The document discusses the Playfair cipher encryption method. It consists of generating a 5x5 key square using the keyword, with each letter appearing only once. To encrypt a pair of letters: if they are in the same column, take the letters below; if the same row, take the letters to the right; otherwise, take the letters at the opposite corners of the rectangle formed by the letters. An example is then provided to encrypt the plaintext "cryptography" using the keyword "playfair".