Answer :
The Playfair matrix is constructed using the keyword "PLAYFAIR". It is a 5x5 matrix where each letter of the keyword is placed in order, followed by the remaining letters of the alphabet in order (excluding 'J').
The matrix is then used for encrypting a message. In this case, the message "thisisourfirsttest" is encrypted using the Playfair matrix to produce the ciphertext "O qm:cn:un:vn:dl:britn:mn:tn O gm:nccn:nv.dil:rb:to:nm:nt O qm:cncn:nv:dl:rbit:nmtn qm:cninc:nv.dl:rb:tn:nm:tn". The Playfair matrix is a fundamental component of the Playfair cipher, which is a manual symmetric encryption technique. It employs a 5x5 matrix of letters to perform substitution encryption. The matrix is constructed using a keyword or phrase, in this case, "PLAYFAIR". First, the keyword is written without repetition, and then the remaining letters of the alphabet (excluding 'J') are appended in order. The resulting matrix is as follows:
P L A Y F
I R E S T
U B C D G
H K M N O
Q V W X Z
To encrypt the message "thisisourfirsttest," the plaintext is divided into pairs of two letters (digraphs). If two letters are the same or a pair contains 'J', a filler letter (e.g., 'X') is inserted between them. The digraphs are then processed according to the following rules: 1. If the letters are in the same row, they are replaced by the letters to their right, wrapping around to the left side if necessary. 2. If the letters are in the same column, they are replaced by the letters below them, wrapping around to the top if necessary . 3. If the letters are neither in the same row nor the same column, they form a rectangle, and each letter is replaced by the letter in the same row but opposite corner of the rectangle. The resulting ciphertext is "O qm:cn:un:vn:dl:britn:mn:tn O gm:nccn:nv.dil:rb:to:nm:nt O qm:cncn:nv:dl:rbit:nmtn qm:cninc:nv.dl:rb:tn:nm:tn".
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