hill cipher in matlab

main.m
% Hill Cipher Encryption and Decryption

% Function to get the inverse mod 26
function invModVal = inverseMod(modValue)
    invModVal = -1;
    for i = 1:26
        if mod(modValue * i, 26) == 1
            invModVal = i;
            break;
        end
    end
end

% Function to encrypt using Hill cipher
function cipherText = hillCipherEncrypt(plainText, keyMatrix)
    % Convert the plain text to numbers (A => 0, B => 1, etc.)
    plainText = upper(plainText) - 'A';
    
    % Pad the plain text with 'X' if its length is not a multiple of the key matrix size
    if mod(length(plainText), size(keyMatrix, 1)) ~= 0
        plainText = [plainText; repmat(23, size(keyMatrix, 1) - mod(length(plainText), size(keyMatrix, 1)), 1)];
    end
    
    % Reshape the plain text into a matrix
    plainTextMatrix = reshape(plainText, size(keyMatrix, 1), []);
    
    % Encrypt each block of the plain text
    cipherTextMatrix = mod(keyMatrix * plainTextMatrix, 26);
    
    % Convert the cipher text to characters
    cipherText = char(cipherTextMatrix + 'A');
end

% Function to decrypt using Hill cipher
function plainText = hillCipherDecrypt(cipherText, keyMatrix)
    % Convert the cipher text to numbers (A => 0, B => 1, etc.)
    cipherText = upper(cipherText) - 'A';
    
    % Reshape the cipher text into a matrix
    cipherTextMatrix = reshape(cipherText, size(keyMatrix, 1), []);
    
    % Compute the inverse of the key matrix mod 26
    keyMatrixInvMod = mod(round(det(keyMatrix) * inverseMod(round(det(keyMatrix)))), 26) * inv(keyMatrix);
    
    % Decrypt each block of the cipher text
    plainTextMatrix = mod(keyMatrixInvMod * cipherTextMatrix, 26);
    
    % Convert the plain text to characters
    plainText = char(plainTextMatrix + 'A');
end

% Example usage
plainText = 'HELLO';                                % Plain text
keyMatrix = [6, 24, 1; 13, 16, 10; 20, 17, 15];      % Key matrix

cipherText = hillCipherEncrypt(plainText, keyMatrix);         % Encrypt the plain text
decryptedText = hillCipherDecrypt(cipherText, keyMatrix);     % Decrypt the cipher text

disp(['Plain Text:    ', plainText]);
disp(['Cipher Text:   ', cipherText]);
disp(['Decrypted Text:', decryptedText]);
2210 chars
62 lines