# Digraph Cipher

- Substitution Cipher

A Digraph Cipher is a cryptographic technique that operates on pairs of letters (digraphs) rather than individual letters. It is a substitution cipher where each digraph in the plaintext is replaced by a corresponding digraph in the ciphertext according to a predefined rule or key.

Here's a general overview of how a Digraph Cipher works:

Key Generation: The sender and receiver agree on a specific encryption rule or key that determines the mapping between digraphs in the plaintext and the corresponding digraphs in the ciphertext.

Splitting into Digraphs: The plaintext is divided into pairs of letters (digraphs). If the plaintext contains an odd number of letters, a padding character (such as 'X') may be added at the end to form a complete digraph.

Encryption: Each digraph in the plaintext is replaced by the corresponding digraph in the ciphertext according to the encryption rule or key.

Decryption: The receiver uses the same encryption rule or key to decipher the ciphertext. Each digraph in the ciphertext is replaced by the corresponding digraph in the plaintext.

The encryption and decryption rules in a Digraph Cipher can vary depending on the specific algorithm or key chosen. Some common approaches include:

Digraph Substitution: Each digraph is replaced with a different digraph based on a substitution table or matrix. For example, the digraph "AB" in the plaintext might be replaced by the digraph "XY" in the ciphertext.

Digraph Transposition: The order of the digraphs in the plaintext is rearranged according to a specific transposition rule or permutation. This rearrangement can be based on the positions of the letters within the digraphs or other predetermined patterns.

Digraph Combination: The encryption process may involve a combination of substitution and transposition techniques, where digraphs are both substituted and rearranged.

The security of a Digraph Cipher depends on the complexity and randomness of the encryption rule or key. It is important to use a sufficiently large set of possible digraph mappings to ensure resistance against cryptanalysis techniques such as frequency analysis.

Digraph Ciphers have been used historically as a way to enhance the security of simple substitution ciphers by introducing an additional layer of complexity. However, they are generally considered relatively weak compared to more modern and sophisticated encryption methods.

It's worth noting that while the concept of Digraph Ciphers provides an interesting perspective on encryption, they are not commonly used in modern cryptography. Instead, more advanced algorithms, such as symmetric key ciphers (e.g., AES) or public-key ciphers (e.g., RSA), are employed to ensure stronger security.

A B C D E
A AA AB AC AD AE
B BA BB BC BD BE