The **Bifid Cipher** is a Fractionating Transposition cryptographic technique that was invented by the French amateur cryptographer Félix Delastelle in 1901. It is a fractionating transposition cipher that combines elements of both substitution and transposition methods.

The Bifid Cipher operates on a square grid known as the Polybius square or Polybius checkerboard. This square consists of a 5x5 grid containing the letters of the alphabet (usually excluding the letter 'J').

The encryption process in the Bifid Cipher involves the following steps:

- Key Generation: A keyword or passphrase is chosen, and any duplicate letters are removed. The remaining unique letters of the keyword are then combined with the remaining letters of the alphabet to form the key.
- Polybius Square: A 5x5 Polybius square is created, with the letters of the key filling the first row, and the remaining letters of the alphabet filling the subsequent rows in order.
- Message Conversion: The plaintext message is divided into individual letters and each letter is replaced with its corresponding row and column numbers in the Polybius square.
- Fractionation: The row and column numbers obtained in the previous step are combined into pairs, creating a new sequence of numbers.
- Transposition: The pairs of numbers obtained from the fractionation step are written as a single sequence and divided into two equal parts.
- Encryption: The row and column numbers corresponding to each part are located in the Polybius square, and the corresponding letters are identified.

The resulting encrypted text is a substitution cipher, where each letter in the plaintext is replaced with a pair of letters in the ciphertext. To decrypt the Bifid Cipher, the process is reversed, using the same key and Polybius square.

The Bifid Cipher offers a level of security by fractionating the plaintext, making it more resistant to frequency analysis and other classical cryptographic attacks. However, modern cryptanalysis techniques have rendered the Bifid Cipher relatively easy to break.

Despite its susceptibility to modern cryptographic analysis, the Bifid Cipher remains an interesting historical cipher and serves as a foundation for more complex cryptographic systems.