Bifid

Explanation

The Bifid cipher is a type of poly-alphabetic substitution cipher that combines elements of both transposition and substitution.

It was invented by Felix Delastelle in 1901 and was considered one of the strongest ciphers at the time due to its complexity.

Encryption is done by creating a 5x5 grid, known as the Polybius square, is created. The letters of the alphabet (excluding 'J' in most cases) are arranged in the grid, typically starting with 'A' and ending with 'Z', omitting 'J'.

Each letter of the plaintext is first converted into its corresponding row and column numbers in the grid. For example, the letter 'C' may correspond to the row 1, column 3. These numerical representations are then combined into pairs of numbers.

The pairs of numbers are then arranged into a single line of digits. For example, if the plaintext is 'HELLO', and the corresponding pairs of numbers are (1,3), (4,1), (2,4), (2,4), and (4,2), then the resulting line of digits would be '134142442'.

The resulting line of digits is then transposed using a keyword or keyphrase. This typically involves writing the digits into a grid of a certain width determined by the length of the keyword, and then reading the columns of the grid in the order specified by the keyword.

The transposed digits are then converted back into pairs of row and column numbers, which correspond to the letters in the Polybius square. The resulting pairs of numbers are then used to retrieve the ciphertext letters from the grid.

The Bifid cipher provides a higher level of security compared to simple substitution ciphers due to its combination of fractionation and transposition. However, it can still be susceptible to cryptanalysis, especially if the keyword is short or if there are patterns in the plaintext.

Overall, the Bifid cipher is a historically significant encryption technique that demonstrates the use of both fractionation and transposition in cryptography.

Facts

The Bifid cipher was used by the French military during World War I.