Which cipher uses a 5x5 table containing a keyword or key phrase to encrypt pairs of letters?

The Playfair cipher is a historic encryption method that employs a 5x5 grid or table filled with letters derived from a keyword or key phrase. Each letter of the keyword is placed in the grid, and any repeating letters are removed. The remaining spaces are filled with the rest of the letters in the alphabet, combining 'I' and 'J' into a single cell to fit into the 5x5 structure.

In this cipher, plaintext is processed in pairs of letters. When a pair of letters is to be encrypted, their positions in the grid determine the corresponding ciphertext letters; if both letters of a pair are in the same row, each letter is replaced by the letter to its immediate right, wrapping around to the start of the row if necessary. If they appear in the same column, each letter is substituted with the letter directly below it, again wrapping around as needed. If the letters form a rectangle within the grid, the letters are substituted according to their diagonal positions.

This method provides a significant improvement in security over simpler ciphers that operate on individual letters, making it more resistant to frequency analysis. The unique structure of the Playfair cipher and its reliance on pairs of letters fundamentally distinguish it from the other ciphers listed, such