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Bifid Cipher

Tool to crypt/decrypt the bifid cipher. The bifid cipher uses coordinates of letters and mixes them in order to get new coordinates. This tommographic process is described in Cryptography Elementary Treaty by Delastelle.

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Bifid Cipher -

Tag(s) : Polygrammic Cipher

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# Bifid Cipher

## Bifid Encoder

Tool to crypt/decrypt the bifid cipher. The bifid cipher uses coordinates of letters and mixes them in order to get new coordinates. This tommographic process is described in Cryptography Elementary Treaty by Delastelle.

### How to encrypt using Bifid cipher?

Bifid encryption requires a grid (or a keyword to generate the grid) and (optional) a number N which will serve as a period/block length (otherwise take N=1).

Example: Encrypt the plain message DCODE with the grid (generated with the word SECRET):

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

Encryption first consists in splitting letters in blocks of size N. This text-splitting is not mandatory, but simplifies encryption/decryption for long texts.

Example: A period of length N=3 for DCODE gives DCO,DE (if the block is incomplete, it does not matter)

For a non splitted encryption, imagine a period size of N=1 (or a period size equal to or greater than the number of letters in the plain message)

For each letter of the block, write the coordinates of the letters (row, column) in a table.

Example: Take the first block DCO. D=(2,3), C=(1,3), O=(4,2) and write it in a table:

 D 2 3 C 1 3 O 4 2

To get new coordinates, read the numbers of the table vertically by columns.

Example: The vertical reading gives 2,1,4,3,3,2 or the coordinates (2,1),(4,3),(3,2).

Replace the coordinates with the corresponding letters in the grid.

Example: (2,1) for A, (4,3) for P and (3,2) for I.

These steps are repeated for each block.

Example: The final encrypted message is APIAI

### How to decrypt Bifid cipher?

Bifid decryption begins identically to encryption.

Example: The message DBAKS has been encrypted with a period N=3 and the grid (generated with the word MESSAGE):

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

The message is split into period/block of size N

Example: The message is decomposed in block of 3: DBA,KS

Each letter is associated with its position in the grid as coordinates (row, column)

Example: The letters of the block D,B,A have the respective coordinates (2,3),(2,1),(1,4).

Write the coordinates on 2 lines (and thus N columns, eventually except the last block)

Example:

 2 3 2 1 1 4

Example: You get 2,1,3,1,2,4 or (2,1),(3,1),(2,4).

The new coordinates are then associated with the corresponding letters in the grid.

Example: You find the plain letters (2,1)=B, (3,1)=I and (2,4)=F

These steps are repeated for each block.

Example: The plaintext message is BIFID.

### How to recognize Bifide ciphertext?

The message has a low coincidence index around 0.04 to 0.05.

If the grid is 5x5 then it can have at most 25 distinct characters.

### What does bifid means?

Bifid means 'that is cut in half lengthwise', which applies to the Delastelle cipher: the coordinates are cut in 2 during the encryption and decryption steps.

### Why the period should not be N=1?

With a period of 1, the encryption or decryption algorithm has no effect (because the principle of column writing and row reading does not produce a shuffling effect)

### When Bifid have been invented ?

Delastelle described this process in 1902.

## Source code

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