Digraph 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 Digraph Cipher is a type of substitution cipher that encrypts pairs of letters (digraphs) from the plaintext, rather than single letters as in simpler substitution ciphers. The Playfair cipher is one well-known example of a digraph cipher.

Below is an example of a Playfair Cipher table (which is a type of Digraph Cipher) using the keyword "CIPHER." The table is constructed by placing the keyword at the start and filling the rest of the 5x5 grid with the remaining letters of the alphabet (I and J are typically treated as the same letter in Playfair).

Step 1: Construct the Playfair Table

Keyword: CIPHER

1. Remove duplicate letters from the keyword.
   • CIPHER → C, I, P, H, E, R
2. Fill the table with the keyword first, followed by the remaining letters of the alphabet (excluding J, or combining I/J):

Playfair Cipher Table Example:

Step 2: Encrypting with the Digraph Cipher

To encrypt with the Playfair cipher, you follow these basic rules:

1. Pair the letters of the plaintext into digraphs (groups of 2). If a digraph contains the same letter twice (e.g., "LL"), insert an "X" between them.
2. Locate the letters of each digraph in the table.
   • If both letters appear on the same row, replace them with the letters immediately to their right (wrapping around if needed).
   • If both letters appear in the same column, replace them with the letters immediately below them (wrapping around if needed).
   • If the letters form a rectangle, replace them with the letters at the other corners of the rectangle.

Example

Suppose we want to encrypt the word "HELLO":

1. Convert to digraphs: "HE LL O"
   • "LL" becomes "LX" (to break the repeated letters)
   • So, now we have the pairs: "HE", "LX", "LO"
2. Use the Playfair table to find and replace each digraph:

• "HE": H and E are in the same row. The letter to the right of H is E, and the letter to the right of E is C. So, "HE" becomes "EC".
• "LX": L and X form a rectangle. Replace L with X and X with L, so "LX" becomes "PX".
• "LO": L and O form a rectangle. Replace L with G and O with R, so "LO" becomes "GR".

3. The final encrypted message is: "EC PX GR"

This is how a Digraph Cipher using the Playfair cipher works with a simple table and a keyword.