below:

[ENCRYPTION and DECRYPTION]
*First, we need a polybius square.

1 2 3 4 5
1 A B C D E
2 F G H I/J K
3 L M N O P
4 Q R S T U
5 V W X Y Z

*Our message is "WATSON"

* To encipher a message, write the row and column numbers of the letters in two rows below the message:

W A T S O N
5 1 4 4 3 3
2 1 4 3 4 3

*Then the digits are read off by rows, in pairs, and converted back to letters:
51 44 33 21 43 43  VTNFSS

*So the cipher-text is "VTNFSS".

*Deciphering is the inverse operation.

*Our cipher-text is "VTNFSS"

*Encrypt it using Polybius Square

VTNFSS --> 51 44 33 21 43 43

*Obviously, we have a six pairs of numbers since the cipher-text are consist of 6 letters.

*Then, separate every whole number to each other.

5 1 4 4 3 3 2 1 4 3 4 3

*Lastly, count the numbers into six (since the cipher-text are consist of 6 letters) and group them into two (above and below row)

5 1 4 4 3 3
2 1 4 3 4 3

52 - W
11- A
44 - T
43 - S
34 - O
33 - N

*So the message is "WATSON"

[EXERCISE]

Now, let's try what you learnt. Decrypt this: "EDWNLSADGUDSPEIOBXPYNSFSS".

PM Swordcane for your answer.

- RMPM -

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